Photothermal depth profiles of mechanically and electrolytically polished NiTi shape memory alloys (abstract)

Abstract

Machining of NiTi shape memory alloys (SMA) is difficult due to the required special tools, techniques, and the wear of cutting tools. Thus metal injection molding (MIM) of NiTi powders followed by polishing processes may be an alternative fabrication process for SMA components. Transient heat input across the surface and heat transport inside SMA components are important aspects for their functional efficiency. In this work the influence of polishing processes on the thermal depth profiles of SMA materials and the thermal bulk properties of MIM samples are analyzed with the help of photothermal IR radiometry. The effects of polishing have been studied both for polycrystalline nearly equiatomic NiTi alloy and MIM samples. Bulk samples, cut from a polycrystalline ingot of nearly equiatomic NiTi, had first been heat treated and flash cooled to reduce the concentration of nonequiatomic precipitations. In the second step, sample 1 was polished mechanically with a plane grinder, sample 2 was polished electrochemically in an electrolytic bath, and sample 3 was first polished electrolytically and then mechanically. The thermal depth profiles have been measured by frequency dependent photothermal radiometry using an intensity modulated argon-ion laser pump beam. The PTR amplitudes and phases have been calibrated with the signals recorded for glassy carbon. We will show the frequency variation of the inversely normalized amplitudes which correspond to the effusivity depth profile. The polished samples exhibit different depth profiles: that of electrolytic polishing (2) is most distinct from any other, while that after mechanical polishing (1) and that of combined electrolytic and mechanical polishing (3) are similar. The depth dependence of the effusivity can roughly be approximated by a three-layer model consisting of a surface layer of about 10 μm, a subsurface layer extending about 100 μm into the sample, followed by the bulk material at large penetration depths. In the case of mechanical polishing, the bulk and surface layer have similar effusivity values, whereas the effusivity of the intermediate layer is always increased. Supplementary measurements of the surface topology reveal surface roughening produced by the electrolytic treatment, which may explain the strongly decreasing effusivity near the surface. The higher effusivity in the intermediate layer of a depth of up to 100 μm may be related to local demixing of the alloy or stress release, produced by friction effects and heat generation during polishing. This work was performed in the frame of SFB 459.