Correlations between processing technology, microstructure and properties of a Ni-rich Ti–Ni shape memory alloy

Abstract

A Ti-50.8Ni (at.%)/Ti-55.9Ni (wt.%) VIM processed ingot was rotary swaged and rolled in parallel, obtaining bars with a total Area Reduction above 90% (from forged/rolled raw state one). Throughout the successive stages, the microstructural evolution, thermal and mechanical properties were compared. Deformation bands, grain morphology, precipitates and oxidation were evaluated (by OM and SEM/EDS). An essentially (∼100%) austenitic phase was detected at room temperature (via XRD/Rietveld method), while the Martensitic Transformation temperatures occurred at negative temperatures (via DSC). The rolled bar got through the process more regularly, with homogeneous and refined structure. Typical defects such as superficial microcracks and creases were relevant in the last stages of the two-dies rotary swaging with inductive heating. The work hardening level (via Hardness Test) was similar in both metal forming processes, being 5–7% more pronounced at the edge area of the bars due to redundant work.

Keywords

• Ni–Ti Forming
• Microstructure and properties
• Martensitic transformation