Low Temperature Ageing Effects in NiTi Shape Memory Alloys

This paper reports the comprehensive study of the characteristics of the R-phase transformations observed in low temperature aged (@ 150℃) Ti-49.85 atomic percent Ni shape memory alloys (SMAs). The stabilizations of the Rphase transformations occurred with two separate transformation peaks were observed in DSC study in prolonged aged conditions. In conventional ageing temperature ageing at 300℃, R-phase transformations exhibit an increase in Af temperature; but in low temperature aged NiTi shows that measured Af temperature is lower than the solutionized sample and R-phase appeared well below the original B2→B19' transformation temperature initially; this result is very peculiar for this ageing treatment. The energy required for the nucleation of this phase ΔHB19' →R on heating cycle was 2.24 J/g, which was much lower than the reverse transformation (on cooling) ΔHB19' →R=-12.6 J/g. This loss of about -8.55 J/g, is believed to be covered by the transformations between the two peaks of B19' →R and R→B2 in heating. The occurrence of the Rphase transformation is attributed to precipitation-induced inhomogeneity of the matrix, both in terms of composition and internal stress fields.

並列關鍵字

Austenitic Start (A_s) ； Martensitic Start (M_s) ； Austenitic Finish (A_f) ； Martensitic Finish (A_f) ； Endothermic Energy ； Shape Memory Effect and R-phase Transformations

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