UDK 669.715:539.42 Original scientific article/Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 42(5)191(2008) THE EFFECT OF COMPOSITIONAL VARIATIONS ON THE FRACTURE TOUGHNESS OF 7000 Al-ALLOYS VPLIV SPREMEMB V SESTAVI NA ŽILAVOST LOMA ALUMINIJEVE ZLITINE VRSTE 7000 Maja Vratnica1, Zorica Cvijovic2, Nenad Radovi}2 1 Faculty of Metallurgy and Technology, University of Montenegro, 81000 Podgorica, Cetinjski put b. b., Montenegro 2 Faculty of Technology and Metallurgy, University of Belgrade, 11000 Belgrade, Karnegijeva 4, Serbia majav@cg.ac.yu Prejem rokopisa — received: 2008-02-12; sprejem za objavo - accepted for publication: 2008-04-03 To provide an understanding of how compositional variations affect the microstructural parameters associated with coarse intermetallic (IM) particles and the fracture toughness in AA 7000 aluminum forgings, a microstructural and fractographic analysis as well as mechanical tests were carried out on three industrially produced Al-Zn-Mg-Cu alloys with different contents of impurities (Fe+Si). Light optical microscopy and image analysis were used to assess the volume fraction, size and distribution of all the soluble and insoluble coarse (> 0.1 |im) IM particles identified in the corresponding R-C and L-R planes for T73-type heat treatments by selective etching and energy-dispersive X-ray spectroscopy. These quantitative data were correlated with the plain-strain fracture toughness, KIC, with the results being used to produce useful information on alloy design and thermomechanical processing via microstructural control. The scanning electron microscope observation of fracture surface features and an estimation of the area fractions of different fracture modes in the plastic zone segments of a test specimen showed that multiple failure mechanisms occurred with coarse voiding at the intermetallics becoming more important as the fraction of coarse IM particles increases. A quantitative assessment of the relevant microstructural and fractographic parameters will be utilized for developing and verifying a multiple micromechanisms-based model for fracture toughness. Key words : 7000 Al- alloys, chemical composition, microstructure, fracture toughness S ciljem, da se ugotovi, kako spremembe v sestavi vplivajo na parametre mikrostrukture, odvisne od velikih delcev intermetalnih spojin (IM), in na žilavost loma v izkovkih iz zlitine AA 7000, so bile izvršene mikrostrukturne, mikrofraktografske in mehanske preiskave pri treh industrijskih zlitinah Al-Zn-Mg-Cu z različno vsebnostjo nečistoč (Fe + Si). Optična mikroskopija in analiza slike sta bili uporabljeni za določitev volumenskega deleža, velikosti in porazdelitve topnih in netopnih velikih (>0,1 |im) IM-zrn v ustreznih R-C- in L-R-ploskvah po toplotni obdelavi T73 s selektivnim jedkanjem in z disperzivno spektroskopijo rentgenskih žarkov. Ti kvantititivni podatki so bili korelirani z žilavostjo loma KIC, dobljeni pa so bili tudi podatki, ki so koristna informacija za načrtovanje zlitin in termomehansko obdelavo s kontrolo mikrostrukture. Analiza prelomnih površin z vrstičnim mikroskopom in določitev površine različnih deležev preloma v plastični zoni preizkušancev je pokazala več vrst mehanizmov preloma in nastanek tem več velikih jamic ob IM-zrnih, čim večji je bil delež teh velikih zrn. Kvantitativna ocena relevantnih mikrostrukturnih in mikrofraktografskih parametrov bo uporabljena za razvoj in za verifikacijo modela žilavosti loma na podlagi več mahanizmov preloma. Ključne besede: aluminijeva 7000 zlitina, kemična sestava, mikrostruktura, žilavost loma 1 INTRODUCTION High-strength aluminum alloys of the AA 7000 (Al-Zn-Mg-Cu) series are widely used for structural applications due to their good combination of specific strength and fracture toughness1. However, the critical fracture toughness properties, especially in the short transverse direction, may be seen as questionable, since the fracture resistance is influenced by a number of parameters, including a range of microstructural features that are controlled by the chemistry and processing1,2,3,4. Furthermore, the microstructural anisotropy associated with wrought materials may influence the failure mode depending on the load and crack orientation2. It is now recognized that the coarse particles of intermetallic (IM) phases are generally detrimental to the fracture properties. This is associated with the fact that although the fracture processes in precipitation-hardened AA 7000 alloy products involve multiple micromecha-nisms, the decohesion and fracture of these particles, which are brittle and have weak interface bonding, is the first step in a sequence of events that lead to the overall material fracture125. The remaining fracture path is partitioned between intergranular fracture and micro-void-induced transgranular fracture. The undesirable coarse particles with sizes in the range of 1 |m to 20 |m are IM phases of two types: (a) insoluble Fe- and Si-bearing phases formed during the solidification process, and (b) normally soluble phases containing alloying elements that do not completely dissolve during the homogenization and solution treatment1. In order to improve the toughness, it is necessary to achieve the lowest levels of coarse IM particles. The removal of excess amounts of the soluble particles is possible by controlling all the stages of processing. But, the limits on the reduction of the Fe and Si impurities are set by the cost and the availability of high-purity materials. Consequently, these impurities are always present in commercial alloys. They react with Al and alloying elements such as Mg and Cu to form a large Materiali in tehnologije / Materials and technology 42 (2008) 5, 211-214 211 M. VRATNICA ET AL.: THE EFFECT OF COMPOSITIONAL VARIATIONS ON THE FRACTURE TOUGHNESS ... number of phases6. In addition, these alloys contain Mn and Cr, which may also be present in the form of coarse IM particles, since they combine with Fe, Si and Al. Therefore, it is of interest to predict the variation in the fracture properties as a function of the microstructural parameters, such as the volume fraction of coarse IM particles, their size and their spatial distribution. However, most of the available information is concerned with the properties of wrought alloys. Systematic and in-depth quantitative microstructural and fractographic examinations of commercial AA 7000 alloys in the form of thick plates cut out from forgings have not been widely conducted. It is the purpose of this contribution to report on a microstructural and fractographic investigation of the effect of compositional variations on the attributes associated with coarse IM particles and the fracture toughness of modern AA 7000 alloy forgings (the high-zinc variant) in the over-aged condition as a function of test orientation. The failure mechanisms are identified and the individual contributions to the overall fracture are quantitatively assessed. The data are then used to obtain a relationship between the microstructural parameters and the plane-strain fracture toughness, with the results being utilized for the modeling of toughness. 2 EXPERIMENTAL Three industrial alloys with Zn, Mg, and Cu levels broadly in the range of the AA 7049 composition were received in the form of hot-forged ~50-mm-thick pancake-type plates. The chemical composition of each alloy is given in Table 1. The amounts of alloying elements are very near to the nominal ones. The only difference in composition between the alloys is the total (Fe+Si) content, which increases gradually from alloy 1 through alloy 3. Figure 1 shows the cutting of the tests specimens from the received plates. All three alloys were solution treated at 460 °C for 1 h, water-quenched, and aged to a T73 temper. The two-step T73 over-aged treatment consisted of the aging of the specimens for 5 h at 100 °C and 5 h at 160 °C. Light optical microscopy (LOM) and image analyses were used to characterize the microstructure of the as-heat-treated plates. Metallographic sections were taken from the corresponding R-C and L-R planes. The specimens were then prepared using standard metallographic techniques. A selective etching and energy-dispersive X-ray spectroscopy (EDS) analysis on a scanning electron microscope (SEM) were used to identify the IM phases present. The volume fraction of all the soluble and insoluble coarse IM particles, VV, their size expressed by the average intercept length, L, and the mean free path, A, characterizing the space distribution were assessed with the line-intercept method. The measurements were carried out on 500 uniformly sampled microstructural frames at a magnification of 1000 times. Plane-strain fracture-toughness tests were performed in accordance with ASTM E399 on the specimens of the corresponding R-C and L-R orientations, i.e., on the single-edge-notched three-point bending specimens (SEB) of R-C orientation and the compact-tension (CT) specimens of L-R orientation. The specimens were fatigue pre-cracked according to the ASTM standard specifications. In all cases, three specimens were tested. The Kic values for the R-C orientation specimens were obtained from the /-integral data. Jic was evaluated with the unloading compliance technique, with a single specimen for each Jic result. The /-integral and the crack growth, Aa, were calculated in accordance with ASTM E1152 and ASTM E813. On a broken specimen an SEM fractographic examination was performed to explain the fracture mechanism. The fracture surface morphology was investigated in the central region of the plastic zone ahead of the fatigue pre-crack. The area fraction of the microvoid-induced transgranular fracture regions, Aai, the intergranular fracture regions, Aai, and the coarse IM particles, Aap, were estimated. The area measurements were performed on SEM fractographs by tracing the areas on a digitizing Figure 1: (a) Schematic illustration of the specimen orientations used for the fracture-toughness tests (L-longitudinal direction, C-circumferential or tangential direction, R-radial direction) and (b) locations of the metallographic planes for the microstructural analysis (b) Slika 1: (a) Shema orientacije vzorcev, uporabljenih za preizkuse žilavosti loma (L-vzolžna smer, C tangencialna smer, R - radialna smer) in (b) mesto odvzema vorcev za mikrostrukturno analizo 192 Materiali in tehnologije / Materials and technology 42 (2008) 5, 191-196 M. VRATNICA ET AL.: THE EFFECT OF COMPOSITIONAL VARIATIONS ON THE FRACTURE TOUGHNESS tablet. These measurements provided the data to quantify the contributions of the different fracture micromecha-nisms to the plane-strain fracture-initiation process as a function of the purity degree and the specimen orientation. 3 RESULTS AND DISCUSSION 3.1 Analysis of the microstructural data Typical microstructures of simple uniaxially forged material after the full heat treatment are illustrated in Figure 2. All the forgings show a deformed dendrite cell structure with coarse IM particles having an average size of 1.27-2.43 pm. As expected, relatively coarse and closely spaced precipitates, mostly situated on the grain boundary surrounded by the precipitate-free zones (PFZs), were also observed (Figure 2a). The TEM characterization of the precipitation in AA 7000 alloys by previous authors1,2 indicates that these particles are variants of the -Mg(Cu,Al,Zn)2 phase. In the Al-rich matrix there was a dense population of uniformly distributed dispersoids1,2,4 and fine precipitates of the and rj' phases12 that contributed to the precipitation hardening. On the other hand, the coarse IM particles are inhomogeneously distributed and aligned in the direction of the prevailing deformation, as observed on the metallographic plane of L-R orientation (Figure 2b). The micrographs also illustrate the IM particles that are irregularly shaped and of different types (Figures 2c and d). The combined use of the metallographic and EDS analyses indicates that these particles are of the following types: (a) soluble Mg(Cu,Al,Zn)2, S-CuMgAl2, and, most often observed, Mg2Si, (b) the Fe-bearing phases Al7Cu2Fe, (Cu,Fe,Mn)Al3 and a very little of (Cu,Fe,Mn)Al6, (c) the Cr-bearing phase (Cu,Fe,Mn, Cr)Al7, and (d) another type of Si-containing phases (Fe,Cr,Mn,Cu)3SiAl12. The identified phases were found in all three alloys; however, variations in the composition caused large changes in their fraction and their morphological characteristics. This observation was also supported by the image analyses. As can be seen from the data presented in Table 2, the alloy 3 had the highest percentage of Table 1: Chemical composition of the investigated alloys (in mass fractions, w/%) Tabela 1: Kemična sestava raziskanih zlitin (v masnih deležih, w/%) Alloy Elements Zn Mg Cu Mn Cr Zr Ti V B Fe Si 1 7.45 2.47 1.53 0.25 0.17 0.15 0.015 0.003 0.003 0.12 0.11 2 7.30 2.26 1.55 0.29 0.18 0.13 0.015 0.007 0.003 0.16 0.09 3 7.65 2.26 1.55 0.25 0.18 0.11 0.017 0.005 0.003 0.26 0.11 Table 2: Results of the image analysis and the plane-strain fracture-toughness tests Tabela 2: Razultati analize slike in določitve žilavosti loma Alloy Plane Kic/ (MPa-m1/2) aa* IM phase characteristics Type Vv, p/% L/|m X/|im 1 R-C 45.50 nd*** MgZn2+S 0.159 1.63 1024.8 Fe-rich** 0.227 2.00 883.5 Mg2Si 0.125 1.74 1394.1 L-R 43.16 P 0.152 MgZn2+S 0.147 1.56 1054.7 I 0.278 Fe-rich** 0.236 2.08 878.0 T 0.570 Mg2Si 0.144 1.94 1350.0 2 R-C 42.63 nd*** MgZn2+S 0.048 1.29 2685.6 Fe-rich** 0.357 1.99 557.0 Mg2Si 0.094 1.70 1806.7 L-R 40.96 P 0.299 MgZn2+S 0.095 1.27 1333.8 I 0.304 Fe-rich** 0.440 2.06 465.5 T 0.397 Mg2Si 0.134 1.92 1425.1 3 R-C 40.53 nd*** MgZn2+S 0.119 1.63 1363.4 Fe-rich** 0.590 2.43 409.8 Mg2Si 0.147 1.95 1318.7 L-R 37.67 P 0.378 MgZn2+S 0.046 1.38 3031.4 I 0.287 Fe-rich** 0.532 2.37 444.3 T 0.335 Mg2Si 0.146 2.09 1434.8 * Area fractions of microvoid-induced transgranular fracture regions (t), intergranular fracture region (i), and coarse constituent particles (p); ** Fe-rich phases = Al7Cu2Fe + (Cu,Fe,Mn)Al3 + (Cu,Fe,Mn)Al6 + (Cu,Fe,Mn,Cr)Al7 + (Fe,Cr,Mn,Cu)3SiAl12 ; *** ND = not determined * delež površine mikrojamičastega transkristalnega preloma (t), interkristalen prelom (i) in velikih IM- zrn (p); ** Z Fe bogate faze = Al7Cu2Fe + (Cu,Fe,Mn)Al3 + (Cu,Fe,Mn)Al6 + (Cu,Fe,Mn,Cr)Al7 + (Fe,Cr,Mn,Cu)3SiAl12 ; *** ND = ni določeno Materiali in tehnologije / Materials and technology 42 (2008) 5, 191-196 193 M. VRATNICA ET AL.: THE EFFECT OF COMPOSITIONAL VARIATIONS ON THE FRACTURE TOUGHNESS ... (C) (d) Figure 2: Optical microstructures of the over-aged alloy 3 (a),(c),(d) and alloy 1 (b) observed in R-C (a),(c) and L-R planes (b),(d) etched in 10 % H3PO4 at 50 °C for 5 min (a) and Keller's reagent at 20 °C for 5 s (b), (c), (d). Type of phases: A = Mg(Cu,Al,Zn)2, B = S-CuMgAl2, C = Mg2Si, D = (Cu,Fe,Mn)Al3 or (Cu,Fe,Mn)Al6, E = Al7Cu2Fe, F = (Cu,Fe,Mn,Cr)Al7, G = (Fe,Cr,Mn,Cu)3SiAl12. Slika 2: Opti~na slika prestarane zlitine 3 (a), (b), (c) in (d) in zlitine 1 (b) v ploskvah R-C (a) in (c) ter L-R v ploskvah (b) in (d), jedkano v 10 % HPO4 5 min (a) in s Keller reagentom 5 s pri 20 °C (b), (c) in (d). Vrste faz: A - Mg(Cu,Al,Zn)2, B - S-CuMgAl2, C - Mg2Si, D -(Cu,Fe,Mn)Al3 ali(Cu, Fe,Mn) Al6, E - Al7Cu2Fe, F - Cu,Fe,Mn,Cr)Al7, G - Fe,Cr,Mn,Cu)3SiAl12 coarse particles, with an average of volume fraction of 0.79 %, while the alloys 1 and 2 had a similar volume fraction, varying between about 0.52 % and 0.58 %. The significant increase in the amount of coarse particles, serving as the crack-initiation sites, is a direct consequence of an increase in the total (Fe+Si) content. For all the alloys, i.e., the volume fraction of phases containing Al, Mg, Cu, and Zn (Mg(Cu,Al,Zn)2 and S-CuMgAl2) is lower than ~ 0.15 %. Note also that, since the Si content is practically unchanged from one alloy to the other, the volume fraction of Mg2Si particles is constant. This implies that the Fe content plays an important role in the formation of coarse particles. The volume fraction of grey particles (Fe-containing phases) increases almost linearly with the increase in the Fe content. As a result, the coarse particles are distributed over shorter distances. These features can provide planes of easy crack growth, thereby reducing the deformation capacity of the matrix. 3.2 Toughness behavior Table 2 shows how reducing the total (Fe+Si) content and thereby removing most of the coarse IM particles improves Kic. As expected, the toughness was the highest for the alloy 1, with the lowest (Fe+Si) content of the mass fraction (w) 0.23 %. By increasing the impurity level from 0.23 % to 0.25 % and in turn the volume fraction of the Fe- and Si-containing particles from the volume fraction ( ) 0.380 % to 0.574 %, the Kic value in the R-C orientation decreased by approximately by 6.5 %. Since in alloy 2 the Si content is w = 0.02 % and lower than that in the alloy 1, it is concluded that large variations in the amount of coarse particles and the fracture toughness can occur with a relatively small change in the Fe content. The toughness decreased further when going to a purity of w = 0.37 %. The drop in the fracture toughness, due to the presence of undesirable particles -