M.J. Witcomb

Constitution and hardnesses of the Al–Ir system

Abstract The Al–Ir phase diagram was investigated using optical and scanning electron microscopy, and X-ray diffraction. The results were found to agree with published literature, with the exception of the congruent melting of Al2.7Ir and a previously unreported eutectic reaction to form Al2.7Ir and AlIr. The AlIr and Al2.7Ir phases had the widest composition ranges: 47–53 and 23–30 at.% Ir respectively. Mechanical tests, in the form of Vickers hardness tests were used to deduce the fracture toughness of the alloys. It was found that the high Al-content intermetallic compounds were very brittle, and the alloys containing AlIr were tougher. This compound was tougher on the Ir-rich side, especially when there was a small amount of the AlIr+(Ir) eutectic between the AlIr dendrites.

The effects of Ti and Cr additions on the phase equilibria and properties of (Pt)/Pt3Al alloys

Abstract Platinum–aluminium alloys with two-phase coherent γ/γ′ microstructures analogous to those of the nickel-based superalloys are being developed. These alloys are being designed as a next generation of high-temperature alloys, since the higher melting point of Pt compared to Ni, means these alloys may be capable of operating in a higher temperature regime. Ternary alloying is necessary in order to stabilize cubic (L12) ∼Pt3Al, and allow for coherency between (Pt) and ∼Pt3Al. In this investigation the effects of Ti and Cr additions on the phase equilibria of the alloys are assessed. Chromium and titanium additions were both found to stabilize the cubic (L12) form of ∼Pt3Al. Titanium partitioned preferentially to ∼Pt3Al, while chromium partitioned to (Pt). Partial isothermal sections of the systems at 1350°C were drawn. An isostructural L12 phase extending from ∼Pt3Al to γ-Pt3Ti is proposed in the Pt–Al–Ti system, and a ternary phase (Pt63:Al19:Ti18) was identified in this system

Martensite-type transformations in platinum alloys

Abstract Depending on the martensitic transformation temperatures and mechanical properties, alloys exhibiting this phenomenon can have a wide range of applications. Such transformations are known to exist in the platinum system. Platinum–iron is reported to exhibit a martensitic transformation, for example, from a disordered fcc (face-centred cubic) to a bcc (body-centred cubic), or from an ordered fcc to a bct (body-centred tetragonal) structure. The transformation temperature is usually in the range 30 to 300°C. A martensite-type transformation has also been observed in platinum–titanium, where a B2 (ordered bcc) to B19 (orthorhombic) transformation occurs, with the transformation temperature in the region of 1000–1070°C. Martensite transformations have been observed in other platinum systems, such as platinum–gallium and platinum–aluminium. This paper describes martensitic-type transformations in the platinum–titanium system.

An investigation of the Al–Re phase diagram

Abstract The Al–Re binary system was studied by analysing arc-melted samples with optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. There was good agreement with the most recent published phase diagram up to 25 at.% Re. The following phases were identified to form from solidification: (Al), Al 12 Re, Al 6 Re, Al 4 Re, Al 3 Re (of nominal composition Al 11 Re 4 ), Al 2 Re 3 , Al 2 Re and (Re). The (Al)+Al 12 Re eutectic was confirmed, and a previously unreported eutectic: L→Al 3 Re+Al 2 Re 3 was identified. The remaining reactions were peritectic, except for the formation of Al 3 Re, which was congruent. Phase limits were derived for all phases, except Al 12 Re. The solubility limit for (Re), ∼83 at.% Re, was found to correspond with the composition for the Al 5 Re 24 , and the h.c.p. structure was confirmed by X-ray diffraction.

The effect of microstructure on hardness measurements in the aluminium-rich corner of the Al–Ni–Cr system

Abstract Arc-melted Al-rich samples from the Al–Cr–Ni system were analysed using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. All of the expected binary phases were observed and identified. The binary phase with the largest extension into the ternary was Al 3 Ni 2 at about 12 at.%. Three separate ternary phases were identified from the microstructures, and were found to agree reasonably with the compositions in the literature, although the phase fields had different shapes. Vicker’s microhardness measurements have indicated that the phase hardnesses in decreasing order are most likely: Al 3 Ni 2 , φ, (αCr), ρ, Al 4 Cr, Al 7 Cr, Al 11 Cr 2 , Al 9 Cr 4 , Al 3 Ni and (Al). The φ, Al 4 Cr, Al 11 Cr 2 and Al 3 Ni phases were brittle in massive form.

Constitution of the Al-Ir-Ru system

Abstract The Al–Ir–Ru phase diagram was investigated using optical and scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. The results were self-consistent but there was little published literature available for comparison. The B2 phase was found to extend across the ternary, from AlIr to AlRu, with a minimum width of ∼2 at.% Al within the ternary. Solid solubilities for the extensions of the following binary phases were observed: 10 at.% Ru for ∼Al 2.7 Ir; 4.6 at.% Ir for ∼Al 2 Ru; 11.3 at.% Ru for ∼Al 13 Ir 4 ; and 12 at.% Ir for ∼Al 13 Ru 4 . A previously unreported ternary phase ∼Al 53 Ir 20 Ru 27 was identified and was shown to form peritectically from B2. Aluminium was found to be soluble up to ∼10 at.% in (Ru), and nearly 20 at.% in (Ir). Although the solubility for the other component is high in the Ir–Ru binary, there was a marked decrease within the ternary. Limited solubility ranges were deduced for most of the other phases. Most of the ternary invariant reactions were derived, and a schematic partial liquidus was presented. The hardness measurements of the B2 phase increased with Ir substitution. Other phases identified with high hardnesses were ∼Al 2.7 Ir, ∼Al 53 Ir 20 Ru 27 and ∼Al 2 Ru.

Solidification phases and liquidus surface of the Al-Ni-Ru system above 50 at.% aluminium

Abstract The Al–Ni–Ru system above 50 at.% aluminium was studied using scanning electron microscopy with energy dispersive X-ray spectroscopy, and X-ray diffraction analyses of as-cast material. The liquidus surface was deduced and the invariant reactions derived. The composition range of the ∼RuNi2Al14 ternary phase was ∼7–11 at.% Ru and ∼70–81 at.% Al, which is wider than hitherto reported. The liquidus surface of the ∼RuNi2Al14 phase was also larger than previously reported, lying between ∼75–95 at.% Al and ∼2–5 at.% Ru.

A metallographic study of the Al-Ni-Re phase diagram

Abstract An investigation undertaken on the Al–Ni–Re system using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction revealed that the largest liquidus surface was due to (Re). The Al 5 Re 24 composition was shown to be the limit of solubility of (Re). No exclusively new ternary phases were found, and the solubilities of the binary phases were limited. Al 3 Re and Al 6 Re had solubilities of up to ∼5 at. % Ni, AlNi up to ∼2 at. % Re (maximum), and AlRe 2 up to 2 at. % Ni. The AlNi 3 , Al 3 Ni 2 , Al 3 Ni, Al 2 Re 3 and Al 4 Re phases had negligible solubility, and Al 12 Re was not observed. Eleven ternary invariant reactions were observed or deduced.

Constitution of the AlNiRu ternary system above 50 at. % aluminium

Abstract A study of the AlNiRu system above 50 at. % aluminium was undertaken using optical microscopy, scanning electron microscopy and X-ray diffraction. The ternary extensions of the Al 3 Ru 2 , Al 2 Ru, Al 6 Ru, Al 3 Ni 2 and Al 3 Ni binary phases were found to be less than 1 at. %, and there was negligible solubility in the aluminium-rich solid solution. Only the Al 13 Ru 4 phase exhibited reasonable extension into the ternary system: a maximum of approximately 10 at. % Ni. A previously unreported ternary phase was found with the composition ∼Al 14 Ni 2 Ru, which formed peritectically. A schematic liquidus surface was derived, with eight ternary invariant reactions.

An investigation of the Pt-Al-Ru diagram to facilitate alloy development

The platinum-rich region of the Pt-Al-Ru system was investigated with a view to stabilizing the displacive (martensite-type) transformation in Pt3Al. In the alloys with low ruthenium contents, ruthenium was found to be partitioned almost exclusively to the fcc Pt phase with extremely limited solubility in Pt3Al. The low-temperature D0′C form of Pt3Al was found to co-exist with the Pt phase. At high Ru additions (greater than 20 at.%), a two-phase mixture of an hcp Ru solid solution and the high-temperature L12 form of Pt3Al was observed. No ternary phases were observed in any of the alloys studied.