Dmytro G. Savvakin

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.

Microstructure Development and Alloying Elements Diffusion during Sintering of Near-β Titanium Alloys

Two near-β alloys, Ti-5Al-5Mo-5V-2Cr-1Fe and Ti-10V-3Fe-3Al, were produced by the blended element powder metallurgy technique. The use of (i) elemental powders with the Al-V master alloy in the case of Ti-5Al-5Mo-5V-2Cr-1Fe and, (ii) the complex Al-Fe-V master alloy in Ti-10V-3Fe-3Al has highlighted the influence of different alloying elements and their combination on microstructure evolution and chemical homogenisation. While Fe has the fastest diffusivity in Ti and its addition improves the density of both sintered alloys it also results in accelerated rates of grain coarsening. The combination of Al and V in the master alloy powder inhibits the diffusion of V into the Ti matrix. The unexpectedly slow diffusion of Cr at the early sintering stage in Ti-5Al-5Mo-5V-2Cr-1Fe was attributed to the formation of surface oxides on the Cr powders.