Obtaining TRIP/TWIP effects in metastable beta Ti alloys with high oxygen content - field assisted sintering technique as high throughput method

Abstract

Recently, TRIP/TWIP effects causing high ductility due to martensitic transformation (TRIP) or twinning (TWIP) during loading were found to take place in finely tuned metastable beta Ti alloys. These effects, well known from steels, are present in alloys that are in the proximity to beta -> alpha" martensitic transformation after quenching.

Typically, such alloys exhibit high ductility and very pronounced deformation strengthening, but their yield point is below 700 MPa. High levels of oxygen cause a substantial improvement of strength in beta Ti alloys, therefore, oxygen addition was studied as a way to obtain TRIP/TWIP alloy with high yield strength.

Ti-Nb-Zr-O system has been selected for its potential applicability in medicine (all the elements are biocompatible). High throughput method, consisting of layering mixtures of elemental powders (and TiO2 for oxygen addition) and their compaction and homogenization via field assisted sintering technique (FAST), was utilized for preparation of such alloys.

Concentrations of Zr and O were kept constant within each sample, while the Nb content was varied. Individual layers were studied by scanning electron microscopy and microhardness mapping.

Compression tests were performed on the layered samples with simultaneous in-situ measurement of acoustic emission and digital image correlation (DIC). The deformed microstructures were analyzed by electron back-scatter diffraction.

Stress induced martensite and {332} twinning was found in layers, containing from 26 wt% to 32 wt% Nb in sample with 0.5 wt% of oxygen and 7 wt% of Zr.