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This paper aims to investigate the high-speed machinability and phase transformation behavior of nickel-titanium shape memory alloys considering the process and post-process perspective. The tool wear results showed that PCD tool has great potential over carbide considering the limitation of quick tool wear in the high cutting speed range. Maximum tool life was achieved at the cutting speed of 130 m/min. The results obtained from differential scanning calorimetry indicated that the latent heats and transformation temperatures of nickel-titanium shape memory alloy are highly cutting speed dependent. Decrease of the cutting speed resulted in increased impair on these properties. It was found that the decrease of the cutting speed causes a decrease in the transformation enthalpy. The results revealed that increased transformation hysteresis is highly cutting speed related. Two hundred fifty m/min and 70 m/min cutting speed values resulted in 31.5 and 37.8 °C post-machining transformation hysteresis respectively. Compared to the unmachined sample, a minimal change in functional properties was achieved at the cutting speed of 250 m/min. Likewise, the lowest cutting speed (70 m/min) led to maximum subsurface hardening and deeper machining-induced layer according to metallurgical investigations. Residual austenite formation was evident on the subsurface of the machined specimens. [ABSTRACT FROM AUTHOR]

The Ti9Nb1, Ti8.5Nb1.5, Ti8Nb2 and Ti7.5Nb2.5 alloys were developed by powder mixing, compaction and sintering followed by heat treatment. Phase transformation on the sintered and heat-treated Ti (10–25 wt.%) Nb alloys were investigated using the X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The DSC analysis shows an endothermic peak at ~ 800 °C in Ti9Nb1 alloy. For Ti8.5Nb1.5 and Ti8Nb2 alloys, both endothermic peaks appeared with a slight variation ~ 660 °C while the Ti7.5Nb2.5 alloy occurred at 490 °C. All these endothermic peaks were reversible upon cooling and revealed exothermic peaks which confirm the α ↔ β phase transformation. These Ti–Nb alloys were all subject to heat treatment at 1100 °C (β-zone). The FCC Ti–Nb phase was detected by the XRD analysis in as-sintered Ti7.5Nb2.5 alloy. Moreover, a BCC phase coexisted with the HCP in Ti9Nb1, Ti8.5Nb1.5 and Ti8Nb2 alloys. Optical microscopy analysis revealed the equiaxed grains composed of the light β-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. The EDS and Eltra techniques confirmed the chemical compositions and O contents. Generally, the densities of the Ti–Nb alloys increased with increasing Nb content. [ABSTRACT FROM AUTHOR]