UDK 621.785.5:669.14.018.8 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 42(4)157(2008) LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS: MECHANISMS, MICROSTRUCTURES AND HIGH TEMPERATURE OXIDATION BEHAVIOUR NITRIRANJE KOVINSKIH ZLITIN S FLUKSOM Z MAJHNO ENERGIJO IN VELIKO GOSTOTO: MEHANIZMI, MIKROSTRUKTURE IN VISOKOTEMPERATURNO OKSIDACIJSKO VEDENJE Fernando Pedraza Université de La Rochelle. Laboratoire d'Etudes des Matériaux en Milieux Agressifs (LEMMA, EA 3167). Avenue Michel Crépeau, 17042 La Rochelle cedex 01, FRANCE fpedraza univ-lr.fr Prejem rokopisa — received: 2007-09-17; sprejem za objavo - accepted for publication: 2008-06-07 Nitridation is typically carried out to improve wear and erosion of different metal and alloy substrates. In the case of "stainless" alloys, the nitridation temperature needs to be lowered to avoid the precipitation of CrN that would reduce the overall corrosion resistance. Low energy - high flux nitridation allows to nitride relatively thick layers in short times at low temperatures depending on the substrate crystal structure and chemical composition as shown for pure Ni, a Ni-20Cr model alloy, a conventional AISI 304L stainless steel and an ODS FeAl intermetallic alloy. The mechanisms of nitridation, the phases and microstructures are discussed in this work with the support of X-ray diffraction, atomic force, scanning and transmission electron microscopy techniques. The high temperature oxidation behaviour of the nitrided matrices is thereafter evaluated in air and the results are compared to non nitrided specimens. The oxidation kinetics are determined with thermogravimetry and the mechanisms are discussed in light of the oxide phases and microstructures resulting from the previous nitridation treatment. It will be shown that a reduction of the high temperature oxidation resistance occurs for the shortest oxidation times because of trapping of the protective elements. Key words: nitridation, ion implantation, nitrided layer, austenite alloys, ODS Fe-Al alloys, surface oxidation Nitriranje poveča obrabno in erozijsko odpornost podlag iz kovin in zlitin. Pri nerjavnih jeklih je treba znižati temperaturo nitriranja, da bi se izognili izločanju CrN, ki bi zmanjšalo splošno korozijsko odpornost. Nitriranje s fluksom z majhno energijo in veliko gostoto omogoča, da se ustvarijo relativno debeli sloji v kratkem času in pri nizki temperaturi, odvisno od mikrostrukture in kemijske sestave podlage, kot je prikazano za čisti Ni, modelno zlitino Ni-Cr20, konvencionalno jeklo AISI 304 L in za intermetalno zlitino FeAl ODS. V tem delu razpravljamo o mehanizmu nitriranja, fazah in mikrostrukturah na temelju rezultatov difrakcije rentgenskega sevanja, opazovanja atomske sile ter vrstične in presevne elektronske mikroskopije. Ocenili smo visokotemperaturno vedenje nitriranih matic na zraku in ga primerjali z nenitriranimi vzorci. Kinetiko oksidacije smo ugotovili s termogravimetrijo in o rezultatih razpravljamo z upoštevanjem oksidnih faz in mikrostruktur, ki so nastale pri nitriranju. Ugotovili smo, da se zmanjša visokotemperaturna oksidacijska odpornost pri najkrajših časih oksidacije zaradi ujetja varovalnih elementov v pasti. Ključne besede: nitriranje, ionska implantacija, nitrirana plast, avstenitne zlitine, Fe-Al ODS zlitina, oksidacija površine 1 INTRODUCTION Nitriding of austenitic stainless steels has been extensively studied owing to the significant improvements in surface hardness and tribological behaviour 1 as well as in corrosion resistance 2 so long as precipitation of CrN is avoided 3. All these improvements obtained at moderate temperature (T < 450 °C) seem to be associated with the formation of an interstitial solid solution of nitrogen in the steel matrix: face centred cubic (fcc) yN or "expanded austenite". Various studies suggest that the yN would correspond to a fcc phase with a high density of stacking faults likely induced by the internal stresses in the nitrided layer 4-6. However, the effect of the nitriding process to other alloy systems has been poorly investigated to date. For high temperature applications, Ni-base superalloys are typically employed as they show good corrosion and oxidation resistance and excellent resistance to creep and rupture at high temperatures 7. However, they exhibit poor wear resistance. Therefore, plasma nitriding studies have been carried out for instance on Inconel 718 (containing the mass fraction of Cr 20 %) at temperatures between 550 °C and 750 °C leading to precipitation of chromium nitride, CrN, and subsequent increase in Knoop hardness 8 and wear resistance until the nitrided layer is worn away 9. Further studies on plasma assisted nitriding of Inconel 690 (containing the mass fraction of Cr 30 %) have been carried out at temperatures between 300 °C and 400 °C 10 where the different depths of nitrogen diffusion have been related to the grain orientations and the anisotropic dependence of stress on strain 11. The low energy-high flux nitrogen implantation approach has rarely been addressed. This is also known Materiali in tehnologije / Materials and technology 42 (2008) 157, 131-133 131 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS as an implantation-diffusion technique at relatively low temperatures to promote nitrogen diffusion while arresting CrN precipitation in stainless steels 5 6,12,13. Williamson et al. 14 studied a collection of 16 fcc metals nitrided under the same conditions (0.7 keV, 2 mA cm-2, 400 °C and 15 min). It was shown that the Ni-rich alloys contained much less nitrogen with correspondingly thinner layers than the Fe-rich alloys. Besides, no nitrogen could be detected in the pure Ni specimens but an isolated diffracted peak corresponding to the M3N phase. The second study dealt with the tribological properties of Inconel 600 (containing the mass fraction of Cr 16 %) in comparison with the AISI 316 stainless steel, both nitrided at 400 °C for 1 h under 1.2 keV and 1 mAcm-2 15. Again, a thin layer with a maximum concentration of the mole fraction of N 9 % was found in the Ni-rich substrates compared to a 25 at% in the stainless steel, but still offering an increase in hardness and a reduction in wear rate. Despite the extensive use of Ni base superalloys, their significant weight is a limitation in the aeronautic domain as fuel consumption must be reduced. To this end, various intermetallic alloys based on TiAl and on FeAl represent solid alternatives to replace the heavier Ni superalloys 16. In these materials, the nitridation of TiAl have received most of the attention concerning the treatment itself 17-19, their corrosion properties 20 or their high temperature behaviour 21-24. However, little is known on the nitridation of FeAl intermetallic alloys. To the best of our knowledge, only the oxidation kinetics and the likely mechanisms of a nitrided ODS FeAl alloy were reported by Dang et al. 25. Contrary to most of the studies devoted to wear and erosion, the purpose of this work is to review the mechanisms of nitridation by implantation-diffusion (also called low energy-high flux nitridation) in different model (pure Ni, Ni20Cr), commercial (AISI 304L) and candidate materials (ODS FeAl) and the effect on their high temperature oxidation behaviour. The roles of "physics" (crystal structure, grain orientation) and "chemistry" (alloying elements) will be discussed to elucidate the mechanisms involved upon nitridation. On the basis of the resulting phases and microstructures, the high temperature oxidation behaviour will thereafter be interpreted. 2 EXPERIMENTAL Table 1 gathers the base composition and crystal structure of the materials of study. The samples consisted of round coupons of varying diameter and 1 mm thick cut from the bars. The main surfaces were mechanically polished to a final roughness of 0.01 pm. They were then ultrasonically degreased in acetone and rinsed in 96 % ethanol. Low energy - high flux nitrogen (N2+, N+) implantation was carried out at LMP (Poitiers, France) with a Table 1: Substrates major composition ((/%) and the initial crystal structure Tabela 1: Osnovni sestavni elementi v odstotkih in začetna kristalna struktura podlag ODS-zlitine, utrjene z disperzijo oksidov substrate Fe Cr Ni Al Y2O3 matrix Ni - - ~ 100 - - fcc Ni20Cr - 20 80 - - fcc AISI 304L 70 20 10 - - fcc ODS* FeAl 60 - - 38 2 ordered B2 * ODS = oxide dispersion strengthened Kaufman type ion source at 1.2 keV and a current density of about 1 mAcm-2 for 1 h, corresponding to an estimated dose of about 2.25 ■ 1019cm-2. The temperature of the samples was carefully controlled with a thermocouple attached on the back of the samples. Prior to the nitridation treatment, Ar+ sputtering (1.2 keV, 0.5 mA cm-2 for 15 min) was carried out on each main coupon face to remove the rigid oxide layer that precludes nitridation 26. The backing pressure in the chamber upon the nitriding process was better than 10-2 Pa. Implantation was carried out on both principal coupon faces for the subsequent oxidation experiments, representing about 85 % of the overall surface. Oxidation of the nitrided specimens was conducted in a Setaram TG92 thermobalance of 10-6 g of accuracy at 800 °C for 24 h under synthetic air. Heating and cooling rates were fixed at 50 °C/min. Thermodynamic calculations have been performed using the HSC Chemistry software 27 to assess the thermodynamically stable compounds expected to form within the different matrices. The calculations have been carried out at equilibrium conditions at 10-2 Pa (implantation conditions) and at atmospheric pressure (after implantation) disregarding collision cascades and sputtering of the surfaces. Only the gas species N2+ (g) or N2 (g) have been considered to react with the substrates, thus taking into account the splitting of the molecules into 2 nitrogen atoms and the corresponding energy release. The characterisation of the implanted and the oxidised specimens was undertaken using contact mode atomic force microscopy (AFM) with an Autoprobe CPR (Veeco Instruments), by X-ray diffraction in a Bruker AXS D-5005 equipment in the 0-20 configuration and grazing incidence (GIXRD) using Cu K1 (X = 0.15406 nm) radiation as well as by scanning electron microscopy (SEM) coupled to energy-dispersive spectrometry (EDS) in a JEOL JSM-4510 LV. Cross sections of the implanted specimens were also prepared for transmission electron microscopy (TEM) studies in a JEOL-JEM 2010 operating at 200 kV. For such purpose, careful mechanical polishing in SiC# 4000 emery paper was performed down to a thickness of about 50 pm. Then, Ar bombardment at 3 keV was carried out in a GATAN PIPS™ (precision ion polishing system) model 691 at 158 Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS ... 55-, 5.0 45 a 40 Q. O 3.5 3.0 r fi 2.5 ■r ZO 1.5 10 (a) - untreated] ■ ODSFeAl • AIS304L ▲ N20Cr ★ H iV 2 3 4 5 Estimated depth, hip.m 24 2118£ 154 0 n 1 1 6 3 0 (b) nitrided I, ODSFeAl ' AISI304L M20Cr H 2 3 4 5 Estimated depth, hipm Figure 1: Evolution of Vickers microhardness with estimated depth of the (a) untreated specimens and (b) nitrided by implantation-diffusion -NID- Slika 1: Evolucija mikrotrdote po Vickersu z ocenjeno globino; (a) nenitriran vzorec (b) nitriran z ionsko implantacijo in difuzijo - NID different angles. Vickers microhardness measurements were also performed at increasing loads to get acquainted of the effects of the implantation. 3 RESULTS AND DISCUSSION 3.1 Nitridation by implantation-diffusion After nitridation, all the substrates undergo increased surface microhardness compared to the untreated specimens as depicted in Figure 1. In comparison with the untreated specimens, the hardness increase is of about (8, 20, 250 and 280) % for pure Ni, Ni20Cr, AISI 304L and ODS FeAl, respectively. From these results, it can be considered that nitridation does not effectively occur in pure Ni. This can be due to two interconnected mechanisms. The first one is due to the incorporation of N as an interstitial solid solution and/or to the formation of hard metal nitrides, i. e. "structural deformation", i. e. the appearance of harder crystalline phases. The second one is related to an increased plastic deformation typically occurring upon implantation, i. e. "microstructural deformation", i. e. surface roughness. Regarding the crystallographic phases, the XRD patterns after implantation clearly reveal various features and striking differences among the different substrates as shown in Figure 2. In the case of pure Ni [Figure 2(a)] the patterns of the untreated and the nitrided specimens are rather similar. Calculations of the lattice parameters of both untreated and nitrided substrates leads to the iT™ (a) Y200 I Y220 J NID 3 ' ' !■■ untreated 40 60 80 100 2 0/degrees 120 40 60 80 100 2 0/degrees 120 2 0/degrees 46 48 95 96 97 2 0/degrees 99 100 Figure 2: X-ray diffraction patterns of the different substrates untreated and nitrided by implantation diffusion -NID- (a) pure Ni, (b) Ni20Cr, (c) AISI 304L and (d) ODS FeAl Slika 2: Diagrami difrakcije rentgenskega sevanja za različne podlage, nenitrirane in nitrirane z implantacijsko difuzijo (NID): (a) cisti Ni, (b) NiCr20, (c) AISI 304 in (d) ODS FeAl Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 159 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS same results (a0 ~ 0.351 nm) hence indicating no expansion of the matrix volume. The only remarkable changes involves an attenuation of the <111> directions after nitridation compared to the untreated Ni. Williamson et al.14 also claimed the absence of yN peaks in pure Ni at a lower energy and a higher flux than in our studies. However, they observed a hexagonal Ni3N phase and detected a small shift to lower angles, thus implying retention of a very small amount of nitrogen. Contrary to pure Ni, the nitrided Ni20Cr and AISI 304L substrates [Figure 2(b)] exhibit a fcc yN phase 28 at lower diffraction angles and the original phase peaks have shifted towards higher diffraction angles 29. For the sake of comparison between both implanted Cr-con-taining substrates a rough estimation of the retained nitrogen has been carried out using the Vegard's law for substitutional solid solution as follows: a n — a+a • Cn, where an and a are the lattice parameters for the N-containing and N-free phases, respectively, and is the Vegard's law constant (0.00072 for Fe alloys, also assumed for Ni alloys in this study 14). The concentration of nitrogen is the mole fraction in x(N)%. The results are gathered in Table 2. Table 2: Lattice parameters of the N-containing /n and N-free y austenite phases, the relative expansion induced, and their corresponding average atomic nitrogen contents, x(N)%, as a function of the diffraction plane (hkl) in Ni20Cr and AISI 304 L Tabela 2: Mrežni parametri avstenitnih faz y-faz z dušikom in brez njega, relativna inducirana razširitev in ustrezna povprečna atomska vsebnost dušika x(N)% za različne difrakcijske ravnine (hkl) v Ni20Cr in AISI 304L hkl 111 200 220 311 Ni20Cr a N/nm 0.3580 0.3637 0.3589 0.3612 a/nm 0.3538 0.3540 0.3545 0.3548 expansion/% 1.2 2.8 1.2 1.8 x(N)/% ~6 13.5 6 ~9 AISI 304L aN/nm 0.3666 0.3716 0.3666 0.3683 a/nm 0.3572 0.3583 0.3583 0.3583 expansion/% 2.6 3.7 2.3 2.7 x(N)/% 13 18.5 11.5 14 Table 2 shows that the retained amount of nitrogen is highly anisotropic. In Ni20Cr the N content is significantly lower than in the AISI 304L steel regardless of the crystallographic plane. In both substrates however, the highest amount of nitrogen seems to concentrate in the (200) planes and the lowest in the (220). The different partitioning of nitrogen in the various planes also brings about different expansion of the lattice, which in turn may induce strains and stresses. Menthe et al. 30 suggested that a tetragonal distortion of the fcc phase had occurred whereas Fewell et al. 31 proposed a triclinic distortion. Marchev et al. 32,33 considered instead the formation of a martensitic phase. However, any of these would imply the presence of extra peaks never observed on the diffraction patterns. A new structural model nitrogen expanded austenite has been recently proposed by Blawert et al. 4 assuming the effects of deformations and twin faulting commonly observed in fcc metals or alloys. The expanded austenite would correspond to a fcc phase with a high density of stacking faults likely induced by the internal stresses existing in the nitrided layer 5 6. Indeed, it has been shown that the presence of stacking and twin faults in a perfect fcc lattice produces angular displacements of peaks in XRD patterns 34. The three nitrogen solid solutions observed by Leroy et al. 10 after plasma nitriding of the Ni base alloy Inconel 690 (Ni-30Cr-10Fe, w/%) has not been observed in this work using low energy-high flux implantation. In the ODS FeAl intermetallic, the major contribution arises from the (110) and (220) reflections before and after nitridation. At grazing incidence, the hexagonal AlN appears as inferred by three XRD peaks (20 — 33.2°, 36.1° and 38°) and a large and high (110) peak corresponding to the substrate matrix 25. In this alloy, the chemical affinity of N to Al is much greater than that to Fe (e. g., AHf° — -318.0 and -10.5 kJ mol-1 for AlN and Fe4N, respectively) 35 and thus iron nitride formation was not expected to occur. The surface state after nitridation is also quite different among the substrates as shown by plane view SEM in Figure 3. In pure Ni some grains are darker and the orientation of the dislocation slipping bands composing each grain is underpinned; while other grains are lighter in colour and of smoother appearance. In addition, a significant number of protrusions appear throughout the entire surface, especially at grain boundaries. AFM investigations confirm that the roughness can vary between 17.5 nm and 27.5 nm and the aligned bands can be ascribed to the slipping bands due to the presence of stress, as also reported in fcc AISI 316 L stainless steel 36. In Ni20Cr the surface is rather uniform and smooth with no protrusions but with relatively coarse pores. The average roughness is of about 5-8 nm but more significant height differences among grains compared to nitrided Ni. The AISI 304L surface is the most heterogeneous of all three fcc nitrided substrates. Some grains are very smooth and deeper and contain large pores thus reminding of the Ni20Cr grains, whereas other grains resemble more the nitrided Ni by underlining the slipping bands, hence being rougher. A common feature observed on the three fcc alloys is the occurrence of twinning within the grains, but again the morphology of twins differs from one matrix to the other. On the contrary, the ordered B2 cubic structure ODS FeAl, the surface seems very uniformly implanted with no twins but some protrusions at the external surface and remaining porosity. This latter feature can be mainly explained by the manufacturing process of this material, which is powder metallurgy. The elongated shape of the protrusions would be related to "softer" areas of the base material, where the strengthening effect 160 Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS ... A [SI 304L ODS Fe AI Figure 3: SEM surface morphology after low energy-high flux nitridation of (a) pure Ni, (b) Ni20Cr, (c) AISI 304L and (d) ODS FeAl Slika 3: SEM-morfologija povr{ine po nitriranju s fluksom z majhno energijo in veliko gostoto pri (a) ~istem niklju, (b) NiCr20, (c) AISI 304 L in (d) ODS FeAl of Y2O3 particles is less important, as revealed by AFM studies [Figure 4(a)]. This microstructure is accompanied by the highest roughness values, which can attain up to 50 nm. According to the work of Pranevicius et al. 37, the surface roughness can derive from the competition between surface kinetics and bulk diffusion. Nucleation of roughness would first occur by relocation of adatoms, formation of surface vacancies and removal of atoms, which in turn lead to the appearance of clusters of atoms in other regions of the surface. The development of surface roughness subsequently occurs by further relocation and sputtering of atoms displaced by the ion beam. Thereafter, diffusion of nitrogen seems to occur mainly along grain and sub-grain boundaries creating com-pressive stresses 38. Within the metallic substrate, atomic nitrogen can then recombine as molecular nitrogen, raising locally the pressure and inducing plastic deformation. Therefore, the amount of deformation would depend on the yield stress of the host material. As a result, a blistered surface appears 39 40. Due to the recession of the metal surface upon implantation, the blisters are peeled off and the pores are then clearly visible in pure Ni and in Ni20Cr [Figure 4(b)]. Since the solubility of nitrogen in nickel is very low the observed porosity is rather shallow. The larger number of pores and blisters are however found at the grain and twin boundaries rather than within the grains as also inferred in a previous study41. This seems to support the idea that diffusion of nitrogen might be more prone to occur along these short circuit paths, which also become readily Figure 4: AFM images of (a) nitrided ODS FeAl showing ridges pinned by Y2O3 particles (b) nitrided Ni20Cr showing the resulting porosity (views of (10 X 10) |im areas) Slika 4: AFM-sliki (a) nitrirani ODS FeAl, ki prikazuje grebene, zasidrane z delci Y2O3 in (b) nitriranega NiCr 20, ki prikazuje nastalo poroznost (ploskvi (10 X 10) |im Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 161 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS saturated in nitrogen inducing significant plastic deformation. Indeed, EDS microanalyses indicate that no nitrogen has been retained in pure Ni either within the grains or at the grain boundaries where more protrusions are observed. Conversely, in the Cr-bearing alloys the distribution of nitrogen is uneven and confirms the XRD results. For instance, whereas about the mole fraction of N 10 % is present at the surface of Ni20Cr regardless of the location, in AISI 304L stainless steel some of the grains only incorporate about 12 % N and some others contain up to 17 % N, which is close to the chromium content in the substrate. Because of the anisotropic incorporation of N, different compressive stresses are generated. This leads to distortions, plastic deformation and even lattice rotations in an anisotropic fashion 42. As a result of the anisotropic deformation, heterogeneous diffusion will occur modifying the nitrogen ingress rate 36. On the contrary, in the FeAl intermetallic alloy the average composition is Fe-25Al-20N (X/%). This suggests that the N content being introduced could be limited by the Al amount at the surface of the substrate and therefore is only dependent on Al diffusion 43. The SEM cross section morphologies clearly reveal that the only well defined nitrided layers appear on the AISI 304L and the ODS FeAl substrates after a chemical etch (Figure 5). However, the EDS composition profiles (Figure 6) indicate that N has effectively been incorporated in the Ni20Cr matrix. The maximum N content is found for the ODS FeAl alloy but the depth is the lowest because of N inward diffusion is arrested by the formation of AlN. On the contrary, the shape of the N content is similar in Ni20Cr and AISI 304L. As higher N contents are present in the steel, the nitrided layer is about 1 pm thicker in the steel than in the Ni20Cr alloy. At the substrate/nitrided layer interface, a steep N drop occurs in the steel in comparison with the Ni20Cr alloy. Some explanations can be found from thermodynamic calculations and TEM analyses. Nitrogen has a very low solubility 44 and permeability 45. Upon nitrogen implantation chromium shows a strong tendency to form either the fcc CrN (AH° = -40 kJ mol1) or the hcp &2N (AH° = -38 kJ mol-1) phases, which have not been observed experimentally in Ni20Cr. However, the hexagonal Cr2N phase seems to precipitate at the nitrided layer / AISI 304L interface as shown by cross section TEM and selected area diffraction patterns (SADPs) (Figure 7, Table 3). Fe2N nitride could be also present at the nitrided layer/steel interface but its heat of formation (-18 kJmol-1) suggests that &2N should be the major nitride. This means that the formation of metal nitrides at the nitrided layer/substrate interface would arrest further N inward diffusion and could explain the steep drop of the N content shown in Figure 6. This may indicate that Cr allows to significantly increase the N solubility in Ni. Because nickel rejects nitrogen, the nickel-rich substrate (Ni20Cr) incorporates less nitrogen. On the other hand, from a thermodynamic point of view the free enthalpy (AG) is more negative 3025- ■ B ■ N in Ni20Cr ■ ® ■ N in AISI 304L ■ ^ ■ N in ODS FeAl Figure 5: SEM cross section of the nitrided (a) AISI 304L stainless steel and (b) ODS FeAl showing protrusions and the nanograined structure of the substrate Slika 5: SEM-prerez nitriranega (a) nerjavnega jekla AISI 304 in (b) ODS FeAl s protruzijami in nanozrnata struktura podlage 20 15105 Distance from surface, ds/pm Figure 6: N profile from EDS microanalyses of the cross sections of the nitrided materials. (NB: EDS of ODS FeAl from TEM cross sections) Slika 6: N-profil iz EDS-mikroanalize na prerezu nitriranih materialov (Opomba: EDS ODS FeAl iz TEM prereza) 0 162 Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS ... 004 302 304 302" 300 .002 •300 302 002 Table 3: Data from the selected area diffraction patterns (SADPs) shown in Figures 7 (b) and (c) and the corresponding compounds identified by TEM Tabela 3: Podatkih iz difrakcijskih slik izbranih ploskev (SADPs), ki jih prikazuje slika 7 (b) in (c), in ustrezna spojina, identificirana s TEM experimental ^-spacing 7N (experimental) Cr2N (JCPDS 79-2159) Fe2N (JCPDS 73-2102) ^-spacing hkl ^-spacing hkl ^-spacing hkl 2.40c 2.37 (110) 2.39 (110) 2.25b 2.21 (002) 2.21 (002) 2.10c 2.10 (111) 2.09 (111) 2.10 (111) 1.86c 1.86 (200) 1.86 (201) 1.87 (201) 1.52c 1.55 (210) 1.48 (211) 1.46c 1.46 (211) 1.47 (003) 1.35 1.37 (300) 1.38 (300) 1.16 b 1.16 (302) 1.17 (302) 0.92c not assigned not assigned not assigned 0.89c 0.89 (400) Figure 7: (a) TEM cross section of the nitrided AISI 304L stainless steel. SADPs of the (b) innermost zone corresponding to a single grain oriented [010]cr2N; and (c) outermost zone representative of various grains Slika 7: (a) TEM-prerez nitriranega nerjavnega jekla AISI 304 L. SDAP (b) notranje cone, ki ustreza enemu zrnu z orientacijo 010 Cr2N, in (c) zunanja cona, ki ima razli~na zrna b data from Figure 7 (b) and c from Figure 7 (c) 0.20 0.15- iT 0.10 C "ST 0.05 0.00 H ■ CrN in Ni20Cr ® CrNinAISI304L ^ ■ Cr2N in Ni20Cr A-Cr2N inAISI304L 340360380400420440460480500520 Temperature, T/°C 8.0x107- 6.0x107- "oT 4.0x10-7 - C "sr 2.0x107- 0.0 ^Fe2NinAISI304L 340 360 380 400 420 440 460 480 500 520 Temperature, T/°C (thus, more spontaneous reaction) upon the formation of chromium nitrides than that of iron nitrides (Figure 8). However, the iron effect cannot be neglected if the chemical potential of the species is also taken into account; i. e. when one mole of nitrogen encounters the substrate surface 70 % of the atoms are composed of iron Figure 8: Evolution of mole of metal nitride produced per mole of N2+(g) as a function of temperature at 10-2 Pa according to the HSC thermochemical calculations 27 (a) chromium nitrides formation in Ni20Cr and AISI 304L and (b) iron nitrides in AISI 304L Slika 8: Evolucija molarnosti kovinskega nitrida na mol N2 (g) v odvisnosti od temperature pri 10-2 Pa na podlagi termokemičnih izračunov 27 (a) nastanka kromovih nitridov v NiCr20 in AISI 304 L in (b) nitridi železa v AISI 304 L Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 163 Fe.NinAISI304L F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS and only 20 % of chromium. As a result, iron can also enhance incorporation of nitrogen at least to some extent. Indeed, Rivière et al. 5 found that nitrogen was always detected in a nitride type state and that it was preferentially bound to chromium, without specific nitride formation, which agrees well with the trapping-detrapping mechanism proposed by Möller et al. 46. Similarly, a small amount of iron atoms showed the same nitride type bonding but only at the outermost surface. Therefore, iron interaction together with a lower nickel content (which rejects nitrogen) results in higher nitrogen supersaturation in the superficial layers of AISI 304L than in Ni20Cr. Thereafter, because of the difference in chemical potentials between the external layer and the bulk, diffusion will be enhanced. As a result, the Fe-based alloy, which incorporates more nitrogen, will exhibit a higher degree of deformation. This induces significant swelling of the grains, thus developing rougher surfaces than Ni20Cr. For the ODS FeAl intermetallic alloy, the nitrided layer has a nanostructured morphology and at the nitrided layer / substrate interface an iron band segregates (Figure 9). Diffraction patterns of the different ^- m areas point out the different features observed in these samples such as the nanometre scale of the nitrided layer characterised by the typical rings corresponding to FeAl as well as some spots at shorter distances belonging to AlN. As summarised in Table 4, some of the distances may also correspond to a-Fe. Sanghera and Sullivan 35 found that nitrogen implanted at low energy and low flux into pure aluminium did not render stoichiometric AlN because the radiation damage induced many vacancies, interstitials and defects. From our EDS analyses, only the outermost layers would contain enough nitrogen to produce the hexagonal AlN phases massively and therefore, once the average values of nitrogen decrease, a mixture of FeAl containing dispersed particles of AlN occurs closer to the nitrided layer/substrate interface. From the TEM results a combined mechanism of nitrogen diffusing inwardly and aluminium outwardly during the nitridation treatment would occur. This countercurrent diffusion would be promoted by the creation of short-circuit diffusion paths, i.e. the grain boundaries of the nanostructured layer. Indeed, diffusion of indium (isoelectronic with aluminium) has been found to be faster than that of iron by a factor of about two in Fe66Al34 and Fe50Al50 47, which helps in corroborating the suggested mechanism. Table 4: Experimental rf-spacings obtained with 0.15 |im-diaphragm SADPs at the nitrided layer/substrate interface in the as-nitrided intermetallic alloy and their correspondence to the planes of the identified compounds Tabela 4: Eksperimentalne rf-razdalje, izmerjene pri SDPS z 0,15 |im veliko zaslonko, na medpovršini nitridna plast/podlaga v nitrirani intermetalni spojini in njihova lega glede na ploskev indentificiranih spojin Figure 9: TEM cross section showing (a) the nanostructured morphology of the nitrided layer and (b) the nitrided layer/substrate interface. The band of -Fe segregated at this interface is indicated between arrows Slika 9: TEM-prerez, ki prikazuje (a) nanostrukturirano morfologijo nitridne plasti, in (b) medpovršina nitrirana plast/podlaga. Plast segregiranega a-Fe na tej medpovršini je prikazana med puščicami Experimental rf-spacing, nm FeAl JCPDS 33-20 -Fe JCPDS 89-4186 AlN JCPDS 25-1133 0.252 — — 002 0.207 110 110 — 0.160 111* — 110 0.143 200 200 — 0.119 211 211 202 * superstructure peak 3.2 High Temperature oxidation behaviour Because of their specific uses, the oxidation tests were conducted at different temperatures and the results will be therefore presented independently. 3.2.1 Oxidation of Ni and Ni20Cr: 700 °C and 800 °C Figure 10 shows the mass gain curves against time for both untreated and nitrided specimens. It can be observed that in nitrided Ni no significant difference is observed at both temperatures. On the contrary, in Ni20Cr nitridation increases significantly the overall mass gain. Assuming parabolic behaviour, the oxidation constants have been calculated by the (AM/S)2 vs. time 164 Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS ... 1.5 untreated-700 °C -800 °C 800 °c (a) nitrided- ■ 700°C 800 °C i 1.0- £ 700 °C S 0.5-§ < 0.0 5 10 15 20 Oxidation time, foxid/h 25 0.08 5 10 15 20 Oxidation time, f0xid/h 25 Figure 10: Isothermal oxidation at 700 °C and 800 °C for 24 h in synthetic air (a) untreated and nitrided Ni and (b) untreated and nitrided Ni20Cr Slika 10: Izotermna 24-urna oksidacija pri 700 °C in 800 °C v sintetičnem zraku; (a) nenitriran in nitriran Ni in (b) nenitriran in nitriran NiCr20 3 aï m c 3 c 40 45 50 2 ©/degrees 55 Figure 11: Selected range of the obtained on the untreated, as-nitrided and nitrided and oxidised at 700 °C and 800 °C Ni20Cr substrates. N.B: Only the matrix peaks are indicated. See text for further information concerning oxide species. Slika 11: Izbrana področja na nenitriranem, nitriranem in nitriranem ter oksidiranem Ni20Cr podlagah. (Opomba: Prikazani so le vrhovi matice. V tekstu je pojasnilo o vrstah oksidov). homogeneous oxide scales, the oxide layers spall off or oxide plates develop in nitrided Ni [Figure 12(a) and (b)]. In Ni20Cr oxidation occurs preferentially depending on the grain orientation and grain boundary. At the lowest temperatures, the Ni20Cr samples are distinctively covered of oxides [Figure 12 (c) and (d)], which are more developed at 800 °C [Figure 12(e) and (f)], method. In Ni, the parabolic rate constant (kp) values of 4 ■ 10-12 g2 cm-4 s-1 and 2.5 ■ 10-11 g2 cm-4 s-1 are found for 700 °C and 800 °C, respectively. However, in Ni20Cr the kp values increase about one order of magnitude from (1.0 ■ 10-15 to 8.3 ■ 10-15) g2 cm-4 s-1 at 700 °C and from (2.3 ■ 10-14 to 2.3 ■ 10-13) g2 cm-4 s-1 at 800 °C after the whole oxidation test. The XRD patterns have revealed the formation of NiO oxides in both untreated and nitrided Ni samples, together with some weak peaks of the substrate, indicating a relatively thick oxide layer at both temperatures. The oxide species developed on Ni20Cr are the same for both the untreated and nitrided specimens at either temperature and these include NiO, M&2O4 and &2O3. At the highest temperatures, more contribution of Cr2O3 oxide is found to occur. However, the substrate/oxide intensity ratios are always higher at any temperature than in the nickel substrates. This means that a thinner oxide layer is obtained in the Ni20Cr samples after 24 h of isothermal oxidation. Regarding the expanded austenite (y^ phase (Figure 11) oxidation at 700 °C for 24 h brings about shifting of the yN and y peaks towards the original y phase (20 = 44.28°) giving rise to the observed doublet. This clearly implies redistribution of nitrogen in the matrix but no nitride phase can be derived from the XRD results. The SEM morphologies are also completely different. Whereas the untreated specimens develop even and Figure 12: SEM surface morphologies developed at high temperature on (a) and (b) nitrided Ni at 700 °C and 800 °C; (c) and (d) nitrided Ni20Cr at 700 °C and (e) and (f) Ni20Cr at 800 °C Slika 12: SEM-morfologija površin, ki so nastale pri visoki temperaturi na (a) in (b) nitriranem Ni pri 700 °C in 800 °C, (c) in (d) nitriranem Ni20Cr pri 700 °C in (e) ter (f) pri 800 °C 0 0 Materiali in tehnologije / Materials and technology 42 (2008) 4, 157-169 165 F. PEDRAZA: LOW ENERGY-HIGH FLUX NITRIDATION OF METAL ALLOYS thus suggesting that the N implantation effect is lost at the highest temperature, as confirmed on the cross sections by SEM and EDS microanalyses. Indeed, the N content drops from about the mole fraction 10 % at the surface of the as-nitrided specimens to 3.5 % and 0 % after 24 h of oxidation at 700 °C and 800 °C. At 800 °C, some tiny metal nitrides precipitate (about 3 % N). 3.2.2 Oxidation of AISI 304L: (400, 450, 500 and 550) °C Figure 13 shows the mass gain curves as a function of time for both the untreated [Figure 13 (a)] and the nitrided [Figure 13 (b)] specimens. Oxidation is more significant in the nitrided samples than in the untreated steel upon the first oxidation times at any temperature as a result of both a chemical and physical effect 48. The first one is related to the amount of implanted nitrogen, whereas the second refers to the defects induced upon implantation. The XRD patterns of the untreated steel show mainly the substrate peaks, i. e. austenite and ferrite phases are observed, indicating the low thickness of the scale. The small participation of the ferrite phase has been previously reported to occur as a result of both plastic deformation induced upon grinding 49 and after high temperature exposure due to chromium outward diffusion, which partially destabilise the austenitic phase until oxide formation is accomplished 50. Only in 0.035 0.030 ST- 0.025 o 0.020 TO iL 0.015 <0 0.010 0.005 aooo ao4 5 10 15 20 Oxidation time, fo*id/h E o en E