The hydrogen interaction properties of mechanically alloyed mixtures of Ti, Mg, and Ni have been shown to be strongly dependent on the time and energy of ball milling. Mixtures ball milled at a ball to powder ratio (by mass) of 20:1 were able to absorb 3–4 wt% hydrogen. Mixtures ball milled at a ball to powder ratio of 70:1 were able to absorb up to 11 wt% hydrogen. The dependence of hydrogen uptake on ball milling time and energy is shown to be related to the microstructural morphology of the ball milled powders. The original Mg powder particles were heavily deformed by the ball milling process and contained nano-sized inclusions of both Ti and Ni at the surface. The higher energy ball milled samples showed a more refined microstructure and a greater hydrogen capacity. In addition, there was an inverse relationship between the amount of hydrogen uptake and the size of inclusions at the surface of the particles.

Mo-Si-B intermetallics consisting of the phases Mo3Si, Mo5SiB2, and α-Mo (Mo solid solution) can be designed to exhibit some degree of oxidation resistance, fracture toughness, and creep strength, but not necessarily all of these at the same time. For example, microstructures that enhance the oxidation resistance are typically associated with low fracture toughness. Examples will be given illustrating the oxidation resistance, fracture toughness, and creep strength of Mo-Si-B intermetallics as a function of their phase volume fractions as well as the topology and length scale of their microstructures. Microstructures containing either individual α-Mo particles or a continuous α-Mo matrix will be described. The examples provide possible ways to control the composition and microstructure of Mo-Si-B alloys such as to optimize the desired balance of properties.

Computer simulation of the core structure and glide of ordinary 1/2<110] dislocations and <101] superdislocations in L10 TiAl has been performed using the recently constructed BondOrder Potentials. This description of atomic interactions includes explicitly, within the tight-binding approximation, the most important aspects of the directional bonding, namely d-d, p-p and d-p bonds. The ordinary dislocation in the screw orientation was found to have a non-planar core and, therefore, high Peierls stress. The superdislocation was found to possess in the screw orientation either a planar (glissile) or a non-planar (sessile) core structure. However, the glissile core transforms into the sessile one for certain orientations of the applied stress. This implies a strong asymmetry of the yield stress and the break down of the Schmid law when the plastic flow is mediated by superdislocations. At the same time, this may explain the orientation dependence of the dislocation substructure observed in the single-phase γ-TiAl by electron microscopy.

The addition of transition metal elements can significantly modify physical properties of intermetalic compounds. We studied the influence of Molybdenum and Vanadium additives on thermal expansion coefficient (CTE) of Fe3Al and FeAl over the wide range of temperatures. The site preference of both transition metals was determined by full-potential LMTO method within the grandcanonical formalism. At low temperatures CTEs were found directly from the FP-LMTO calculations by incorporating them into the Debye model of a solid. The obtained thermal expansion for pure Fe3Al and FeAl is within 10% of its experimentally measured values. At high temperatures we performed Molecular Dynamics (MD) simulations based on our many-body atomistic potentials. The parameters were fitted to reproduce the total energy of a crystal under various types of deformations obtained by FP-LMTO method and were tested with respect to different structures and vacancy formation energies. Our calculations show that addition of V decreases the CTEs of both iron-aluminides, while the addition of Mo makes Fe3Al DO3 structure unstable.

Defects formation in LaNi5-based alloys was investigated by X-ray diffraction (XRD). In-situ XRD data measured along the first hydriding-dehydriding P-C isotherms were analyzed by the Rietveld method. Lattice strain and crystallite size were evaluated from the peak profile. In LaNi5 hydride phase formed accompanying with 2 % of anisotropic lattice strain in <hk0> direction caused by dense dislocations. Coexisting solid solution phase was not affected by the strain in the hydride phase. Lattice parameter a became smaller after cycles than before hydriding, which is related with vacancies formation.

Only 5 % of Al substitution for Ni dramatically changed the defects formation behavior. Hydride phase did not show lattice strain in the first hydriding but showed small strain during dehydriding. Formation of both dislocations and vacancies are strongly affected by substitution of other elements for a part of Ni.

Atomistic microstructures of long-period superstructures in Al-rich TiAl alloys were investigated by high-resolution transmission electron microscopy. The multi-slice image simulation revealed that Al-atoms in Ti-rich (002) layers of the superstructures are imaged as very bright dots under appropriate observing conditions. By enhancing the contrast of the very bright dots using an image processing technique, partially ordered long-period superstructures were clearly characterized. For the short-range order state, A4B, A2B and A3B type ordered-clusters are generated in the Ti-rich (002) layers. These ordered-clusters are in contact with each other, and form local microdomains of various superstructures, such as Al5Ti3, h-Al2Ti and so on. For the Al5Ti3 and h-Al2Ti ordered states, the ordered-clusters are tiled periodically to form the Al5Ti3 or h-Al2Ti domains and characteristic anti-phase boundary structures.

In two-phase titanium aluminide alloys, the implementation of precipitation reactions is a widely utilized concept to control the microstructure and strengthen the material. A study has been made on the influence of carbide and boride precipitates on dislocation mobility and strengthening at 300 K. Compression tests were carried out for characterizing the mechanisms determining flow stress and dislocation glide resistance. The interaction mechanisms between the precipitates and dislocations were assessed by thermodynamic glide parameters and transmission electron microscopy. It has been shown that small titanium boride precipitates and carbide precipitates of perovskite type act as long-range dislocation glide obstacles. The interaction between the dislocations and the borides and carbides mainly leads to an athermal stress contribution. However, the dislocation-particle interactions are quite different. Small groups of borides are encircled by dislocations. This gives rise to the formation of loop structures the density of which increases with strain. On the contrary, the carbide precipitates are shearable and can be overcome without Orowan looping. This different behaviour is also reflected in the work hardening characteristics. Whereas the work hardening coefficient of the boron doped material increases with increasing B-concentration, it is independent of concentration in the case of the carbon-doped material.

Texture evolutions of cold-rolled thin foils of binary stoichiometric Ni3Al single crystals were examined as a function of the initial crystal orientation. In the cases of the initial rolling direction (RD) close to <001>, the rolling texture above 90% reduction is composed of two {110}<112> textures, which resulted from the formation of the banded structure with two types of differently oriented matrix bands. The macroscopic shape of these cold-rolled foils is straight and simply elongated along RD keeping their rectangular shape. In contrast, when the initial RD is close to <112>, the texture and the microstructure are rather uniform but the foils are curved, twisted, and eventually cracked from the side edge of the samples. Based on the analysis of the texture evolution and the microstructure observation, the operative slip systems were determined.

The structure of E21 is a derivative of L12 structure with an interstitial carbon in the body center or the octahedral interstitial site. In the Fe-Al-C ternary system E21-Fe3AlC intermetallic is known to precipitate in γ-austenite alloys. For the high temperature application, the stability of microstructure is a key issue and the control of the lattice misfit is essential. By adding some elements to Fe-Al-C alloys, the lattice constant can be controlled, and enhancement of the stability of γ-austenite + E21-Fe3AlC two phase structure is expected. In this study, phase stability and lattice parameter of E21 phase in the (Fe-Mn)-(Al-M)-C system (M=Si, Ge) are investigated. In both systems, no significant changes in microstructures were observed within the composition range of concern. The lattice constant of E21-Fe3AlC phase, however, does not show decrease with the addition of Si or Ge. This result is different from what was expected. By WDS analysis, it was revealed that Si tends to distribute mainly in γ phase, and Ge concentrates in E21-Fe3AlC phase. It was found that the lattice constants of E21 phases are very close to those of L12 structure estimated from the atomic radii of constituent elements which are evaluated from those in binary alloys and compounds. The distribution behavior of Si and Ge can be related to tolerance factor which strongly affects the stability of phases.

In homogeneous phase transformations the order parameter proceeds towards equilibrium uniformly in all microvolumes of the system. However, defect-mediated diffusion (vacancy mechanism) involving local atomic jump processes during the early stages of transformation kinetics can produce discrete regions within which the order parameter has changed significantly embedded in an unperturbed matrix. This effect is evident in order-order transformations in B2 FeAl as measured by residual resistivity. An estimate of the heterogeneity regime is calculated in terms of vacancy diffusion parameters.

The effect of transition metal substitution for Mo on the phase stability and multi-phase microstructures in the Mo-Si-B ternary system has been examined. The metal-rich portion of the ternary Mo-Si-B system at equilibrium is comprised of thermally stable BCC Mo(ss) phase, a ternary-based Mo5SiB2 (T2 phase), binary-based metal-rich silicides (Mo3Si [the A15 phase] and Mo5Si3 [the T1 phase]) and borides (Mo2B and MoB phases). Systematic alloying with selected transition metals which are substitutional in both Mo(ss) and T2 phases such as Cr, V, Nb, W, Ti and Hf, has been performed to elucidate the roles of the substitution on the stability of the three phase fields of Mo(ss) + T2 + A15 and T2+ T1 + A15. The potential of the alloying effects on the microstructure design and control of the solidification pathways is further detailed.

Effects of ruthenium (Ru) substitution on constituent phases, phase transformation temperatures and mechanical properties were investigated for Ti-Ni shape memory alloys. Ti50Ni50-XRuX alloys with Ru contents (X) from 0mol% (binary TiNi) to 50mol% (binary TiRu) were systematically prepared by Ar arc-melting followed by hot-forging at temperatures from 1173K to 1673K depending on chemical composition. Phase stability was assessed by DSC (differential scanning calorimetry), XRD (X-ray diffractometry) and TEM (transmission electron microscopy). Mechanical properties were investigated using hardness and tensile tests at room temperature. With increasing Ru content, it was found that the lattice parameter of B2 phase increases, the martensitic transformation temperature slightly decreases, and the melting temperature increases monotonously. Besides, R-phase appears for Ti-Ni alloys containing 3mol% and 20mol%Ru but no diffusionless phase transformation is seen in Ti-Ni alloy containing 5mol%Ru. Vickers hardness shows the maximum at an intermediate composition (HV1030 at 30mol%Ru); this suggests that large solid solution hardening is caused by Ru substitution for the Ni-sites in TiNi.

The formation mechanism and morphology of the tetragonal compound Ni3V (D022) quenched from the disordered fcc (A1) state have been examined by TEM using a Ni-25at%V. The alloy is fully transformed to D022 in the quenched state. The resulting microstructure exhibits a fine brick-like “multi-variant structure (MVS)” consisting of one major and two minor variants with c axes perpendicular each other, having coherent interfaces of {102)D022 and {100)D022. The major variant particles with a quadrangular-prism shape are alternately arrayed with two minor variants, one parallel to {102)D022 and the other {100)D022, respectively. The thermal APDs are formed in the all variants, but the average size within the major is larger than that in the minors. The c/a ratio of the D022 in the MVS is 2.006, much smaller than the value (2.04) in equilibrium state. From all these facts, the transformation of A1→D022 is assumed to proceed through a selective nucleation and growth of tetragonal A6 phase, followed by ordering to D022. The formation of the tetragonal phase produces an anisotropic strain field due to the contraction and expansion of the a and c axes against the cubic matrix. The strain field makes {102)D022 and {100)D022 with the invariant lines become habit, and also drives the selective nucleation and growth of minor variants to minimize the elastic strain energy, thereby leading to the multi-variant structure.

Hydrogen absorption/desorption of Ti3Al-based alloys with α2 single-phase, β0 single-phase and β/O two-phase microstructures was investigated to obtain systematic understanding microstructure effects. On exposing all the alloys to hydrogen, β (βH) and γ (γH) hydrides are formed. The βH and γH have bcc and bct (close to fcc) based structures, respectively. The hydrogen/metal ratio of the γH is larger than that of βH. Thus, solute hydrogen in Ti3Al based alloys stabilizes the bcc-based phase just like Nb, regardless of their original structure before hydrogenation. On the other hand, reversible hydrogen absorption/desorption around 100°C and related reversible βH-γH transformation was observed in β/O two-phase (Ti,Nb)3Al and α2-Ti3Al alloys, but not obviously observed in a β0-(Ti,Nb)3Al alloy. In forming the βH, the expansion of the β0 and β phases occurred isotropically keeping their structures B2 and bcc, respectively. In contrast, the O and α2 phases expanded in anisotropic manner. The lattice deformation for the βH→γH transformation can be described similarly to those for the O→βH and α2→βH transformations. Such lattice deformation in anisotropic manner most likely agrees with that observed in a martensitic displacive transformation, because of the surface relief observed on the γH after the βH→γH transformation in the β0-(Ti,Nb)3Al alloy and many twins formed during the α2→βH transformation in the α2-Ti3Al alloy. Mechanisms of the reversible βH-γH transformation in the β/O two-phase (Ti,Nb)3Al and α2-Ti3Al alloys was proposed on the basis of similarities in the lattice deformation and ordering of hydrogen atoms required for the βH-γH, O-βH and α2-βH transformations.

Pseudoelasticity in Fe3Al single crystals with the D03 structure was investigated focusing on the type of thermally-induced antiphase domain boundaries (APDB) developed in the crystals. Fe3Al single crystals containing 23.0at%Al and 28.0at%Al were produced by a floating zone method. Fe-23.0at%Al single crystals with an appropriate heat treatment demonstrated perfect strain recovery during unloading after compression to plastic strains below 5.0%. In contrast, Fe-28.0at%Al single crystals exhibited little strain recovery. The ordering process and the type of thermal APDB depended strongly on the alloy composition. As a result, B2-type APDB having a displacement vector (R) of 1/4<111> was frequently observed in Fe-23.0at%Al, while D03-type APDB with R=1/2<111> was dominant in Fe-28.0at%Al. In Fe-23.0at%Al, the strong interaction between B2-type APDB and 1/4<111> superpartial dislocations led to the individual motion of the superpartials dragging deformation-induced antiphase boundary (APB). The restoring force due to the deformation-induced APB resulted in the pseudoelasticity in Fe-23.0at%Al single crystals. Moreover, the refinement of antiphase domains surrounded by B2-type APDB was found to be effective in the enhancement of shape recovery in the crystals. Thus, the thermally-induced APDB in the D03 phase played an important role in the pseudoelasticity in Fe3Al single crystals.

Al5Ti3 single-phase single crystals with the stoichiometric composition of Ti-62.5at.%Al were obtained by the floating zone method and subsequent heat treatment at 750°C for 48h. The crystals contain no L10 phase, but compose of anti-phase domains (APD) surrounded by the anti-phase boundary (APB) on the basis of the Al5Ti3 long-period superstructure.

Orientation dependence of plastic deformation behavior and operative slip system were examined in compression in a wide temperature range between RT and 750°C using the single-phase single crystals in comparison with Al-rich TiAl single crystals with the L10 matrix and Al5Ti3 precipitates. In the wide crystal orientation area, {111)<110] ordinary slip appeared independent of the tested temperature, while {111)<101] superlattice slip was operative only at around [001] and [110] axes. This is because the critical resolved shear stress (CRSS) for the ordinary slip is lower than that for the superlattice slip and that for the ordinary slip in other Al-rich TiAl crystals containing L10. This implies that existence of the L10 matrix with the Al5Ti3 phase must be closely related to strengthening for the ordinary slip, similarity in Ni-base super-alloys consisting of the Ni-based matrix and L12 precipitates. The CRSS for both slips gradually decreased or was kept constant with temperature, showing no remarkable anomalous strengthening.

In this paper, deformation mechanism in Al5Ti3 single-phase single crystals will be discussed focusing on condition of the Al5Ti3 superstructure.

In this study Ti-46Al-9Nb (at%) sheet material processed by a powder metallurgical route was examined. Subsequent to hot rolling the sheets were subjected to a stress-relief treatment at 1273K for 3 hours. During this heat treatment a fine-grained near gamma microstructure has been formed. 100 hours tensile creep tests under constant load were carried out at 700°C in rolling direction, transverse direction as well as 45° direction. Using the method of load changes a stress exponent of 4.1 was determined. Furthermore, the apparent activation energy was determined in the temperature range of 715 – 775°C. Both stress exponent and activation energy suggest that diffusion assisted dislocation climb is the dominant creep mode. A comparison of these results with those of so-called conventional or so-called “2nd generation” γ-TiAl based alloys, e.g. Ti-46.5Al-4(Cr,Nb,Ta,B) (at%) and Ti-47Al-4(Cr,Mn,Nb,Si,B) (at%), indicates a significantly better creep resistance and a higher activation energy of the high Nb containing alloy. Additionally, internal friction experiments were conducted in order to analyze the deformation behavior under very small strains at elevated temperatures.

The elastic energies associated with hydride formation have been calculated using coherent elasticity theory. The energies modify the required condition for hydride nucleation to lower temperatures or higher hydrogen gas pressures. Since the elastic energies are quite large, hydride cannot form in a crystal or on a planer surface without assistance of a lattice defect. Hydride can form with reasonable excess hydrogen gas pressure only at a corner of specimen at which the most part of elastic energy is released.

Effect of combined addition of elements on the formation of high-temperature β-Ti (bcc) phase in Ti-45Al-M1-M2 quaternary alloys has been examined by converting the quaternary system to quasi-ternary system using M1-equivalency of M2: [M1] eq = k [M2]. The k value is evaluated from the ratio of critical concentration, XcM, to form the β phase in the isothermal section of each ternary system at 1473 K: kM1/M2= XcM1/XcM2. In case that M1 and M2 are in the same group element, Mo-equivalency of Cr, kMo/Cr = 0.46 and Nb-equivalency of V, kNb/V = 0.57. The critical concentrations to form the β phase in the quaternary systems become lower than those estimated from these values, and the experimentally determined equivalencies become 0.8 and 1.0, respectively. These results suggest an existence of negative interaction between M1 and M2 to stabilize the β phase even if they were in the same group element. Experimentally determined quaternary phase diagram revealed that the combined addition makes the β+α two-phase region expand to lower temperatures, thereby effective in improving hot workability for development of wrought gamma alloys.

In order to study the role of vacancy-type defects in the unique deformati on-induced phase transition mechanisms of B2-L10 and B2-A1 of intermetallic compound Fe-Rh alloys, high-speed impact compression deformation has been carried out. Induced phases and substructures were examined by XRD and TEM. The relation between the phase transitions and vacancy-type defects was investigated by the positron annihilation Coincidence Doppler Broadening (CDB) method. The results showed that intermixed phases of L10 and A1 were always formed in the B2 phase, and the vacancy concentrations were rather decreased in the specimens deformed by higher strain rates, suggesting that vacancies were more consumed by the phase transition to A1 structure.