Kenki Hashimoto1 and Tokuzou Tsujimoto1
1National Research Institute for Metals, Tokyo
Precipitation behaviour in Ni-8%Ti and Ni-10%Ti alloys aged at 550∼950°C has been studied by means of transmission electron microscopy and X-ray diffraction. The results are summarized as follows:
(1) Stable η -phase plates are formed directly from the supersaturated solid solution at high temperature aging, while metastable α 2-phase cuboidal particles appear at the early stage of aging at low temperatures. The latter process is observed at temperatures less than 675°C in the Ni-8%Ti alloy and at temperatures lower than 940°C in the Ni-10%Ti alloy.
(2) The distribution of α 2-phase particles is rather regular on aging at temperatures lower than 575°C in the Ni-8%Ti alloy and at temperatures lower than 900°C in the Ni-10%Ti alloy, while the distribution is at random at higher temperatures.
(3) The lattice parameter of α 2-phase changes to smaller values as the aging temperature increased.
(4) From the above results, the upper limit line of α 2-phase precipitation, Ni-rich solvus line of α 2-phase and the upper limit line of modulated structure formation are drawn on the Ni-rich side of the Ni-Ti diagram.
(5) X-ray side-bands are observed at the stage where the regular array of α 2-phase particles is formed.
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