Solid-phase equilibria in the Ti-rich part of the Ti---Al system

The solid-state-phase equilibria in the Ti-rich portion of the Ti---Al system (0–50%Al) have been investigated at temperatures between 800°C and 1415°C by electron probe microanalysis (EPMA) and by transmission electron microscopy (TEM) on diffusion couples as well as on annealed alloys. The TEM observations on the diffusion couples show that the phase field ₂, (ordered hexagonal DO₁₉) extends up to the β (bcc A2) phase field, leading to two peritectoid transformations, β + → ₂ at T = 1210 ± 10°C and β + ₂→ at T = 1160 ± 10°C (: hexagonal A3). The diffusion-couple experiments yield the tie lines of the observed phase equilibria. The present results confirm the existence of a eutectoid transition → ₂ + λ (λ: ordered fcc Ll₀) at about T = 1120 ± 10°C. An updated phase diagram based on these data is presented.     

Alloy design of gamma titanium aluminides based on phase diagrams

Phase stability in the Ti---Al---X (X = Cr, Mo and W) system has been investigated at 1473 and 1573 K by the following methods: microstructure observation of quenched specimens, diffusion coupling experiments, thermal analysis of DTA and in situ observation of X-ray diffraction. The ternary phase diagrams of Ti---Al---X system are proposed; additions of Cr, Mo and W stabilize the β phase in the ternary phase diagrams. The tendency of ‘β stabilizer’ is in the following order: 6 at% Cr = 3 at% Mo = 1 at% W 10 vol% β phase. In these Ti-Al-X ternary systems, the + β + γ three-phase coexisting region is close to the Ti---Al binary line and sifts slightly toward a higher concentration of Al as temperature increases. Based on the ternary phase diagrams, (γ + β) TiAl having a super-plastic capability and partial lamellar (P-L) microstructure which shows relatively well balanced mechanical properties from room temperature to elevated temperatures have been rationalized.     

Eutectoid transformations accompanied by ordering

Eutectoid transformations accompanied by ordering, unlike ordinary ones, proceed through non-pearlitic modes of transformations. Eutectoid invariants are classified into two categories in binary systems. The eutectoid invariant of A3()→D0₁₉(₂) + L1₀(γ) in the Ti-Al binary system belongs to the first category, in which one product phase has an ordered structure of a parent phase. Its transformation product exhibits a γ/₂ lamellar structure consisting of nearly perfectly aligned alternate lamellae of γ and ₂, which is formed by precipitation of γ plates in either or ₂ matrix with the Blackburn orientation relationship. The eutectoid invariant of A1(γ)→D0₂₂(γ″) + L1₂(γ′) in the Ni₃V-Ni₃Al pseudo-binary system is an example of the second category, in which both product phases have different ordered structures of a parent phase. The transformation of a 75Ni-18V-7Al alloy results in a ‘checkerboard’ pattern consisting of a periodic array of columns of γ′ and two γ″ orientation variants, which are formed by phase separation simultaneous with ordering.     

Influence of deformation on precipitation process in Mg–15 wt.%Gd alloy

Mg–Gd is a promising light hardenable alloy with a high creep resistance at elevated temperatures. The supersaturated solid solution of Gd in Mg decomposes in a sequence of the following phases: β″ (D0₁₉) → β′ (c-base centered orthorhombic-c-bco) → β (fcc) stable. Formation of the metastable β′ phase causes a strong hardening. Dislocations facilitate nucleation of precipitates. Dislocation density is, therefore, an important parameter which influences the precipitation process. This effect was examined in the present work by comparison the decomposition sequences in Mg–15 wt.%Gd alloy cold rolled to various thickness reductions. It was found that precipitation of the β′ phase starts at lower temperatures in the cold rolled specimens.     

Structural phase transformations of rare-earth modified transition alumina to corundum

The effect of rare-earth dopant on transformations of the γ→θ→ phases in fine alumina powders under vacuum was investigated by in situ neutron diffraction from 500 to 1300 °C. La-doped (1 mol%) Al₂O₃ powders (surface area 170 m²/g) were prepared by an impregnation technique. Below 800 °C both samples contain a dominant γ-phase. Above 1000 °C, transformation of the γ-phase to the intermediate θ-phase, and then completely to the -phase (corundum) was observed. Addition of 1 mol% of La in Al₂O₃ effectively shifts the -phase formation temperature from 1125 °C for pure alumina to 1250 °C, probably due to the larger size of La compared to Al ions, which hinders ionic diffusion in the processes of sintering and transformation. Consequently, doping La in alumina improves the surface-area and thermal stability at high temperatures, which is important for catalytic applications.     

Structural chemistry and phase relations in intermetallic systems Ti–{Pd,Pt}–Al

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing arc melting, levitation melting under argon or by powder reaction sintering in closed crucibles. Three ternary compounds were observed at 950°C in the Ti–Pd–Al system: τ₃-(Ti,Pd)(Ti,Pd,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁ with a filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pd,Al)(Ti,Pd,Al)₃ with AuCu₃-type. Due to the wide extension of the Laves phase field, there is no compatibility among γTiAl and τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁. The Ti–Pt–Al system at 950°C contains three ternary compounds: τ₃-(Ti,Al)(Ti,Pt,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁ with the filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pt,Al) with Cu-type. Compatibility exists for Al-rich γTiAl and τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁. The typical feature for both alloy systems studied is the three-phase equilibrium: ₂Ti₃Al+γTiAl+τ₃-(Ti,Pd/Al)(Ti,Pd/Pt,Al)₂. The solid solubility of palladium and platinum in the binary titanium aluminides, as observed from EMPA and X-ray data, is rather small and at 950°C accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new oxide compounds Ti₃PdAl₂O_x and Ti₃PtAl₂O_x with a filled Ti₂Ni-type are observed in both quaternary systems.     

Synthesis kinetics of Mg2Sn in Mg-Sn powder mixture using non-isothermal differential scanning calorimetry

The non-isothermal heating process of Mg-Sn powder mixture was studied by differential scanning calorimetry(DSC) technique and the synthesis kinetics of Mg₂Sn was evaluated by the model-free and model-fitting methods. The activation energy and conversion function of Mg₂Sn synthesis reaction are calculated to be 281.7 kJ/mol and g(α)=[−ln(1−α)]^1/4, respectively. The reaction mechanism of 2Mg+Sn→Mg₂Sn under non-isothermal condition is regarded as “nucleation and growth”. During the non-isothermal heating process, the phase transformation occurred in the Mg-Sn powder mixture was analyzed by XRD and the microstructure evolution of Mg₂Sn was observed by optical microscopy, which is in good agreement with the reaction mechanism of 2Mg+Sn→Mg₂Sn deduced from the kinetic evaluation.     

The ternary system Al–Ni–Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation

Phase relations in the ternary system Al–Ni–Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1x_Al0.7. The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam melting of proper elemental powder blends. The existence of four ternary compounds, τ₁ to τ₄, is confirmed, however, in contrast to earlier investigations at significantly different compositions and with different shape of the homogeneity regions. This is particularly true for the phase regions of τ₃-Al₃NiTi₂ with the MgZn₂-type structure ranging from Al₃₀Ni₂₈Ti₄₂ (composition lowest in Al) to Al₅₀Ni₁₆Ti₃₄ (composition richest in Al) and for τ₂-Al₂NiTi. The complex atom site substitution mechanism in τ₃ changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diffraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{Al_xTi_1-x}₂+τ₃, we observed two closely adjoining three-phase equilibria: ₂-AlTi₃+Ni{Al_xTi_1-x}₂+ τ₄-AlNi₂Ti and ₂-AlTi₃+τ₃-Al₂NiTi₂+τ₄-AlNi₂Ti. The earlier reported “homogeneous phase at Al₂₃Ni₂₆Ti₅₁′” was shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic. The experimental results are in fine agreement with the thermodynamic calculation.     

Strengthening of iron aluminide alloys by atomic ordering and Laves phase precipitation for high-temperature applications

The ternary system Fe–Al–Ta allows the formation of the hard and brittle ternary Laves phase Ta(Fe_0.5+x,Al_0.5−x)₂ with hexagonal C14 structure. The present study concentrates on Fe–Al–Ta alloys with small Ta contents between 2 and 6 at.% and various Al contents between 0 and 45 at.%. The phase equilibria in the ternary Fe–Al–Ta system at 1000 °C are studied experimentally for determination of the solubility limits of Ta in iron aluminide matrices and types of phases and structures which may occur at high temperatures. It is observed that small amounts of Laves phase together with atomic ordering increase the yield stress and affect ductility in a complex way.     

Structural anelasticity of NiTi during two-stage martensitic transformation

The two-staged thermoelastic martensitic transformation (TMT) B2→R→B19′ in polycrystalline equiatomic NiTi has been studied by means of measurements of strain amplitude-independent and amplitude-dependent internal friction (ADIF), Young’s modulus and amplitude-dependent modulus defects. The internal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent on the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelasticity. Based on the data obtained, the anelasticity is associated with the dislocations inside the martensitic variants, not with the interfaces or interface dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in the B19′ martensitic phase. This observation is explained by a much higher density of the dislocations inside the variants of the R phase as compared with that of the B19′ phase.