Investigating the influence of Ti powder purity on phase evolution during NiTi sintering using in-situ neutron diffraction |
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| Affiliation: | 1. Dalhousie University, Halifax, NS B3H 4R2, Canada;2. Canadian Nuclear Laboratories, Chalk River, ON K0J 1J0, Canada;1. R&D Scientist, Nuclear Safety Experiments Branch, Canadian Nuclear Laboratories, Chalk River, ON, Canada;2. R&D Scientist, Fuel Development Branch, Canadian Nuclear Laboratories, Chalk River, ON, Canada;3. Project Engineer, Stern Laboratories, Hamilton, ON, Canada;1. Multi-disciplinary Materials Research Center, Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi׳an Jiaotong University, Xi׳an 710049, China;2. Ferroic Physics Group, National Institute for Materials Science, Tsukuba 305-0047, Ibaraki, Japan;3. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA |
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| Abstract: | The influence of Ti powder purity on phase evolution during the reactive sintering of elemental Ni and Ti powders to form NiTi was studied using differential scanning calorimetry (DSC) and in-situ neutron diffraction. Reaction between the Ni and Ti is not significant until 600 °C. From 600 to 700 °C, Ti2Ni forms in mixtures made from high (HP) and low purity (LP) Ti powder. The Ni3Ti phase also grows in this temperature range in the LP mixture. The most significant phase evolution takes place between 700 and 920 °C. The α to β phase transformation in (Ti) begins at the eutectoid temperature (765 °C) and ends at 820 °C. The highest growth rates for all three intermetallic phases, including NiTi, and the decay rate of the elemental Ni occur in this temperature range. At approximately 1000 °C, all reactants are consumed and homogenization occurs, with NiTi continuing to grow at the expense of the other intermetallic phases. The Ti rich intermetallic phase persists above its melting point, due to the formation of a solid-solution with oxygen (i.e. Ti2Ni(O)). From 1100 to 1200 °C, the microstructure becomes a stable mixture of NiTi with a small fraction of Ti2Ni(O). The phase evolution is similar in the LP and HP mixtures. However, the rate of reaction is higher in the LP mixture due to the influence of impurities (O, Fe and Ni) on the diffusivities in the many phases involved. |
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| Keywords: | Shape-memory alloys In-situ Phase transformation Reaction synthesis Differential scanning calorimetry Diffraction/scattering |
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