Synthesis and behavior of nanostructured TiNbTaZr alloy via mechanical alloying and field assisted sintering technique

In this study, we focus on the synthesis and microstructural evolution of a metastable beta-titanium with a predetermined composition Ti-36Nb-2Ta-3Zr by severe plastic deformation (SPD). Literature has revealed the potential of Ti-Nb-Ta-Zr (TNTZ) alloy as a biomedical joint replacement material due to extraordinary mechanical behavior, particularly the markedly low elastic modulus which may bridge the gap to the modulus of human bone. However, little effort has been made in terms of the development of TNTZ with an ultra-fine grained (UFG) structure. Starting with commercially pure elemental powder blend, the TNTZ alloy is processed in the order of cryomilling, field-assisted sintering technique (FAST), and high-pressure torsion (HPT) to achieve a bulk UFG structure with excellent homogeneity of phase distribution. Grain refinement and increase of lattice defect density during cryomilling promote atomistic interdiffusion between different particles and provides extra diffusion paths in the subsequent processing. The high energy sintering and electrical current effect during FAST activate solid state reactions of the HCP Ti phase and BCC Nb/Ti phase while maintaining the UFG microstructure due to the fast heating rate. Mechanical alloying is further improved during HPT as the microstructure of the TNTZ disk sample shows significantly homogeneous distributions of elements and phases. Introduced and stored strain energies are calculated for both cryomilling and HPT processes, and the results indicate comparable values for both methods. Thermal analyses are performed based on binary couple models in support of the observed results from electron imaging of the TNTZ alloy with a UFG structure.