347 PART FOUR: SELECTED MECHANICAL PROPERTIES 1. HARDNESS Since the hardness of a metal depends on the ease with which it plastically deforms, there is an empirical relationship between hardness and strength. Since the hardness test is much simpler than a tensile test it is therefore often used by engineers to obtain an easily measured and specified quantity which indicates something about the strength and heat treatment of a metal [1988Die, 1990Smi]. 1.1. Experimental procedure The hardness of the alloys was measured using a Vickers Ltd. Vickers hardness tester with a 10 kg load, with at least six analyses carried out to obtain an average hardness value. 1.2. Results and discussion 1.2.1. As-cast The results of hardness measurements on the annealed samples are given in Table 4.1. The average values are superposed on the solidification projection in Figure 4.1. Table 4.1. Hardness values of as-cast Pt-Cr-Al alloys (HV10). Alloy As cast hardness Phases present Pt10:Al10:Cr80 704 ? 16 (Cr), ~PtAl Pt10:Al30:Cr60 675 ? 17 (Cr), ~PtAl2 Pt10:Al60:Cr30 699 ? 63 ~Cr5Al8, ~PtAl2 Pt10:Al80:Cr10 367 ? 20 ~CrAl5, ~Pt8Al21, (Al), ~PtAl2 Pt20:Al20:Cr60 760 ? 36 (Cr), ~PtAl Pt20:Al40:Cr40 777 ? 7 ~PtAl2, ~PtAl, (Cr) Pt30:Al50:Cr20 806 ? 8 ~PtAl2, ~PtAl, (Cr) Pt40:Al10:Cr50 749 ? 13 ~CrPt, ~Pt2Al Pt40:Al30:Cr30 743 ? 13 T1 Pt40:Al50:Cr10 841 ? 76 ~Pt2Al3, ~PtAl 348 Table 4.1.(cont.) Hardness values of as-cast Pt-Cr-Al alloys (HV10). Alloy As-cast hardness Phases present Pt50:Al10:Cr40 572 ? 61 ~CrPt, ~Pt2Al Pt65:Al5:Cr30 322 ? 21 ~Pt3Cr, ~Pt3Al Pt3:Al65:Cr32 670 ?32 ~Cr4Al9, ~PtAl2 Pt3:Al35:Cr62 637 ?26 ~Cr2Al, ~PtAl2, T2?Cr3Al2 Pt10:Al50:Cr40 762 ?16 T2?Cr3Al2, ~PtAl2, ~Cr5Al8 Pt12:Al18:Cr70 758 ?50 (Cr), ~PtAl2, ~PtAl Pt17:Al68:Cr15 666 ?16 ~PtAl2, ~CrAl4, ~Cr4Al9 Pt18:Al2:Cr80 767 ?29 ~CrPt, ~Cr3Pt, (Cr) Pt28:Al12:Cr60 462 ?4 ~CrPt, (Cr), T1 Pt28:Al70:Cr2 653 ?89 ~PtAl2, ~Pt8Al21 Pt35:Al55:Cr10 822 ?39 ~PtAl2, ~Pt2Al3, ~PtAl Pt38:Al22:Cr40 717 ?27 T1, ~CrPt Pt43:Al52:Cr5 818 ?69 ~Pt2Al3, ~PtAl Pt50:Al35:Cr15 739 ?14 T1 Pt58:Al10:Cr32 697 ?18 ~Pt3Al, ~CrPt Pt60:Al2:Cr38 333 ?35 ~CrPt Pt63:Al32:Cr5 591 ?65 ~Pt2Al, T1 Pt63:Al22:Cr15 673 ?6 ~Pt3Al, ~Pt2Al Pt78:Al17:Cr5 407 ?6 ~Pt3Al, (Pt) Pt75:Al10:Cr15 880 ?10 ~PtAl2, ~PtAl, (Cr) Pt3:Al79:Cr18 425 ? 111 ~CrAl4, T3?CrAl3, ~CrAl5, ~PtAl2 Pt5:Al52:Cr43 726 ? 30 T2?Cr3Al2, ~PtAl2, ~Cr5Al8 Pt13:Al45:Cr42 688 ? 42 ~PtAl2, ~Cr2Al Pt15:Al5:Cr80 632 ? 4 (Cr), ~CrPt, T1 Pt34:Al27:Cr39 670 ? 9 T1, (Cr) Pt37:Al59:Cr4 707 ? 32 ~Pt2Al3 Pt45:Al35:Cr20 628 ? 38 T1 Pt52:Al43:Cr5 788 ? 45 T1 Pt55:Al25:Cr20 450 ? 75 ~Pt2Al Pt55:Al20:Cr25 414 ? 61 ~Pt2Al Pt64:Al16:Cr20 532 ? 15 ~Pt3Al, ~Pt3Cr Approximately 75% of the alloys in the as-cast condition had hardnesses above 600 HV10, indicating that most phases in this system are fairly hard. Only alloy Pt76:Al5:Cr19 had a hardness of less than 300 HV10, being single phase ~Pt3Cr. The high Al-containing alloys Pt3:Al79:Cr18 and Pt10:Al80:Cr10 were also relatively low in hardness. Five alloys had exceptionally high hardnesses (> 800 HV10) compared to the rest, namely alloy Pt30:Al50:Cr20, which contained a majority of ~PtAl2; alloys Pt40:Al50:Cr10 and Pt43:Al52:Cr5 which contained ~Pt2Al3 and ~PtAl; and alloys Pt35:Al55:Cr10 and Pt75:Al10:Cr15 (actual Pt34:Al49:Cr17), which 349 contained ~PtAl2. Thus, ~PtAl2, ~PtAl and ~Pt2Al3 were very hard phases compared to the others in the system. There was a variation in the hardness of the single-phase T1 alloys, from 628 ? 38 to 788 ? 45 HV10, with no composition-related trend. PtAl CrPt Pt3AlPt2AlPtAlPtAl2 Pt2Al3 Pt3Cr (Cr) (Cr) Cr3Pt Cr2Al Pt5Al21 Pt6Al21 Pt8Al21 Pt5Al3 ~CrAl5 CrAl4 Cr5Al8 Cr4Al9 T2~Cr3Al2 T3~CrAl3 T1 Pt3Al (Pt) Pt2Al Cr 704 767 758 760675 637 462 749 777 762 717 572 333 322 697743 699 670 367 666 653 806 880 818 822 739 673 591 407 425 726 688 632 670 707 628 788 450 414 532 246 Figure 4.1. Hardness values of as-cast Pt-Cr-Al alloys superposed on the solidification projection. 1.2.2. Annealed at 1000qC The results of the hardness measurements on the 1000?C annealed samples are given in Table 4.2, compared to the relevant alloys? as-cast hardness values. The only alloys whose hardness did not change significantly during annealing at 1000?C were alloys Pt65:Al5:Cr30, Pt63:Al32:Cr5 and Pt55:Al25:Cr20, even though, especially in the case of Pt55:Al25:Cr20, the 350 microstructure had changed significantly. On the other hand, an alloy like Pt10:Al30:Cr60 hardly changed during annealing, yet its hardness fell from 675 ? 17 to 241 ? 19 HV10. This alloy was in fact very porous and the Vickers diamond indentor sank into the sample, and it is believed that the given hardness value is not a true indication of its hardness. The same argument could be used for the hardness of alloy Pt50:Al10:Cr40 which seemed softer than expected. Over 75% of the alloys? hardness values decreased. Table 4.2. Hardness values of Pt-Cr-Al alloys annealed at 1000?C for 1000 hours (HV10) compared to as-cast hardness values. Alloy Annealed hardness and standard deviation As-cast hardness and standard deviation Phases present in 1000qC annealed samples Pt10:Al30:Cr60 241 ? 19 675 ? 17 (Cr), ~PtAl2, ~Cr2Al Pt10:Al80:Cr10 575 ? 42 367 ? 20 (Al), ~CrAl5, ~PtAl2 Pt30:Al50:Cr20 715 ? 12 806 ? 8 ~PtAl2, ~Pt2Al3, (Cr) Pt50:Al10:Cr40 320 ? 42 572 ? 61 ~CrPt Pt65:Al5:Cr30 289 ? 7 322 ? 21 ~Pt3Al Pt3:Al65:Cr32 556 ? 27 670 ?32 ~Cr4Al9, ~PtAl2 Pt3:Al35:Cr62 432 ? 32 637 ?26 (Cr), ~PtAl2, ~Cr2Al/T2?Cr3Al2 Pt18:Al2:Cr80 882 ? 10 767 ?29 ~Cr3Pt, ~CrPt, (Cr) Pt35:Al55:Cr10 755 ? 5 822 ?39 ~PtAl2, ~Pt2Al3, (Cr) Pt38:Al22:Cr40 629 ? 4 717 ?27 T1, ~Cr3Pt, (Cr) Pt43:Al52:Cr5 714 ? 20 818 ?69 ~PtAl, ~Pt2Al3, (Cr) Pt50:Al35:Cr15 652 ? 19 739 ?14 T1, ~PtAl Pt58:Al10:Cr32 581 ? 15 697 ?18 ~CrPt, ~Pt3Al Pt63:Al32:Cr5 652 ? 23 591 ?65 ~Pt2Al Pt63:Al22:Cr15 621 ? 11 673 ?6 ~Pt2Al, ~Pt3Al Pt75:Al10:Cr15 750 ? 18 880 ?10 ~Pt2Al3, ~PtAl, (Cr) Pt15:Al5:Cr80 544 ? 18 632 ? 4 ~Cr3Pt, (Cr), ~PtAl Pt55:Al25:Cr20 458 ? 22 450 ? 75 T1, ~Pt3Al, ~Pt2Al Just over half the alloys had hardnesses above 600 HV10, indicating that most phases in this system are fairly hard, which could be expected from intermetallic compounds. Only alloys Pt50:Al10:Cr40 and Pt65:Al5:Cr30 had hardnesses of less than 400 HV10. Only three alloys had exceptionally high hardness (> 750 HV10) compared to the rest, namely alloy Pt18:Al2:Cr80, which contained a majority of ~Cr3Pt; alloys Pt35:Al55:Cr10 which contained ~PtAl2 and ~Pt2Al3; and Pt75:Al10:Cr15 (actual Pt33:Al51:Cr16), which contained ~PtAl2 and ~PtAl. As was 351 the case in the as-cast state, it was obvious that ~PtAl2, ~PtAl and ~Pt2Al3 are very hard phases compared to the others in the system. ~Cr3Pt is also very hard. It was also interesting to see that ~75% of the alloys that were annealed at 1000?C softened during annealing. The only ones that became harder were Pt10:Al80:Cr10 (~Pt8Al21 disappeared during annealing), Pt18:Al2:Cr80 (the alloy almost turning into single-phase ~Cr3Pt) and Pt63:Al32:Cr5 (turning into single-phase ~Pt2Al, T1 disappearing). The average values are superposed on the 1000?C isothermal section in Figure 4.2. T1 Cr4Al9 Cr PtAl CrPt Pt3AlPt2AlPtAlPtAl2 Pt2Al3 Pt3Cr (Cr) (Cr) Cr3Pt Pt8Al21 Pt5Al3 CrAl4 Cr5Al8 (Pt) ~CrAl5 322 241 575 715 289556 432 882 755 629 714 652 581 652 621 750 458 544 L Figure 4.2. Hardness values of Pt-Cr-Al alloys annealed at 1000?C for 1000 hours, superposed on the 1000?C isothermal section. 352 1.2.3. Annealed at 600qC The results of the hardness measurements on the 600?C annealed samples are given in Table 4.3 (and superposed on the 600?C isothermal section in Figure 4.3), compared to the relevant alloys? as-cast and 1000?C annealed hardness values. Table 4.3. Hardness values of Pt-Cr-Al alloys annealed at 600?C for 2500 hours (HV10) compared to as-cast and 1000?C hardness values. Alloy Annealed (600qC) hardness and standard deviation As-cast hardness and standard deviation Annealed (1000qC) hardness and standard deviation Phases present in 600qC annealed samples Pt10:Al30:Cr60 760 ? 17 675 ? 17 241 ? 19 ~PtAl2, (Cr) Pt10:Al60:Cr30 728 ? 41 699 ? 63 Not annealed ~PtAl2, ~Cr5Al8, ~Cr4Al9 Pt30:Al50:Cr20 853 ? 28 806 ? 8 715 ? 12 ~PtAl2, ~PtAl, (Cr) Pt50:Al10:Cr40 708 ? 56 572 ? 61 320 ? 42 ~CrPt, ~Pt2Al Pt65:Al5:Cr30 327 ? 6 322 ? 21 289 ? 7 ~Pt3Al, ~Pt3Cr Pt3:Al65:Cr32 645 ? 31 670 ? 32 556 ? 27 ~Cr4Al9, ~PtAl2 Pt3:Al35:Cr62 576 ? 15 637 ? 26 432 ? 32 (Cr), ~PtAl2, ~Cr2Al Pt18:Al2:Cr80 962 ? 62 767 ? 29 882 ? 10 ~CrPt, ~Cr3Pt, (Cr) Pt35:Al55:Cr10 947 ? 30 822 ? 39 755 ? 5 ~PtAl2, ~PtAl, ~Pt2Al3 Pt38:Al22:Cr40 950 ? 28 717 ? 27 629 ? 4 T1, ~CrPt, ~Cr3Pt Pt43:Al52:Cr5 914 ? 34 818 ? 69 714 ? 20 ~PtAl, ~Pt2Al3 Pt75:Al10:Cr15 810 ? 116 880 ? 10 750 ?18 ~PtAl2, ~Pt2Al3, ~PtAl Pt50:Al35:Cr15 960 ? 14 739 ? 14 652 ? 19 ~PtAl, T1 Pt58:Al10:Cr32 854 ? 6 697 ? 18 581 ? 15 ~Pt3Al, ~Pt3Cr, ~CrPt Pt63:Al32:Cr5 617 ? 24 591 ? 65 652 ? 23 ~Pt2Al, T1 Pt63:Al22:Cr15 798 ? 22 673 ? 6 621 ? 11 ~Pt3Al, ~Pt2Al Pt15:Al5:Cr80 901 ? 13 632 ? 4 544 ? 18 (Cr), ~CrPt, T1, ~Cr3Pt Pt55:Al25:Cr20 650 ? 33 450 ? 75 458 ? 22 ~Pt2Al, T1 Over 70% of the alloys had hardnesses above 700 HV10, once again an indication of the hard intermetallic compounds present in the system. Only alloys Pt3:Al35:Cr62 and Pt65:Al5:Cr30 had hardnesses of less than 600 HV10. Several alloys had exceptionally high hardness (> 900 HV10) compared to the rest, namely alloys Pt15:Al5:Cr80, Pt18:Al2:Cr80, Pt75:Al10:Cr15 (actual Pt33:Al50:Cr17), Pt38:Al22:Cr40, Pt43:Al52:Cr5 and Pt50:Al35:Cr15. It is unclear why alloy Pt75:Al10:Cr15 showed such a large standard deviation on hardness - maybe subsurface porosity played a role. The presence of intermetallic compounds like ~PtAl2, PtAl and ~CrPt are responsible for these high values, as well as the presence of T1. Only alloys Pt3:Al65:Cr32, 353 Pt3:Al35:Cr62 and Pt75:Al10:Cr15 softened during annealing at 600?C. The softening is probably due to the loss of ~Pt2Al3 during annealing in the latter, and the disordering of ~Cr2Al into (Cr) in alloy Pt3:Al35:Cr62. It is unclear why alloy Pt3:Al65:Cr32 softened. Cr4Al9 Cr PtAl CrPt Pt3AlPt2AlPtAlPtAl2 Pt2Al3 Pt3Cr (Cr) (Cr) Cr3Pt Pt8Al21 Pt5Al3 CrAl4 Cr5Al8 (Pt)~CrAl5 708 760 853 327645 576 962 810 950 914 960 854 617 798 947 650 901 728 T1 Figure 4.3. Hardness values of Pt-Cr-Al alloys annealed at 600?C for 2500 hours, superposed on the 600?C isothermal section. 1.3. Conclusions Platinum is significantly hardened by alloying with Cr and Al. To put it in perspective, all the measured hardnesses were significantly higher than the 40-50 HV of pure Pt in the soft state [2007Int]. Most of the alloys were harder than the 360 HV10 hardness of Pt84:Al11:Ru2:Cr3 [2004S?s3], the most promising Pt-based alloy thus far for potential application in high temperature environments. In the case of Pt84:Al11:Ru2:Cr3, the strengthening is the result of precipitation hardening, while the high hardnesses of alloys in the Pt-Al-Cr system is caused by the formation of high hardness intermetallic compounds. 354 2. TOUGHNESS By classifying the deformation lines around a hardness indentation, one can get a basic idea about the toughness of a material (i.e. resistance to fracture). Cracking around an indentation is obviously an indication of brittleness. Straight lines around an indentation indicate planar slip on the primary slip system (moderate toughness), while wavy lines (wavy slip) indicates deformation on multiple slip systems whereby more increased plastic deformation is possible. 2.1. Experimental procedure In order to get a qualitative evaluation of the alloys? toughness, photographs were taken of the hardness indentations via a JVC TK138E Color Video Camera connected to an Olympys Vanox-T light microscope, and the slip mode around the indentations classified. Note that the classification ?no discernible slip? refers to cases where the relevant slip mechanism could not be identified because neither wavy nor planar slip was observed under the optical microscope, although slip would obviously have occurred. 2.2. Results and discussion 2.2.1. As-cast Figure 4.4 summarises the results of the measurements on the as-cast alloys, showing the relevant hardness indentations and slip modes. Pt10:Al10:Cr80 Cracking Pt10:Al30:Cr60 Major cracking and wavy slip Figure 4.4. Optical micrographs for comparison of the hardness indentations in as-cast Pt- Cr-Al alloys. 355 Pt10:Al60:Cr30 Major cracking Pt10:Al80:Cr10 Major interdendritic cracking Pt20:Al20:Cr60 Major cracking Pt20:Al40:Cr40 Major cracking Pt30:Al50:Cr20 Major cracking Pt40:Al10:Cr50 Wavy slip Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. 356 Pt40:Al30:Cr30 No discernible slip Pt40:Al50:Cr10 Major cracking Pt50:Al10:Cr40 Wavy slip Pt65:Al5:Cr30 No discernible slip Pt3:Al65:Cr32 Major cracking Pt3:Al35:Cr62 Wavy slip and major cracking Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. 357 Pt10:Al50:Cr40 Major cracking Pt12:Al18:Cr70 Major cracking Pt17:Al68:Cr15 Major cracking Pt18:Al2:Cr80 Wavy slip Pt28:Al12:Cr60 Wavy slip Pt28:Al70:Cr2 Major cracking Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. 358 Pt35:Al55:Cr10 Major cracking Pt38:Al22:Cr40 Wavy slip Pt43:Al52:Cr5 Major cracking Pt50:Al35:Cr15 Cracking Pt58:Al10:Cr32 Wavy and planar slip Pt60:Al2:Cr38 No discernible slip Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. 359 Pt63:Al32:Cr5 Planar slip Pt63:Al22:Cr15 Wavy slip Pt78:Al17:Cr5 Wavy slip Pt75:Al10:Cr15 Major cracking Pt3:Al79:Cr18 Major irregular cracking Pt5:Al52:Cr43 Major cracking Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. 360 Pt13:Al45:Cr42 Major cracking Pt15:Al5:Cr80 Wavy slip Pt34:Al27:Cr39 No discernible slip Pt37:Al59:Cr4 Major cracking Pt45:Al35:Cr20 No discernible slip Pt52:Al43:Cr5 Major cracking Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. 361 Pt55:Al25:Cr20 Major irregular cracking Pt55:Al20:Cr25 Major irregular cracking Pt64:Al16:Cr20 Planar slip Pt76:Al5:Cr19 Cracking Figure 4.4. (cont.). Optical micrographs for comparison of the hardness indentations in as- cast Pt-Cr-Al alloys. It was interesting to note "pin-cushioning" of the hardness indentation of alloy Pt60:Al2:Cr38, an anomaly typically observed with annealed material, when metal is sinking in around the faces of the indentor [1990Die]. Apart from alloy Pt65:Al5:Cr30, alloy Pt60:Al2:Cr38 was the softest in the as-cast condition (333 HV10). It is obvious that most alloys in the system are very brittle, since many of the alloys exhibited major cracking. The only alloys that stood out as having better toughness were the ones that contained ~CrPt (alloys Pt18:Al2:Cr80, Pt15:Al5:Cr80, Pt38:Al22:Cr40, Pt40:Al10:Cr50, and Pt50:Al10:Cr40), Pt3Cr (Pt65:Al5:Cr30 and Pt76:Al5:Cr19) and ~Pt3Al (Pt78:Al17:Cr5, Pt63:Al32:Cr5, Pt63:Al22:Cr15, Pt64:Al16:Cr20 and Pt60:Al2:Cr38) showing wavy slip in most cases. Considering 362 the alloys Mintek is targeting for high temperature applications are based on high Pt content ~Pt3Al containing alloys, this was most encouraging. The different slip modes of the as-cast alloys are superposed on the solidification projection in Figure 4.5. PtAl CrPt Pt3AlPt2AlPtAlPtAl2 Pt2Al3 Pt3Cr (Cr) (Cr) Cr3Pt Cr2Al Pt5Al21 Pt6Al21 Pt8Al21 Pt5Al3 ~CrAl5 CrAl4 Cr5Al8 Cr4Al9 T2~Cr3Al2 T3~CrAl3 T1 Pt3Al (Pt) Pt2Al cracking wavy wavy and major cracking wavy wavy wavy wavy wavy no discernible slip wavy and planar planar major cracking major cracking major cracking major cracking major cracking major cracking major cracking major crackingmajor cracking major cracking major cracking major cracking major cracking major cracking wavy planar major cracking cracking major cracking no discernible slip no discernible slip Figure 4.5. Summary of the slip modes of as-cast Pt-Cr-Al alloys, superposed on the solidification projection. 2.2.2. Annealed at 1000qC Figure 4.6 summarises the results of the measurements on the annealed alloys, showing the relevant hardness indentations and slip modes. 363 Pt10:Al30:Cr60 No visible slip mechanism Pt10:Al80:Cr10 Cracking Pt30:Al50:Cr20 Cracking Pt50:Al10:Cr40 Major irregular cracking Pt65:Al5:Cr30 Wavy slip Pt3:Al65:Cr32 Major cracking Figure 4.6. Optical micrographs for comparison of the hardness indentations in Pt-Cr-Al alloys annealed at 1000?C for 1000 hours. 364 Pt3:Al35:Cr62 Major cracking Pt18:Al2:Cr80 Cracking Pt35:Al55:Cr10 Cracking and wavy slip Pt38:Al22:Cr40 Wavy slip Pt43:Al52:Cr5 Cracking and wavy slip Pt50:Al35:Cr15 Cracking (interdendritic) Figure 4.6. (cont.) Optical micrographs for comparison of the hardness indentations in Pt- Cr-Al alloys annealed at 1000?C for 1000 hours. 365 Pt58:Al10:Cr32 Wavy slip Pt63:Al32:Cr5 Cracking and planar slip Pt63:Al22:Cr15 Planar slip Pt75:Al10:Cr15 Cracking and wavy slip Pt15:Al5:Cr80 Wavy slip Pt55:Al25:Cr20 Major cracking Figure 4.6. (cont.) Optical micrographs for comparison of the hardness indentations in Pt- Cr-Al alloys annealed at 1000?C for 1000 hours. 366 It was interesting to note "barrelling" of the hardness indentation of alloy Pt63:Al22:Cr15, an anomaly normally observed with cold-worked material, when metal is piling up around the faces of the indentor [1990Die]. The alloy had quite a high annealed hardness (621 HV10). The different slip modes of the annealed alloys are superposed on the preliminary 1000?C isothermal section in Figure 4.7. cracks + planar Cr4Al9 Cr PtAl CrPt Pt3AlPt2AlPtAlPtAl2 Pt2Al3 Pt3Cr (Cr) (Cr) Cr3Pt Pt8Al21 Pt5Al3 CrAl4 Cr5Al8 (Pt) ~CrAl no visible slip mechanism cracks cracki wavy cracks + wavy wavy cracks + wavy wavy planar wavy cracks + wavy cracks cracki cracks cracki cracks cracki cracks cracki cracks cracki L T1 Figure 4.7. Summary of the slip modes of Pt-Cr-Al alloys annealed at 1000?C for 1000 hours, superposed on the 1000?C isothermal section. It is obvious that most alloys in the system are very brittle, since many of the alloys exhibited failry major cracking. The only alloys that stood out as having better toughness were the ones that contained ~Pt3Al (Pt58:Al10:Cr32 (wavy slip), Pt63:Al22:Cr15 (planar slip) and Pt65:Al5:Cr30 (wavy slip). Once again, this was encouraging for the Pt-based alloys that are being developed by Mintek. 367 2.2.3. Annealed at 600qC Figure 4.8 summarises the results of the measurements on the 600?C annealed alloys, showing the relevant hardness indentations and slip modes. Pt10:Al30:Cr60 Cracking Pt10:Al60:Cr30 Cracking Pt30:Al50:Cr20 Cracking Pt50:Al10:Cr40 Minor cracking and wavy slip Pt65:Al5:Cr30 No discernible slip Pt3:Al65:Cr32 Major cracking Figure 4.8. Optical micrographs for comparison of the hardness indentations in Pt-Cr-Al alloys annealed at 600?C for 2500 hours. 368 Pt3:Al35:Cr62 Cracking Pt18:Al2:Cr80 No discernible slip Pt35:Al55:Cr10 Major cracking No picture taken Pt38:Al22:Cr40 Pt43:Al52:Cr5 Cracking Pt75:Al10:Cr15 Major cracking Figure 4.8. (cont.) Optical micrographs for comparison of the hardness indentations in Pt- Cr-Al alloys annealed at 600?C for 2500 hours. 369 Pt58:Al10:Cr32 Minor planar slip Pt63:Al32:Cr5 Cracking Pt63:Al22:Cr15 Planar slip Pt50:Al35:Cr15 Cracking Pt15:Al5:Cr80 No discernible slip Pt55:Al25:Cr20 Major cracking Figure 4.8. (cont.) Optical micrographs for comparison of the hardness indentations in Pt- Cr-Al alloys annealed at 600?C for 2500 hours. 370 It was interesting to note "pin-cushioning" of the hardness indentation of alloy Pt65:Al5:Cr30, an anomaly typically observed with annealed material, when metal is sinking in around the faces of the indentor [1990Die]. The alloy was the softest in the 600?C annealed condition (327 HV10). The different slip modes of the 600?C annealed alloys are superposed on the preliminary 600?C isothermal section in Figure 4.9. It is obvious that most alloys in the system are very brittle, since many of the alloys exhibited fairly major cracking. The only alloys that stood out as having better toughness were Pt58:Al10:Cr32 (planar slip), Pt63:Al22:Cr15 (planar slip) and Pt50:Al10:Cr40 (wavy slip). The latter alloy contained ~CrPt, while the other two contained ~Pt3Al. cracks Cr4Al9 Cr PtAl CrPt Pt3AlPt2AlPtAlPtAl2 Pt2Al3 Pt3Cr (Cr) (Cr) Cr3Pt Pt8Al21 Pt5Al3 CrAl4 Cr5Al8 (Pt)CrAl5 cracks cracki major cracks wavy cracks planar planar cracks + wavy cracks cracki cracks cracki cracks + wavy major cracks T1cracks cracki no discernible slip major cracks cracking cracks cracki major cracks no discernible slip Figure 4.9. Summary of the slip modes of Pt-Cr-Al alloys annealed at 600?C for 2500 hours, superposed on the 600?C isothermal section. 371 2.3. Conclusions Based on the examination of hardness indentations, alloys in the Pt-Al-Cr system generally seemed to be brittle regardless of their state of heat treatment. This was evident from the cracking that was observed in the majority of samples. This was expected considering the high hardness values that were obtained for the system. Most of the intermetallic compounds in the system are hard and decrease the toughness of most alloys. Generally, those alloys containing ~CrPt and in particular ~Pt3Al showed much better toughness. The fact that alloys containing ~Pt3Al showed better behaviour with regard to toughness was encouraging for the Pt-based alloys that are being developed by Mintek. 372 PART FIVE: GENERAL CONCLUSIONS ? After using SEM, EDX and XRD on a selection of alloys, the following diagrams were successfully constructed for the Pt-Al-Cr system: ? solidification projection ? liquidus surface projection ? isothermal sections at 600?C and 1000?C. ? It was concluded that all phase regions were identified correctly since the results were self-consistent. ? None of the diagrams were dominated by any particular phase. ? Three ternary phases were found in the Pt-Al-Cr system. ? The ~CrPt/~Pt3Cr and ~Pt3Cr/(Pt) boundaries were shown as dashed lines, indicating either the uncertainty of where the ordering reactions of Massalski [1990Mas] occur, or the estimated positions of the ~CrPt/~Pt3Cr and ~Pt3Cr/(Pt) two-phase regions of Zhao [2005Zhao]. ? 19 ternary invariant reactions were identified in the Pt-Al-Cr system. ? It was confirmed that the two eutectic temperatures in the published Pt-Cr system [1990Mas] are the wrong way around [2004S?s1, 2005Nzu, 2005S?s, 2006S?s1]. ? Single-phase alloys suggested the congruent melting of ternary phase T1, which confirmed a ternary phase, rather than a peritectically formed binary phase. Work is being done to get more precise crystallographic data for the phase [2007Ngw]. ? Based on the examination of hardness indentations, alloys in the Pt-Al-Cr system generally seemed to be brittle regardless of their state of heat treatment due to the presence of hard intermetallic compounds. ? Alloys containing ~Pt3Al showed better behaviour with regard to toughness which was encouraging for the Pt-based alloys that are being developed by Mintek. ? A thermodynamic database was developed for the Pt-Al-Cr system using Thermo-Calc. The database could be used to get a very good prediction of phase relations between 600?C and 1000?C, and even up to temperatures close to the melting point, reasonable results would be obtained. 373 ? The calculated results complimented the experimental work significantly and the value of the CALPHAD method as a tool in alloy design has been clearly demonstrated. ? However, the match between the calculated and experimental diagrams could be improved. ? The fact that the stability range of Pt3Al, (Pt), Pt3Cr and CrPt (the ordered and disordered fcc phases in the Al-Pt and Cr-Pt systems) were inconsistent with regard to the experimental results clearly showed that the 4SL-CEF model description for L12 was problematic. This was underlined when attempting to calculate a liquidus surface projection using both Thermo-Calc and Pandat. ? As with the Cr-Pt-Ru system, problems with the constituting binary systems seems to be the major cause for problems encountered in the modelling. ? Currently, it would be a waste of time to optimise the databases (both Pt-Al-Cr and Cr-Pt- Ru) for the intermetallic phases any further because there are too many unknowns in the binary systems. ? Only once the Al-Pt and especially the Cr-Pt and Cr-Ru binary phase diagrams are confirmed more rigorously, the calculated ternary phase diagrams could be worked on with more confidence, which should make extrapolation into the quaternary not only easier, but also more valid. ? It is therefore recommended to: ? Undertake slow scanning rate DTA for samples in the Cr-Pt and Cr-Ru systems to obtain reaction temperatures. ? Undertake phase diagram studies in the Cr-Pt and Cr-Ru systems to obtain better phase equilibria data. ? 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Cornish, Proc. Microsc. Soc. South. Afr., 35 (2005) 8. [2005Pri] S. N. Prins, L.A. Cornish and P. Boucher, J. Alloys and Compounds, 403 (2005) 245-252. [2005S?s] R. S?ss and L.A. Cornish, Proc. Microsc. Soc. South. Afr., 35 (2005) 9. [2005Zha] J.-C. Zhao, X. Zeng and D. Cahill, Materials Today, October 2005, 28-37. [2006Cor] L.A. Cornish, R. S?ss, A. Watson and S.N. Prins, Building a database for the prediction of phases in a Pt-based Superalloys, Southern African Institute of Mining and Metallurgy SAIMM conference ?Platinum Surges Ahead?, Sun City, 8th ? 12th October 2006, Symposium Series S45, 91-102. [2006Pre] J. Preussner, M. Wenderoth, S. Prins, R. V?lkl and U. Glatzel, Platinum alloy development - the Pt-Al-Cr-Ni system, Southern African Institute of Mining and Metallurgy SAIMM conference ?Platinum Surges Ahead?, Symposium Series S45, 91-102, Sun City, 8th ? 12th October 2006, p. 103-4. [2006Sch] R. Schmid-Fetzer, D. Andersson, P.Y. Chevalier, L. Eleno, O. Fabrichnaya, U.R. Kattner, B. Sundman, C. Wang, A. Watson, L. Zabdyr and M. Zinkevich, Computer Coupling of Phase Diagrams and Thermochemistry, 31 (2007) 38-52. 382 [2006S?s1] R. S?ss, L.A. Cornish and M.J. Witcomb. J. Alloys and Compounds, 416(1-2) (2006) 80-92. [2006S?s2] R. S?ss and L.A. Cornish, Proc. Microsc. Soc. South. Afr., 36 (2006) 14. [2006Tsh] W. Tshawe, A. Douglas, B. Joja and L.A. Cornish, Proc. Microsc. Soc. South. Afr., 36 (2006) 15. [2006Wat] A. Watson, L.A. Cornish, and R. S?ss, Rare Metals, 25(5) (2006) 1-11. [2007Int] Internet URL: http://www.platinum.matthey.com/applications/properties.html (Accessed January 2007). [2007Ngw] T.B. Ngwenya, Extending the x-ray diffraction database for the PGM-based alloys, M.Sc. dissertation, University of the Western Cape, 2007. 383 APPENDICES APPENDIX A Publications APPENDIX B Thermodynamic databases for Pt-Al-Cr (in TDB format) 384 APPENDIX A Publications Conference abstracts (no proceedings) [2004S?s2] R. S?ss, L.A. Cornish and U. Glatzel, CALPHAD XXXIII Program and Abstracts, 34, Krakow, Poland, 30th May - 4th June 2004. [2007S?s] R. S?ss, L.A. Cornish and A. Watson, CALPHAD XXXVI Program and Abstracts, 146, The Pennsylvania State University, State College, Pennsylvania, 6th - 11th May 2007. Published conference proceedings [2003S?s3] R. S?ss, L.A. Cornish and B. Joja. Proc. 2nd International Conference of the African Materials Society, Johannesburg, 2003. p. 138-139. [2005S?s] R. S?ss and L.A. Cornish, Proc. Microsc. Soc. South. Afr., 35 (2005) 9. [2006Cor] L.A. Cornish, R. S?ss, A. Watson and S.N. Prins, Southern African Institute of Mining and Metallurgy SAIMM conference ?Platinum Surges Ahead?, Sun City, 8th ? 12th October 2006, Symposium Series S45, 91-102. [2006S?s2] R. S?ss and L.A. Cornish, Proc. Microsc. Soc. South. Afr., 36 (2006) 14. Published journal papers [2006Wat] A. Watson, L.A. Cornish, and R. S?ss, Rare Metals, 25(5) (2006) 1-11.