The Platinum Development Initiative: Platinum-Based Alloys for High Temperature and Special Applications: Part III
The Platinum Development Initiative: Platinum-Based Alloys for High Temperature and Special Applications: Part III
Under the Platinum Development Initiative, platinum-based alloys were being developed for high-temperature and special applications for good corrosion and oxidation resistance. Work on ternary alloys had previously identified that the best of these systems for both mechanical properties and oxidation resistance were Pt86:Al10:Cr4 and Pt86:Al10:Ru4 (1), although the maximum precipitate volume fraction was only ∼ 40% as opposed to ∼ 70% achieved in nickel-based super alloys. Since Pt86:Al10:Cr4 and Pt86:Al10:Ru4 gave the best results, a range of quaternary alloys were also made, using these compositions as a basis. The optimum composition was found to be around Pt80:Al14:Cr3:Ru3. Subsequently, further additions were made to the quaternary alloys to change selected properties.
This is the third paper of four on the work undertaken under the auspices of the Platinum Development Initiative (PDI), which was in operation from April 1997 to October 2007, and comprised Anglo Platinum, Impala Platinum, Lonmin (previously Lonrho) and Mintek. The background and aims of the PDI are summarised in (1, 2).
As well as the research work itself, the training of postgraduate students, and encouragement of undergraduate and diploma students by the provision of vacation work and experiential training, was considered an important output, since South Africa has a shortage of scientists and engineers. This was seen as a great benefit, as were the collaborations with other institutions, especially those outside South Africa. During the time of the work, collaborations were started with NIMS (formerly NRIM) in Japan, Fachhochschule Jena and Bayreuth University in Germany, and Leeds, Cambridge and Oxford Universities in the U.K., as well as four South African universities, which were the Universities of the Witwatersrand, Cape Town, Limpopo (formerly the North) and the Nelson Mandela Metropolitan University (formerly the University of Port Elizabeth). The work is still ongoing, mainly as student projects, and to date there have been eighteen postgraduate students involved, of whom fourteen have now completed their studies. In total, there has been involvement in seven Ph.D.s and eleven M.Sc.s, and over eighty journal and conference papers have been published.
Development of a Quaternary Alloy
Using the platinum-aluminium system as a basis for ordered face centred cubic (f.c.c.) precipitates in an f.c.c. matrix (analogous to the nickel-based superalloys), it was found that ternary alloying elements, and in particular chromium and ruthenium, had additional benefits. More extensive work was carried out on the phase relations, and Cr was found to stabilise the cubic form of the ∼ Pt3Al phase, whereas Ru acted as a solid solution strengthener (3–5) However, the 2 at.% Ru amount did not stabilise the high temperature L12 form of Pt3Al, and the alloys needed Cr or another L12 stabiliser.
The composition of the quaternary alloy needed to be optimised so that the maximum proportion of the ∼ Pt3Al second phase was achieved. It was ultimately the objective to increase the volume fraction of γ′ to enhance the alloy's creep properties. Several alloys were therefore manufactured with this objective (6), and the compositions were selected based on the results of the ternary Pt-Al-Cr and Pt-Al-Ru systems. The alloys were prepared by arc-melting the pure elements several times to achieve the highest possible homogeneity, and the samples were then heat treated at 1350°C for 96 hours. Hardnesses of the alloys were measured using a Vickers hardness tester with a 10 kg load. Some of the alloys were single-phase, and these showed cracking around the indentations (6). Two alloys, Pt78:Al15.5:Cr4.5:Ru2 and Pt81.5:Al11.5:Cr4.5:Ru2.5, had large areas of ∼ Pt3Al, together with a fine mixture of (Pt), the solid solution based on platinum, and ∼ Pt3Al (Figure 1(a)). Another, Pt84:Al11:Cr3:Ru2, was composed entirely of a fine two-phase mixture, which is the desired microstructure (Figure 1(b)).
More alloys were produced in an attempt to increase the volume fraction of the ∼ Pt3Al precipitates further. After heat treatment (again, 96 hours at 1350°C in air), some oxidation took place, and, due to the small sample size, losses of aluminium also occurred. No improvement in the microstructure was observed. The hardnesses of the alloys were measured and the results are given in Table I. The alloys were reasonably ductile, with no cracking around the indentations, as was found in some of the earlier single-phase quaternary alloys (6).
In an attempt to improve the microstructure of the alloys, a second heat treatment was conducted for 96 hours at 1350°C, after which the alloy Pt81.5:Al11.5:Cr4.5:Ru2.5 showed a clear, fine two-phase microstructure, possibly due to the change in its overall composition. There was no primary ∼ Pt3Al (so the overall composition is that of the two-phase mixture: 85.2 ± 0.3 at.% Pt, 7.1 ± 0.8 at.% Al, 3.1 ± 0.8 at.% Ru and 4.6 ± 0.1 at.% Cr). The loss of Al is concerning, especially after such a short anneal, but this was the only alloy to suffer such a large change. The precipitates in Pt84:Al11:Cr3:Ru2 (Figure 1(b)) were approximately twice as large, but more well-defined than those of Pt85:Al7:Cr5:Ru3 alloys (6).
The Vickers hardnesses of the Pt:Al:Cr:Ru alloys within the composition ranges selected were relatively independent of both chemical content and the number of annealing stages, and fell within the range HV10 ∼ 400 to ∼ 430. Hardnesses were slightly lower after the second anneal. The volume fraction of ∼ Pt3Al was estimated, using image analysis, to be approximately 25 to 30%. The highest hardness was found in the alloy without primary ∼ Pt3Al. In the second batch of quaternary alloys, there was no clear relationship between the hardness and the composition or microstructure. The decrease in hardness after the second heat treatment is likely to be due to the changes in composition due to oxidation (6). The best alloy at this stage was Pt84:Al11:Cr3:Ru2, which had the required fine two-phase structure, with no primary ∼ Pt3Al and reasonable hardness. Similar alloys annealed at 1300°C for 96 hours and quenched in water gave better all-round results for Pt80:Al14:Cr3:Ru3 than Pt86:Al10:Cr4 because there was more of the ∼ Pt3Al phase (7–9), although it was more coarse, as shown in Figure 2.
Since Pt86:Al10:Cr4 was very promising with regard to high-temperature strength and oxidation resistance, it was decided to test a quaternary alloy with Ru as an addition to verify that the good oxidation resistance was retained. More Al was added in an effort to accelerate oxide scale formation (10, 11). After one hour at 1350°C, a thin continuous oxide layer had formed. After 10 hours’ exposure (Figure 3), the scale was already about three times as thick as that observed on Pt86:Al10:Cr4 after the same time period. No zone of discontinuous oxides, nor any other internal oxidation, was observed, as had been seen in some of the earlier ternary alloys (12, 13), Pt:Al:X where X = Re, Ta and Ti. The increased Al content of the alloys clearly accelerated the formation of a continuous layer, and prevented mass loss due to volatilisation. Although this showed good properties for the short test period, in the long term the alloy's oxidation might be too severe. The alloy should ideally form a continuous oxide layer quickly but then behave logarithmically with regard to mass increase.
High-temperature compression and creep results were promising, and the high-temperature compressive strength of Pt84:Al11:Cr3:Ru2 is significantly higher than that of Pt86:Al10:Cr4 (7–9). However, some potential manufacturers asked for tensile data, especially yield and ultimate tensile stress, since no prior data were available. Since results become strain rate dependent very soon at high temperatures, it was decided to only evaluate the room temperature tensile properties of the best ternary alloys, compared with that of the quaternary alloy (14).
Normal macro-scale tensile testing was not attempted because of the high material cost. Smaller specimens than the sub-size specimen described by the ASTM standard for tension testing (15) were therefore used. Small specimen test technology has been successfully utilised in fusion materials development, due to limited availability of effective irradiation volumes in test reactors (16). Utilising the required dimensional ratios of the ASTM standard (15), together with studies of specimen size effects (16–19) to ensure that the test data would be comparable to those from standard specimens, dimensions of 46 mm length and 3 mm thickness were set for these experiments, with a gauge length of 18 mm and a gauge width of 3 mm.
The specimens were prepared from 50 g ingots, manufactured by arc-melting, then aged in air in a muffle furnace at 1250°C for 100 hours, then quenched in water. This treatment produced a homogeneous two-phase microstructure, without primary ∼ Pt3Al. The flat mini-tensile specimens were machined from each ingot by wire spark erosion. Tensile tests were performed at a cross-head speed of 5 mm min−1. Unfortunately, the method of testing and imperfections in some of the specimen shoulders rendered calibration of the extensometer impossible, resulting in the tests being carried out on a gauge length of 12.5 mm only. This meant that strain could not be accurately measured, thus elongation could only be estimated by measurement of the distance between gauge marks before and after testing, the latter being done after the fractured parts of the specimens were fitted together. Also, yield stress could not be determined. After testing, samples were prepared metallographically, and then Vickers hardness tests (20 kg load) were carried out. Some of the specimens failed outside the gauge length, and the results from these specimens were not carried further in the study. The average hardnesses, maximum ultimate tensile strength and estimated elongations are given in Table II. The spread and inconsistencies in the results were disappointing. It would have been more ideal to test a wider range of specimens, but the price of Pt constrained the number of specimens. Additionally, the results were unexpected, since it was anticipated that Ru being a better solid solution strengthener in these alloys than Cr (20) would promote Ru alloys with a higher ultimate tensile strength. Thus, characterisation had to be undertaken to explain this discrepancy.
|Alloy composition, at.%||Hardness, HV10||Maximum ultimate tensile strength achieved, MPa||Elongation, %|
|Pt86:Al10:Cr4||317 ± 13||836||∼ 4|
|Pt86:Al10:Ru4||278 ± 14||814||∼ 9|
|Pt84:Al11:Cr3:Ru2||361 ± 10||722||∼ 1|
Using one half of the broken samples, scanning electron microscopy (SEM) analyses under backscattered electron (BSE) imaging using energy dispersive X-ray (EDX) spectrometry were done. There were no significant differences between the targeted and actual compositions, although the Pt86:Al10:Cr4 alloy had discernable Ru content (less than ∼ 0.1 wt.%) and the Pt86:Al10:Ru4 sample had a similar amount of Cr (21). This contamination probably arose from minor sputtering during melting in the button-arc furnace. The fracture surfaces of the other half of each sample were also examined using SEM in secondary electron (SE) mode. Transmission electron microscopy (TEM) specimens were made and examined in a Philips CM200 TEM (21). X-Ray diffraction (XRD) analyses were conducted on the polished samples using molybdenum Kα radiation.
Microstructures were derived by SEM, XRD and TEM analyses, indicating that all samples contained both (Pt) and ∼ Pt3Al precipitates. The volume fraction of the precipitates varied between specimens and compositions. The Pt86:Al10:Cr4 specimens were stronger than those of Pt86:Al10:Ru4, because there was a very low volume fraction (∼ 5%) of ∼ Pt3Al precipitates in the alloy with Ru. Thus, it was deduced that Pt86:Al10:Ru4 had been annealed above its solvus, which was thought to be between 1250°C and 1300°C. Thus, the higher ductility of this alloy was due to its nearly single phase Pt solid solution. Pt86:Al10:Cr4 was harder and also had a higher ultimate tensile strength than Pt86:Al10:Ru4. Having a significant volume fraction of ∼ Pt3Al, Pt84:Al11:Cr3:Ru2 was the hardest alloy, but had the lowest ultimate tensile strength. The fracture surfaces (Figure 4) showed that only Pt84:Al11:Cr3:Ru2 failed intergranularly, with the ternary alloys failing mainly by intragranular cleavage with some localised signs of dimpling. Thus, it is likely that the lower ultimate tensile strength was related to the intergranular failure mode, which also correlates to its lower elongation.
The results for these Pt-based alloys are summarised in Table III, together with values for pure Pt and selected high-temperature alloys. The values of hardness and ultimate tensile strength for the Pt-based alloys are higher than those of pure Pt in the soft state (i.e. not hardened). Compared to other high-temperature alloys, such as the ferritic oxide dispersion strengthened (ODS) alloy PM2000, γ-titanium-aluminium and CMSX-4 (a nickel-based superalloy), it is clearly demonstrated that these Pt-based alloys are within the range of the high-temperature alloys in terms of ultimate tensile strength at room temperature. This finding is encouraging since the samples had not been optimised in terms of either heat treatment or microstructure.
|Alloy or metal||Hardness range, HV||Ultimate tensile strength at room temperature, MPa||Elongation, %||References|
|Pt-based alloys||300–350||∼ 800||–||(13, 21)|
|Pure Pt (soft state)||∼ 40||∼ 140||–||(22)|
|Ferritic ODS alloy PM2000||–||720||14||(23)|
Creep testing of the Pt84:Al11:Cr3:Ru2 alloy was undertaken, and the results were worse than for Pt86:Al10:Cr4. This was deduced to arise from a different atmosphere being used; initially the tests were undertaken under argon, and latterly the tests were done in air. However, the results of Pt84:Al11:Cr3:Ru2 were slightly worse than a commercial Pt alloy strengthened by dispersion hardening (DPH). The high-temperature compressive strength of Pt84:Al11:Cr3:Ru2 was significantly higher than that of Pt86:Al10:Cr4 (7–9).
Other Alloying Additions
Other additions such as cobalt or nickel have also been tested to improve the properties of the alloys, and decrease their cost and density. For the potential additions, phase diagram work was also undertaken, such as the Pt-Al-Co (26, 27) and Pt-Ni-Ru systems (28). Ni was added to improve the solution strengthening of the matrix, although less solution strengthening was achieved than hoped. Surprisingly, the melting point was increased by Ni additions. The work was not carried further because of the disappointing hardness results, and because work in Pt-Ni-based alloys was ongoing in Germany (29–34).
Pt-Al-Co and Pt-Al-Co-Cr-Ru alloys were subjected to cold rolling on a small mill, and yielded very interesting results. Alloys with hardnesses below 400 HV10 showed good cold formability (> 75% total reduction in thickness), whereas the cold formability was poor (< 40%) for hardnesses above 450 HV10 (Figure 5).
The alloys with good formability were two-phase, with (Pt,Co) and Pt3Al phases, and contained between 5 and 20 at.% Co and less than 20 at.% Al. Excellent formability was obtained for alloys containing (Pt) and CoPt3, whereas the alloys containing other intermetallic phases showed extremely poor formability (< 5% total reduction). The formability (per cent total reduction) of the Pt-Al-Co alloys improved sigmoidally with increasing per cent (Pt + Co) in the range 60 to 90 at.% (Pt + Co) (Figure 6). The cold formability of these alloys is far superior to Pt-Al alloyed with Cr, Ru and Ni.
A range of platinum-based superalloys that show very promising properties has been developed by Mintek in collaboration with workers at South African universities. Two-phase γ/γ′ microstructures, analogous to nickel-based superalloys, consisting of cuboidal ∼ Pt3Al precipitates in a (Pt) matrix, were achieved. The best alloy to date in terms of microstructure is Pt84:Al11:Cr3:Ru2, since it had the required structure with no primary ∼ Pt3Al, and reasonable hardness. The optimum composition range is Pt84:Al11:Cr3:Ru2 to Pt80:Al14:Cr3:Ru3. The oxidation resistance of Pt84:Al11:Cr3:Ru2 is better than either of the Cr- or Ru-containing Pt-Al-based ternary alloys included in the present study. However, there was concern that the alloy formed the protective alumina film too quickly and this might cause problems in the long term. Pt-based alloys have hardnesses in the order of 300–350 HV, with an ultimate tensile strength of ∼ 800 MPa. It is obvious that these Pt-based alloys have ultimate tensile strength values within the same range as other high-temperature alloys such as CMSX-4, which has an ultimate tensile strength of 870 MPa. Pt-Al-Co alloys with hardnesses below 400 HV10 showed good cold formability (> 75% total reduction in thickness), whereas the cold formability was poor (< 40%) for hardnesses above 450 HV10.
Part IV of the present series will appear in a forthcoming issue of Platinum Metals Review, and will cover the corrosion of these platinum-based alloys.
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The financial assistance of the South African Department of Science and Technology (DST), the Platinum Development Initiative (PDI: Anglo, Impala, Lonmin and Mintek) and the DST/NRF Centre of Excellence in Strong Materials is gratefully acknowledged.
This paper is published with the permission of Mintek.
Lesley Cornish is a Professor at the University of the Witwatersrand, South Africa, and is Director of the DST/NRF Centre of Excellence for Strong Materials, which is hosted by the University of the Witwatersrand, and the African Materials Science and Engineering Network (AMSEN). Her research interests include phase diagrams, platinum alloys and intermetallic compounds.
Rainer Süss is a Chief Engineer in the Physical Metallurgy Group in the Advanced Metals Division at Mintek, as well as the co-ordinator of the Strong Metallic Alloys Focus Area in the DST/NRF Centre of Excellence for Strong Materials. His research interests include phase diagrams, platinum alloys and jewellery alloys.
Lesley Chown is a Principal Engineer in the Physical Metallurgy Group in the Advanced Materials Division at Mintek. She has worked on continuously cast steels, platinum alloys and titanium alloys.
Lizelle Glaner is a Principal Technician in the Advanced Materials Division at Mintek, where she is in charge of the Nano Characterisation Laboratory. She has worked on gold catalysts, gold jewellery alloys and platinum alloys.