Microstructural aspects of the martensite phase in an aged Ti-51at%Ni alloy containing Ti3Ni4 precipitates have been studied systematically using optical and electron microscopy and electron diffraction. A morphological difference in the martensite phase formed in the solution treated and the aged specimens was confirmed. In the former, a dominance of variants forming a small triangular grouping was observed. Plate-like martensite variants formed along certain preferential directions were found in the latter. (001)M twins were observed as internal defects in the martensite formed in aged specimens. This phenomenon was more notable in specimens including finer precipitates. In these specimens, the internal defects in stress-induced martensite were also (001)M twins. Upon increasing the precipitate size by prolonged aging, the microstructure of the martensite in aged specimens recovers to that observed in solution-treated specimens. The mechanism for the formation of (001)M twins is discussed qualitatively in view of the interaction between the precipitates and the parent phase.
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