Abstract
Ni-Ti alloys are considered to be very important shape memory alloys with a wide application area including, e.g., biomaterials, actuators, couplings, and components in automotive, aerospace, and robotics industries. In this study, the NiTi46 (wt.%) alloy was prepared by a combination of self-propagating high-temperature synthesis, milling, and spark plasma sintering consolidation at three various temperatures. The compacted samples were subsequently heat-treated at temperatures between 400 °C and 900 °C with the following quenching in water or slow cooling in a closed furnace. The influence of the consolidation temperature and regime of heat treatment on the microstructure, mechanical properties, and temperatures of phase transformation was evaluated. The results demonstrate the brittle behaviour of the samples directly after spark plasma sintering at all temperatures by the compressive test and no transformation temperatures at differential scanning calorimetry curves. The biggest improvement of mechanical properties, which was mainly a ductility enhancement, was achieved by heat treatment at 700 °C. Slow cooling has to be recommended in order to obtain the shape memory properties.
Highlights
The Ni-Ti alloys named NiTinol are well-known shape memory alloys with good mechanical properties and high corrosion resistance, which enables usage as implants, medical devices, and other applications as biomaterials [1]
The the influence influence of of used used sintering sintering temperature temperature on on the the quality quality of of sintering sintering individual individual particles particles was investigated by a porosity measurement
The non-deformed grains were observed on the perpendicular cut whereas the elongated of after grains after cut whereas the elongated shape shape of grains loading loading during remained in the microstructure on the longitudinal cut
Summary
Ordnance Laboratory) are well-known shape memory alloys with good mechanical properties and high corrosion resistance, which enables usage as implants, medical devices, and other applications as biomaterials [1]. The shape memory effects occur due to the reversible solid-state transformation between the high-temperature and low-temperature phases. The high-temperature phase is called austenite with a high-symmetry structure-ordered body-centered cubic phase B2 (CsCl). The low-temperature and low-symmetry phase is called martensite with monoclinic B190 lattice. Reversible strains at about 8% of the initial length are enabled due to the reversible phase transformation as well [1]. To achieve the desired shape memory effects-transformation temperatures, it is necessary to be careful about a chemical composition. The transformation temperatures are very sensitive for the Materials 2019, 12, 4075; doi:10.3390/ma12244075 www.mdpi.com/journal/materials
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