Orest M. Ivasishin

Deformation behavior of beta-titanium alloys

The cold workability of four solution-treated beta-titanium alloys was determined and related to the initial substructure. Two alloys (TIMETAL-LCB and Ti-15-3), which retained a well-developed subgrain structure after solution treatment, exhibited excellent workability, while those alloys which did not (VT22 and TC6) exhibited inferior ductility. The difference in workability was explained in terms of the variation in strain-accommodation mechanism. Other factors that exacerbated the observed low ductility of the latter two alloys were also identified. These included the occurrence of deformation-induced martensite formation and the development of multi-component textures, both of which may lead to localization of deformation at interphase or grain boundaries and subsequent failure.

3D Monte-Carlo simulation of texture-controlled grain growth

A three-dimensional (3D) Monte-Carlo (MC) routine was developed to quantify the interaction of grain growth and texture development during annealing. The program included special software to enable the input of the initial grain structure and texture and incorporated a description of the misorientation-dependence of the grain-boundary mobility. Outputs from the model quantified the evolving texture in terms of pole figures or crystallite orientation distribution functions and statistics on the grain structure such as the grain-size distribution and boundary-misorientation distribution function. The MC routine was applied to establish grain growth and texture development in materials with random or strongly textured starting conditions and isotropic or anisotropic grain-boundary mobility. Depending on the starting condition and material properties, normal grain growth or a behavior characterized by alternating cycles of fast and slow grain growth was predicted.

Synthesis of Alloy Ti ― 6Al ― 4V with Low Residual Porosity by a Powder Metallurgy Method

The possibility of producing titanium alloy Ti ― 6Al ― 4V with minimal residual porosity from mixtures of elemental powders by the method of pressing and sintering without hot deformation during or after sintering was investigated. Various powder mixtures based on titanium and titanium hydride with alloying additions of either elemental powders having different particle sizes, or master alloys, were studied. It was shown that the synthesis of Ti ― 6Al ― 4V from mixtures of titanium hydride and master alloys is optimal with respect to the attainment of high relative density. In this case the sintered material has density up to 99%, homogeneous microstructure with relatively small (100-120 μm) β-phase grains, and a low concentration of impurities, in particular oxygen, which provide a high level of mechanical properties σ(ten = 970 MPa, δ = 6%).

Microstructure, texture, and mechanical properties of electron-beam melted Ti-6Al-4V

Abstract The chemical homogeneity, microstructure, texture, and mechanical properties of Ti–6Al–4V ingots synthesized via electron-beam melting were established. Despite large aluminum losses during melting, very uniform compositions well within the specification for the alloy were obtained in both 200- and 400-mm diameter ingots. The local conditions of melting and solidification produced essentially texture-free as-cast material with a largely equiaxed beta grain structure. Following hot working via rolling at various temperatures, a wide range of microstructures and textures similar to those found in conventionally-processed Ti–6Al–4V was obtained. The resulting mechanical properties were comparable to or better than those found in Ti–6Al–4V synthesized via vacuum arc remelting.

Grain growth and texture evolution in Ti-6Al-4V during beta annealing under continuous heating conditions

Abstract The kinetics of beta grain growth during continuous heating for two texturally-different, but microstructurally-equivalent, lots of Ti–6Al–4V material were determined using a direct-resistance-heating technique. Heating rates of 0.42, 5, 10, and 50 K s−1 were utilized. After reaching the peak temperature, a special cooling procedure was utilized to avoid complete decomposition of the beta phase during cool-down and thus enable direct determination of its texture at the end of high-temperature annealing. It was found that beta grain growth is strongly affected by texture whose evolution can give rise to a behavior which is discontinuous in nature. As a result, dramatic differences in grain-growth behavior were noted in the two lots of material and were explained in terms of variations in beta texture evolution during heat treatment.

Influence on texture on beta grain growth during continuous annealing of Ti–6Al–4V

Abstract The kinetics of grain growth during continuous beta annealing of two different lots of Ti–6Al–4V bar stock with an equiaxed–alpha preform microstructure were determined using an induction-heating technique. Heating rates of 1.4, 5.6, and 18°C s −1 and peak temperatures between 1020 and 1260°C were utilized. Noticeable differences were found in the grain-growth behaviors of the two lots of material with one exhibiting much smaller grain sizes than the other for a given set of processing conditions. A semi-quantitative analysis of the results revealed that the two batches of alloy had similar grain-growth exponents and activation energies but different rate constants. These trends were explained in terms of the beta-phase textures in the as-received materials which persisted during beta annealing. The former material had a moderate strength, cube-like texture, whereas the latter lot of material exhibited a very strong and sharper fiber texture. The grain-growth results were thus explained in terms of the effect of texture on grain-boundary surface energy/mobility.

Diffusion during Powder Metallurgy Synthesis of Titanium Alloys

In the present study titanium alloys were synthesized by the blended elemental press-andsinter powder metallurgy approach using hydrogenated titanium powder. Experimental investigation and modeling of the homogenization processes during synthesis were used to analyze peculiarities of mass transfer and factors affecting diffusion. Processes of alloying elements redistribution during chemical homogenization of powder blends are shown to be strongly dependent on the chemical composition of the initial powders. Optimization of the processing parameters allows to synthesize uniform, nearly-dense material with reduced grain size, at relatively low temperatures and short time. This will provide improvement of mechanical properties simultaneously with better cost-effectiveness of the process.

The Impact of Diffusion on Synthesis of High-Strength Titanium Alloys from Elemental Powder Blends

High strength near-beta titanium alloys are being increasingly used in industry due to their excellent combination of properties. Blended elemental powder metallurgy (BEPM) allows to produce the above alloys and parts from them in a cost-effective manner. However, the alloy synthesis is complicated by a big amount (up to 18 wt.%) of alloying elements which diffusional redistribution between alloying particles and titanium matrix has a strong impact on microstructure evolution. In this paper synthesis of the high-strength alloys from the powder blends based on hydrogenated titanium was studied. It was found that hydrogen strongly affects diffusion controlled processes upon synthesis, such as chemical homogenization, densification and grain growth through its influence on phase composition and defect structure of the blends. Optimization of the processing parameters allowed to produce uniform, nearly-dense alloys with reduced grain size, which mechanical properties met the requirements of corresponding specifications.

Role of Surface Contamination in Titanium PM

The powder metallurgy (PM) approach is widely used for cost-effective production of titanium alloys and articles. In the PM approach the large specific surface of starting powders heightens the risk of excessive impurity presence and, hence, degradation of final alloy properties. The present study analyzes the opportunity to produce sintered commercially pure titanium (CP-Ti) with acceptable impurity content from powder materials. Starting titanium and titanium hydride powders were comparatively examined. The impurity elements (oxygen, chlorine, carbon) and their conditions on the powder particle surface, as well as the surface processes and gases emitted from powders upon heating, have been analyzed by means of surface science techniques. The role of hydrogen emitted from titanium hydride in material purification has been discussed. The opportunity to produce titanium materials with final admissible content of interstitials (O, C, Cl, and H) using starting titanium hydride powder has been demonstrated.

Effect of phase transformation on texture formation in Ti-base alloys

Variant selection during the β→α phase transformation in rolled Ti-64 alloy has been studied by comparing model and experimental textures. It has been shown that the β→α phase transformation during heat treatment of Ti-64 alloy can proceed without or with preferred selection of orientation relationship variants depending on the heat treatment employed. In the later case, the number of variants may be different in different texture components. Possible reasons for variant selection are discussed.