UDK 669.35:621.785:620.181 Professional article/Strokovni članek
ISSN 1580-2949 MTAEC9, 48(4)599(2014)
INFLUENCE OF HEAT TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALUMINIUM BRONZE
VPLIV TOPLOTNE OBDELAVE NA MIKROSTRUKTURO IN MEHANSKE LASTNOSTI ALUMINIJEVEGA BRONA
Peter Slama, Jaromir Dlouhy, Michal Köver
COMTES FHT a.s., Prumyslova 995, 334 41 Dobrany, Czech Republic peter.slama@comtesfht.cz
Prejem rokopisa - received: 2013-10-03; sprejem za objavo - accepted for publication: 2013-10-24
The paper deals with the influence of heat treatment (annealing, quenching and aging) on the microstructure and mechanical properties of pressed bars made from the CuAl10Ni5Fe4 alloy. The microstructures were observed in light and scanning electron microscopes. The appearance and area fractions of the a and K. phases and their influence on the mechanical properties were examined. Using DSC and EBSD methods, the presence of the y2 phase was monitored, as it is a very hard and brittle phase that impairs the material's corrosion resistance. The results were compared with data for the high-alloyed CuAl14Fe5 aluminium bronze, in which high hardness and wear resistance are imparted by the ß + /2 phases.
Keywords: aluminium bronze, heat treatment, microstructure, mechanical properties, DSC, EBSD, y2-phase
Članek obravnava vpliv toplotne obdelave (žarjenje, kaljenje in staranje) na mikrostrukturo in mehanske lastnosti stiskanih palic iz zlitine CuAl10Ni5Fe4. Mikrostruktura je bila opazovana s svetlobnim in vrstičnim elektronskim mikroskopom. Preučevan je bil pojav in površinski delež faze a in K ter njun vpliv na mehanske lastnosti. Z metodama DSC in EBSD je bila pregledana prisotnost trde in krhke y2-faze, ki poslabša korozijsko odpornost materiala. Rezultati so bili primerjani s podatki za močno legiran CuAl14Fe5 aluminijev bron, pri katerem (ß + y2)-faza znižujeta veliko trdoto in odpornost proti obrabi.
Ključne besede: aluminijev bron, toplotna obdelava, mikrostruktura, mehanske lastnosti, DSC, EBSD, y2-faza
1 INTRODUCTION
CuAl10Ni5Fe4 aluminium bronze is a copper alloy that retains its high strength even at elevated temperatures and which possesses a good corrosion resistance and a high wear resistance.1-3 It has a two-phase micro-structure consisting of the poor-formability a phase and the high-temperature ß phase, which exhibits excellent hot formability. The addition of nickel increases the alloy's strength without diminishing its excellent ductility, toughness and corrosion resistance. Typical applications of the alloy include valve seats, plunger tips, marine engine shafts, valve guides, aircraft components and pump shafts. The alloy exhibits limited cold forma-bility (due to rapid work hardening), but excellent hot formability in the a + ß region. The recommended forming temperatures are between 700 °C and 900 °C.1,2
Depending on the cooling rate and the subsequent heat treatment, the ß phase may undergo a martensitic transformation into the unstable phase ß', which, being very hard and brittle, enhances the strength and reduces the ductility of the material. In addition, other phases are found in the microstructure, which are termed k, which consist mostly of Fe and Al or Ni,3-5 or the /2 phase known to occur in Cu-Al binary alloys. The /2 phase in alloys containing less than 11.8 % aluminium forms during slow cooling or in the course of annealing at temperatures below 565 °C. These phases increase the
strength and reduce the ductility of the alloy. The microstructure therefore consists of the a phase and the a + K + /2 eutectoid, as shown in the phase diagram in Figure 1a.6
Several types of the kappa phase have been classified: the sources3-5,7,8 list four types, denoted as KI, KII, KIII, KIV. The KI phase forms large dendrite-shape particles that are rich in Fe, which consist of Fe3Al with Cu and Ni and the cubic structure DO3. The phase KII forms smaller, globular particles with identical composition
Figure 1: a) Vertical section of the phase diagram of nickel aluminum bronze with 5 % Ni and 5 % Fe, b) vertical section of phase diagram of Cu-Al-Fe at 3 % Fe
Slika 1: a) Navpični prerez faznega diagrama nikelj - aluminijev bron s 5 % Ni in 5 % Fe, b) navpični prerez faznega diagrama Cu-Al-Fe pri
3 %Fe
and structure to ki. The phase (precipitates) Km are NiAl-based and have the cubic B2 structure and a plate or lamellar shape. The kiv particles are small precipitates based on Fe3Al. The composition of the 72 phase is Cu9Al4 and its structure is the cubic P-43m type.
Where tough requirements for hardness and wear resistance are to be met, high-alloyed Cu-Al-Fe aluminium bronzes are used with an aluminium level above 13 % and an iron content of up to 5 %. The micro-structure of these alloys consists of two phases as well, as shown in the phase diagram in Figure 1b9: a and 72. The high hardness of these alloys is due to the latter, the 72 phase, which also exhibits a high hardness and brittle-ness, but a lower corrosion resistance. Owing to their hardness, excellent compressive strength and wear resistance, and sliding properties, these alloys are used for forming and drawing stainless steels and for worm wheels, valve guides and seats.
pared with data for the high-alloyed CuAl14Fe5 aluminium bronze (sample of a worm wheel).
The chemical compositions of both alloys are given in Table 1.
The microstructures were observed on conventional metallographic sections in light (LM) and scanning electron microscopes (SEM). The HV hardness of the materials was measured. The evolution of the microstructure and the formation of the precipitates and phases were monitored using DSC (Differential Scanning Calori-metry) with a LINNSEIS DSC PT 1600 instrument and by means of EBSD (Electron Backscatter Diffraction) in a JEOL JSM 7400F scanning electron microscope equipped with a HKL-NORDLYSS camera and Chan-nel5 software.
3 RESULTS AND DISCUSSION
2 EXPERIMENTAL
Hot-pressed bars with a diameter of 32 mm from an alloy containing 10 % Al, 5 % Ni and 4 % Fe were used for the experiment. The material is denoted as CuAl10Ni5Fe4 (CW307G) according to EN 12163.
Specimens of bars were used for investigating the influence of annealing between 500 °C 30 min and 850 °C 30 min, quenching from 930 °C and aging of the quenched specimens at 30 °C 30 min to 400 °C 30 min upon the microstructure and mechanical properties. Attention was also paid to the influence of the slow cooling in the furnace and the subsequent rapid cooling after removal from the furnace. The results were com-
Figure 2: HV30 hardness of annealed specimens Slika 2: Trdota HV30 vzorcev po žarieniu
3.1 CuAlI0Ni5Fe4 Alloy
3.1.1 HV Hardness
The HV30 hardness of the as-received specimens (AR) upon annealing was measured. The results are shown in Figure 2. The graph shows the hardness levels after annealing of the as-received material and upon annealing (tempering) of the quenched material. The hardness of the as-received material is 265 HV30. Quenching increases this value to 305, and aging, to 400 (Table 2).
Table 2: HV30 hardness Tabela 2: Trdota HV30
Condition
cooling
AR
500 °r
600 °r
650 °r
700 °r
750 °r
800 °r
850 °r
Quenched 930 °C 30 min
Quenched + Aged 400 °C 30 min
Annealed
Furnace
Air
265
969
255
256
245
215
198
184
260
253
254
253
239
234
230
305
400
The hardness decreases during annealing; first slowly, while the temperature is below 700 °C, but more rapidly when it reaches 750 °C. A greater drop in hardness was seen in the specimens cooled slowly in a furnace than in those cooled in air.
Table 1: Chemical composition of alloys in mass fractions (w/%) Tabela 1: Kemijska sestava zlitin v masnih deležih (w/%)
Alloy
Cu
Al
Ni
Fe
Mn
Sn
Si
7n
Pb
CuAl10Ni5Fe4

9 9
5.4
4 5
0.77
0.02
0 13
0.02
0 01
CuAl14Fe5
halanrp
14 5
0 12
4 9
0 97
0 06
0 22
0 006
0 07
(a)
(b)
Figure 3: Micrographs of unetched specimens: a) as-received condition (AR), b) 17 - upon quenching
Slika 3: Mikrostruktura nejedkanih vzorcev: a) dobavljeno stanje (AR), b) 17 - po kaljenju
3.1.2 Microstructure
The micrographs of the specimens prior to etching are shown in Figure 3. Unetched sections show grey globular particles of the Kh phase (according to EDS analysis, they contain 55 % iron, 11 % aluminium and nickel, copper, manganese and silicon), light grains of
Figure 4: Micrographs of etched specimens: a) as-received condition, b) condition upon annealing at 750 °C, furnace, c) condition upon annealing at 850 °C, furnace, d) condition upon annealing at 850 °C, air
Slika 4: Mikrostruktura jedkanih vzorcev: a) dobavljeno stanje, b) po žarjenju na 750 °C, ohlajanje v peci, c) po žarjenju na 850 °C, ohlajanje v peci, d) po žarjenju na 850 °C, ohlajanje na zraku
the phase a and a eutectoid consisting of a + k or a balance of ß. Annealing at the quenching temperature of 930 °C dissolves the k phase in the eutectoid (except the globular Kii) and the ß phase transforms to the non-equilibrium ß' phase,10 as seen in Figure 3b.
The microstructures upon etching are shown in Figures 4 and 5. Etching reveals light grains of the phase a, grey to light-blue globular particles of the phase Kii and the dark eutectoid a + Km.7 In the as-received state, the a grain size is very small: 1 pm to 2 pm. With increasing annealing temperature, the size of the a grains grows, as does the size of the eutectoid areas between them. Slow cooling in the furnace leads to coarsening of the k precipitates and to the formation of larger amounts of lamellar Km precipitates in the eutectoid. The phase k also precipitates along the a grain boundaries. With increasing a grain size the strength and hardness decrease. The greatest drop is seen in specimens annealed at 850 °C with slow cooling. However, the elongation decreases as well, which may be attributed to k precipitating more frequently at the a grain boundaries.
Rapid cooling leads to a higher proportion of the eutectoid at the expense of the a proportion. In addition, a certain fraction of ß phase is retained in the micro-structure (more dark areas in the eutectoid). This is shown in Figures 4c and 4d, where short etching with NH4OH+H2O2 revealed an orange ß phase, a bright a phase and the k phase and eutectoid in light-blue colour.



Figure 5: SEM micrographs of etched specimens: a) condition upon annealing at 800 °C, furnace, b) condition upon annealing at 800 °C, air
Slika 5: SEM-posnetka jedkanih vzorcev: a) po žarjenju na 800 °C, ohlajanje v peci, b) po žarjenju na 800 °C, ohlajanje na zraku
In contrast to the products of slow cooling, the k precipitates are finer. Globular precipitates of Kiv formed as well. The decrease in the strength and hardness is not as great as on slow cooling, which can be explained by a greater proportion of eutectoid and residual ß in the microstructure.
The SEM micrographs also confirm that slow cooling leads to coarser k precipitates than rapid cooling (Figures 5a and 5b).
Upon quenching and aging, fine Kiv precipitates form in the martensitic ß' phase, as shown in the light (Figure 6a) and scanning electron (Figure 6b) micrographs. The martensitic ß' phase and fine precipitates provide the quenched and aged material with high hardness.
3.2 CuAl14Fe5 Alloy
3.2.1 Hardness and microstructure
The specimen of CuAl14Fe5 alloy was made from a sample taken from a worm-wheel, in which very high hardness was required. The alloy corresponds to the AMPCO® 22 material by AMPCOMETAL.11 According to the company information, it contains ß and ya phases and has a nominal hardness of 332 HB30.
The specimen was used for an annealing trial at 950 °C 30 min with subsequent cooling in air. The hardness of the as-received material was 422 HV10. During
annealing, it rose to 462 HV10. Light micrographs are shown in Figures 7a and 7c, and a scanning electron micrograph is shown in Figure 7b. The microstructure consists of light-blue grains of y2 embedded in bright ß phase matrix, of large dark Kii particles in the y2 phase and of minute kiv precipitates within the ß phase. In the as-received specimen, the microhardness of the y2 phase is 573 HV0.05 and that of the ß phase reaches 341 HV0.05. The presence of the ß phase can be deduced from the material's high hardness. Due to annealing, the proportion of the y2 phase increased and, as a result of its high hardness, so did the hardness of the
Figure 6: Microstructures of quenched and aged specimen, etched: a) quenched and aged at 400 °C (LM), b) quenched and aged at 400 °C (SEM)
Slika 6: Mikrostruktura hitro ohlajenega in staranega vzorca, jedkano: a) hitro ohlajeno in starano pri 400 °C (SM), b) hitro ohlajeno in starano pri 400 °C (SEM)
Figure 7: LM and SEM micrographs of CuAl14Fe5: a) as-received condition, unetched, HV0.05 hardness testing indentations (LM), b) as-received condition, etched (SEM), c) annealed at 950 °C 30 min, unetched (LM)
Slika 7: Svetlobni in SEM-posnetek CuAl14Fe5: a) dobavljeno stanje, nejedkano, odtiski trdote HV0,05 (SM), b) dobavljeno stanje, jedkano (SEM), c) žarjeno pri 950 °C 30 min, nejedkano (SM)
entire specimen. The SEM observation reveals needles of non-equilibrium ß' phase in the ß phase (Figure 7b).
3.3 DSC Analysis
Using DSC, the precipitation of the k phases and the ß ^ a + y2 eutectoid transformation were monitored.1213 DSC curves were measured at heating and cooling rates of 10 K/min. Figure 8 shows the DSC curves for run 2 (after one cycle heating-cooling). The focus was the changes taking place at temperatures around 500 °C, in the vicinity of the eutectoid transformation ß ^ a+y2 at 565 °C. Small peaks were recorded for the CuAl10Ni5Fe4 alloy. Temperature peaks between 930 °C and 830 °C on the cooling curve correspond to the formation of k phases. (a+ß ^ a+ß+K) The peak at approx. 500 °C corresponding to the eutectoid transformation is very indistinct.
According to7 at 930 °C the globular kii phase (ß ^ ß+KII transformation) precipitates from the ß phase and at 800 °C the a+Kiii lamellar eutectoid forms (a ^ a+Km). At 850 °C fine Kiv precipitates form in the a phase.
In the CuAl14Fe5 alloy, the peaks due to the formation of the k and y2 phases are much more pronounced. At temperatures below 800 °C, two peaks can be observed, which could indicate the formation of the k and y2 phases in the ß phase. The 500 °C peak may correspond to the ß ^ a + y2 eutectoid transformation.
The transformation (formation of phases) at temperatures below 800 °C can be described according to Figure 1b as follows: ß ^ ß+K ^ ß+K+y2 ^ a+K+y2.
The peaks at temperatures of 300-400 °C correspond to the martensitic transformations of the ß phase (formation of ßi, ß').10.13.14
3.4 Identification of phases using EBSD
As the phases of the different structures produce different diffraction patterns (EBSP), the EBSD analysis can be used for identifying the various phases.15 The EBSD analysis was aimed at identifying the k and y2 phases in both aluminium bronzes. The phase maps and band contrast images are shown in Figure 9.
With the CuAl10Ni5Fe4 alloy, a higher band contrast was obtained (better quality of EBSP) in phases identified as a (large grey-coloured areas) and Kii - Fe3Al (small, bright spots). Due to the low band contrast, the phases in the eutectoid were not identified, most probably owing to the lamellar structure of a + Km.
In the CuAl14Fe5 alloy, the phases identified included y2 (large grey areas) and Kii (small bright spots). No regions with the ß phase were identified, probably due to the presence of needles of non-equilibrium mar-tensitic ß' phase within the ß phase.
Figure 8: DSC curves for heating and cooling: a) CuAl10Ni5Fe4 alloy, b) CuAl14Fe5 alloy
Slika 8: DCS-krivulje pri ogrevanju in ohlajanju: a) zlitina CuAl10 Ni5Fe4, b) zlitina CuAl14Fe5
Figure 9: Band contrast and phase maps: a) alloy CuAl10Ni5Fe4, after annealing at 800 °C, air; grey phase - phase a, light phase -phase KII, b) alloy CuAl14Fe5, grey phase - phase y2, light phase -phase KII
Slika 9: Trakast kontrast in razporeditev faz: a) zlitina CuAl10Ni5Fe4 po zarjenju na 800 °C, ohlajeno na zraku; siva faza - faza a, svetla faza - faza KII, b) zlitina CuAl14Fe5, siva faza - faza y2, svetla faza -faza KII
4 CONCLUSIONS
The outcomes of the investigation of the effect of heat treatment on the microstructure and properties of pressed bars from CuAl10Ni5Fe4 alloy can be summarized as follows:
•	Microstructure of the bars upon hot pressing is fine, consisting of a phase grains and the a + k eutectoid. Owing to rapid cooling, the ß phase may form as well.
•	Annealing causes a grains and the eutectoid regions with KIII-type precipitates to coarsen.
•	Maximum hardness was obtained by quenching and aging at 400 °C: 400 HV30. This is due to the fine dispersion of particles k in the martensitic ß' phase.
•	The y2 phase was not detected in the CuAl10Ni5Fe4 alloy by DSC or EBSD analysis.
CuAl10Ni5Fe4 Alloy
•	The hardness of the CuAl14Fe5 is higher. It is determined by the proportion of the very hard Y2 phase (with a microhardness of more than 550 HV0.05) in the ß phase. The as-received specimen had a hardness of 422 HV10. Upon annealing, with subsequent air cooling, the fraction of 72 increased. The hardness increased as well: to a level of 462 HV10.
•	The microstructure consists of ß phases with a greater fraction of 72, particles of Kii within the 72 phase and minute Kiv dispersoids in the ß phase.
•	The presence of k and 72 phases was confirmed by DSC and EBSD analyses.
Acknowledgement
The results presented in this paper were developed under the project "West-Bohemian Centre of Materials and Metallurgy" CZ.1.05/2.1.00/03.0077 co-funded from European Regional Development Fund.
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