Abstract
New metastable β titanium alloys are receiving increasing attention due to their excellent biomechanical properties and machinability is critical to their uptake. In this study, machining chip microstructure has been investigated to gain an understanding of strain and temperature fields during cutting. For higher cutting speeds, ≥60 m/min, the chips have segmented morphologies characterised by a serrated appearance. High levels of strain in the primary shear zone promote formation of expanded shear band regions between segments which exhibit intensive refinement of the β phase down to grain sizes below 100 nm. The presence of both α and β phases across the expanded shear band suggests that temperatures during cutting are in the range of 400–600 °C. For the secondary shear zone, very large strains at the cutting interface result in heavily refined and approximately equiaxed nanocrystalline β grains with sizes around 20–50 nm, while further from the interface the β grains become highly elongated in the shear direction. An absence of the α phase in the region immediately adjacent to the cutting interface indicates recrystallization during cutting and temperatures in excess of the 720 °C β transus temperature.
Highlights
Queensland Centre for Advanced Materials Processing and Manufacturing (AMPAM), Australian Research Council (ARC) Research Hub for Advanced Manufacturing of Medical Devices, St
The solution treated and aged Ti-25Nb-3Mo-3Zr-2Sn alloy shown in Figure 1a with X-ray diffraction (XRD) phase analysis in b consists of β grains with grain sizes in the order of 50 μm and lath shaped α precipitates located primarily around the grain boundaries and protruding into the β grains
Within this cutting regime the chips have segmented morphologies characterised by a serrated appearance with bands of severe plastic deformation, referred to as expanded shear band regions, with more limited deformation in adjoining regions
Summary
Queensland Centre for Advanced Materials Processing and Manufacturing (AMPAM), ARC Research Hub for Advanced Manufacturing of Medical Devices, St. High levels of strain in the primary shear zone promote formation of expanded shear band regions between segments which exhibit intensive refinement of the β phase down to grain sizes below 100 nm. The presence of both α and β phases across the expanded shear band suggests that temperatures during cutting are in the range of 400–600 ◦ C. There has been significant focus on the development of a variety of new metastable β titanium alloys with lower Young’s moduli approaching that of human bone These alloys employ various combinations of elements to stabilise the body-centred cubic β titanium phase and can exhibit both shape memory and pseudoelastic behaviours [2,3]. A metastable Ti-Nb based β titanium alloy (Ti-25Nb-3Mo-3Zr-2Sn wt.%) with excellent mechanical and biological compatibility has recently been the subject of extensive research and development by the authors [4,5,6]
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