Ni cluster formation in low temperature annealed Ni₅₀.₆Ti₄₉.₄

Ageing is an effective way to improve the shape memory and mechanical properties of Ni-rich Ni–Ti alloys due to the formation of Ni4Ti3 precipitates.1,2 It is well known that ageing leads to progressive precipitation with increasing ageing time, temperature, and Ni content.3 The effect of intermediate temperature,4–6 (670–1070 K) and low temperature, 1,7 (470–570 K) ageing on the transformation behavior has been widely investigated before. Ageing at temperatures below 470K has been considered too low to cause precipitation, as reported by Kim and Miyazaki.1 By transmission electron microscopy (TEM), they did not observe any precipitates in a Ni50:9Ti49:1 sample quenched from 1070K and aged at 373K for 3000 h, and in the sample aged at 473K tiny 2–3 nm precipitates were first observed after 100 h. However, Kompatscher et al. reported that if the sample is quenched from higher temperature, i.e. 1270 K, small precipitates of size 1 nm can be detected by means of small angle neutron scattering, even without ageing.8 Zheng et al. ascribed the suppression of the B2–B19' martensitic transformation (MT) in a sample aged below 600K to the atomic rearrangement as a precursor phenomenon for precipitation.3 Kustov et al. observed that room temperature (RT) ageing combined with differential scanning calorimetry (DSC) cycles strongly suppresses the MT for a sample quenched from 1270 K, from which they concluded the formation of nm size precipitates or their nuclei.9 Wang et al. performed a systematic investigation of a different set of samples to decouple the effect of DSC cycling and RT aging.10 They found that only RT aging has no detectable effect on the transformation, while RT aging after thermal cycling can substantially suppress the MT. In the present work, the atomic scale structural changes of the latter experiment are investigated using different TEM techniques.

Jaar van publicatie:2017

BOF-keylabel:ja

IOF-keylabel:ja

BOF-publication weight:1

CSS-citation score:1

Auteurs:International

Authors from:Higher Education

Toegankelijkheid:Closed