Tailoring deformation and superelastic behaviors of beta-type Ti-Nb-Mn-Sn alloys.

A group of Ti-25Nb-xMn-ySn (in wt%; x = 2, 4 and y = 1, 5) alloys were designed using the "BF-d-electron superelasticity" empirical relationship and subsequently were cast in order to investigate their microstructure, deformation and superelastic behaviors. Monolithic ß phase is found in all investigated alloys except in Ti-25Nb-2Mn-1Sn alloy which exhibits α"+ß dual-phase microstructure. During compression testing, the Ti-25Nb-2Mn-1Sn alloy fails and demonstrates sufficient plasticity of ~ 41% and ultimate compressive strength of ~ 1800 MPa, where other alloys do not fail within the load capacity of 100 kN. Among all the investigated alloys, Ti-25Nb-4Mn-1Sn alloy exhibits the highest yield strength (~ 710 MPa) while Ti-25Nb-2Mn-1Sn alloy possesses the highest hardness (~ 244 HV). In this work, yield strength is influenced by solid solution and grain boundary strengthening while hardness is affected by the amount of constituent phases in each alloy. Additionally, Ti-25Nb-4Mn-1Sn shows highest recoverable strain (2.35%) and superelastic recovery ratio (90%) during cyclic loading-unloading up to 3% strain level, with highest total energy absorption among the investigated alloys. Moreover, all the Ti-25Nb-xMn-ySn alloys display shear bands except that Ti-25Nb-2Mn-1Sn alloy displays shear bands together with some cracks on the outer surface of compressively deformed morphologies.

Strengthening mechanism and corrosion resistance of beta-type Ti-Nb-Zr-Mn alloys.

In order to achieve an effective balance between plasticity and strength, a group of Ti-26Nb-xZr-yMn (x = 4, 7, 10 wt% and y = 3, 5 wt%) alloys were designed to evaluate the effects of Mn and Zr on the microstructures, mechanical properties and strengthening effects of the TiNb system. All the investigated alloys illustrate a monolithic ß phase in their microstructure and they all possess substantial true plasticity (~160%) and true maximum strength (~ 950 MPa) without fracture during the compression tests within the load capacity of 100 kN. The contribution of solid-solution, grain-boundary and dislocation strengthening mechanisms have been evaluated using the strengthening model for ß Ti alloys for all the investigated alloys. Among the investigated alloys, Ti-26Nb-4Zr-5Mn demonstrates the highest true yield strength (654 MPa), dislocation density (2.45 × 1015 m-2) and hardness (242 HV) along with improved strain hardening ability in terms of strain hardening indices (0.42 and 0.09). Furthermore, based on the superior mechanical properties among the investigated alloys, the electrochemical performance of Ti-26Nb-4Zr-3Mn and Ti-26Nb-4Zr-5Mn have also been analyzed in this work. The electrochemical measurements show that both alloys have almost similar corrosion potential and corrosion current density in simulated body fluid, i.e., -0.45 V and 0.838 nA/cm2 for Ti-26Nb-4Zr-3Mn, -0.48 V and 0.839 nA/cm2 for Ti-26Nb-4Zr-5Mn, respectively.