Optimization of the Ti-16.2Hf-24.8Nb-1Zr Alloy by Cold Working

The objective of the present work is to characterize the influence of cold work on the thermoelastic martensitic transformation and on the apparent elastic modulus of the Ni-free Ti-21.6Hf-23.7Nb-1Zr alloy in order to determine the key factor that promotes the desired shape memory properties and/or low apparent elastic modulus. A vacuum arc melted button of each alloy was heat treated at 1100 °C during 1.5 h and quenched with a mixture of ethanol/water at 0 °C. Samples of the alloy were cold rolled from 5% up to 95% and, finally, microstructurally and mechanically characterized. The apparent elastic modulus for each condition as well as the reversibility percentages were evaluated by instrumented nanoindentation using a Berkovich tip and a spherical tip, respectively. A higher proportion of martensite was found in the low cold work percentages compared to the untreated material as it was observed by optical and TEM microscopy. A decrease in the apparent elastic modulus was observed when increasing the cold work percentage. The lowest value was found in the 99% cold work condition with 44 GPa, value closer to that of cortical bone. Cyclic nanoindentation tests show an increase in the reversibility percentages in the cold worked condition compared to the untreated material.