A. P. Miodownik

The Young's moduli of in situ Ti/TiB composites obtained by rapid solidification processing

In situ Ti/TiB composites (Ti-6Al-4V matrix reinforced with TiB phase) with different volume fractions of the TiB phase, have been produced by consolidation of rapidly solidified Ti-6Al-4V alloys with different levels of boron addition. The microstructural examination of such composites shows that the reinforcing phase has a fine grain size and a uniform distribution throughout the matrix. The Youngs moduli of the in situ composites have been determined experimentally to study the strengthening effect of the TiB phase. It was found that the Youngs modulus of an in situ composite with 10 vol % TiB phase can be increased to 140 GPa, compared to 116.7 GPa for the matrix alloy. The theoretical predictions are in good agreement with the present experimental results and other results of similar composites obtained by the reactive sintering technique.

A generalized law of mixtures

The mechanical properties of two-phase composites are predicted using a rigorous continuum mechanics analysis and an equivalent microstructural transformation approach. This leads to a generalized law of mixtures which is contrasted with the classical linear law of mixtures which requires some explicit assumptions. The generalized law of mixtures enables prediction of a variety of mechanical properties of a two-phase composite with any volume fraction, grain shape and phase distribution. It is shown that the classical linear law of mixtures is a specific case of the generalized law of mixtures. Examples are given from continuous Cu-W composites, the particulate Co-WC system, Al/SiCp composites, α-β Ti-Mn alloys and α-β Cu-Zn alloys for the predictions of properties such as Youngs modulus, yield strengths, flow stresses, the overall friction stresses and the overall Hall-Petch coefficients. It is shown that the theoretical predictions by the generalized law of mixtures are in very good agreement with the corresponding experimental results drawn from the literature, for both continuous fibre composites and particulate reinforced systems.

Phase formation in co-deposited metallic alloy thin films

Phase formation in co-deposited thin films of Pd-Rh, Cu-Ag and Cu-Sn has been examined. Depending on the substrate temperature, either equilibrium or non-equilibrium phases are formed. It is postulated that the formation of multiphase structures in co-deposited alloy thin films is controlled by the diffusional breakdown of fully intermixed depositing atoms, so that three kinetic regimes are observed: (i) at low substrate temperatures the surface mobility is insufficient for the decomposition of the fully intermixed depositing atoms and the films contain non-equilibrium single-phase structures; (ii) with increasing substrate temperature decomposition to non-equilibrium two phase structures is observed; and finally (iii) with a further increase in substrate temperature the atomic mobility at the surface is sufficient to allow the full atomic rearrangements necessary for the formation of equilibrium phases. By relating the distance an atom can move on the surface during deposition to substrate temperature and deposition rate, it hau proved possible to account for the temperature ranges of the above deposition regimes. A hypothesis is put forward that relates the non-equilibrium phases formed in the first kinetic regime to free energy against composition diagrams at the temperature of the substrate, and this provides the basis for understanding the complex non-equilibrium phase formation found in the Cu-Sn system.

ω-phase formation in V[sbnd]Al and Ti[sbnd]Al[sbnd]V alloys

Abstract ω-phase formation in V-50 at.% A1 alloy and Ti[sbnd]Al[sbnd]V alloys has been studied using electron diffraction. It is shown that both electron and size factors are important in affecting the magnitude of the deviation of diffuse ω maxima from the commensurate ω maxima positions. It is suggested that the addition of the electron-deficient element Al to the later transition metal V will destabilize the high-temperature β-phase and favour ω formation. It is expected by analogy that the w transformation may occur in other later transition-metal based alloy systems such as Nb[sbnd]Al, Mo[sbnd]Al, Cr[sbnd]Al and Mn[sbnd]Al. Structural models have been proposed to interpret the heavy streaking effects in β + ω and B2 + ω structures.

Metastability of the o-phase in transition-metal aluminides: First-principles structural predictions

Abstract A systematic total-energy study has been performed on the ω-phase of transition-metal-aluminide-based alloys using first-principles electronic structure calculations. The calculated o-phase heat of formation for ω-phase against the electron per atom ratio e/a is found to show the same trends as the measured diffuse ω peak for alloys with values of e/a between 3·3 and 4·7. Moreover, we predict that the ω-phase is the most stable amongst competing metastable phases for NbAl. ω collapse studies show a strong correlation between this transformation and a mechanical instability in the related B2 alloys at low temperatures. A partial ω-type shuffle is also predicted for Ni2Al alloys with e/a values close to 7 in the B82 structure type. As a result of these calculations, we are now able to study the phase diagrams of structurally important ternary alloys such as Ti[sbnd]Al[sbnd]V of Ti[sbnd]Al[sbnd]Nb.

The effect of pre-consolidation heat treatment on TiB morphology and mechanical properties of rapidly solidified Ti-6Al-4V-XB alloys

Rapid solidification (RS) of Ti-alloys containing B has been studied for more than a decade. Now, it has been realized that addition of B to Ti-alloy decreases the density and increases the tensile strength and the specific modulus. It has been reported in all cases that consolidation of such alloys results in needle-shaped TiB precipitates with a high aspect ratio, which is attributed to the preferential growth along the b axis of the orthorhombic unit cell. Although the needle-shaped TiB precipitates in RS Ti-alloys have the potential for significant in-situ fiber reinforcement, the ductility is lower compared with conventional Ti-alloys. The objective of this work is to study the possibility of altering the TiB morphology in the rapidly solidified Ti-alloys containing B by an appropriate pre-consolidation heat treatment. This work also aims to evaluate the effect of pre-consolidation heat treatment on the mechanical properties of consolidated alloys.

Phase transformations in Ti-40Al-10V

Phase transformations in arc melted Ti-40Al-10V ingots have been studied. Solidification started with the bcc β phase. The room temperature microstructure consisted of B2,ωord (ordered ω), γ and α2 phases. The α2 phase was generally found in bands of (α2 + γ) and was richer in V and Ti than the γ phase. Orientation and interfacial relationships have been determined. The fine structures of the γ phase and different morphologies of the (α2 + γ) structure have been studied by electron microscopy. It is shown that the phase transformation sequence in the alloy is L → L + β → β → α + β → α + B2 → α2 + γ + B2 → α2 + γ + B2 + ωord.

Microstructural investigation of a rapidly solidified Ti-6Al-4V-1B-0.5Y alloy

A Ti-6Al-4V-1B-0.5Y (wt%) alloy has been prepared by consolidation of the melt-spun alloy fibres. The microstructures of the melt-spun fibre and the consolidated alloy were examined by different techniques. It was found that in the consolidated alloy, titanium boride and yttrium oxide particles have a refined particle size and a uniform distribution in the α(+β) matrix compared with the microstructure of the same alloy obtained by conventional ingot metallurgy. The boride phase in the consolidated alloy mainly has a needle-shaped morphology and has been identified by electron diffraction to be orthorhombic TiB with a B27 structure, while the yttrium oxide has a cuboidal morphology and has been identified as bcc Y2O3. Detailed TEM examination also revealed that yttrium addition has a strong influence on the TiB morphology by comparing the microstructures of Ti-6Al-4V-1B alloys with and without yttrium addition. Under similar processing conditions, the TiB phase in the consolidated alloys without yttrium addition mainly has a nearly equiaxed morphology with a finer particle size, while the TiB phase in the consolidated alloy with yttrium addition will mainly have a needle-shaped morphology. This effect of yttrium addition on the TiB morphology has been discussed in terms of heterogeneous nucleation and the reduced undercooling.

Ordering and decomposition of the β phase in melt-spun TiAl1 − x Vx alloys

Abstract Ordering and decomposition of the β phase in melt-spun and aged TiAl 1 − x V x ribbons were studied using calculated phase equilibria and electron microscopy. The phase transformation paths were dramatically changed by rapid quenching from the melt. In the melt-spun ribbons, γ nucleation in the β/B2 matrix and α → α 2 ordering were completely suppressed. The β → B2 ordering temperature T C was modelled. The results support the predicted decrease in T C with substitution of Al with V. The effective β → B2 ordering temperature was lowered by high cooling rates. The formation of the ω phase in the melt-spun ribbons and of ω and α 2 phases in the aged ribbons is also discussed.

TEM study of metastable β-phase decomposition in rapidly solidified Ti-6Al-4V alloy

A new rationale is presented for various decomposition products obtained from the metastable β-phase found in Ti-6A1-4V alloy produced by hot isostatic pressing comminuted melt-spun fibres and cooled to room temperature by furnace cooling. This alloy has an α-matrix with about 8 vol% retained β-phase, which is supersaturated with β-stabilizers to such an extent that the martensitic transformation has been suppressed. The metastable β-phase decomposes by different modes during continuous cooling, depending on the actual composition of individual β-grains. Less enrichment of vanadium and iron favours the direct formation of the equilibrium α-phase from the β-matrix, while greater enrichment of vanadium and iron leads to a spinodal decomposition of the metastable β-phase, resulting in a β+β′ two-phase structure. During further continuous cooling, the β′-phase which is lean in β-stabilizers will transform into isothermal ω-phase. In addition, an unknown phase has also been observed in the β-phase, which is typified by the appearance of 1/2{112}β reflections in the SAD patterns.