Optimization of the Ti-21.6Hf-23.7Nb 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 elastic modulus of the Ni-free Ti-21.6Hf-23.7Nb alloy in order to determine the key factor that promotes the desired shape memory properties and/or low elastic modulus.

 A vacuum arc melted button of each alloy was heat treated at 1100 ºC during 2 hours and quenched with a mixture of ethanol/water at 0 ºC.  Samples of the alloy were cold rolled from 5% up to 95% and finally, microstructurally and mechanically characterized. The elastic modulus for each condition as well as the reversibility percentages were evaluated with nanoindentation tests 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 elastic modulus was observed when increasing the cold work percentage. The lowest value was found in the 95% cold work condition with 48 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.

 

 Keywords: Nanoindentation, shape memory effect, .Ni-free Ti alloys,  low elastic modulus.