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 degree C) Ti-49.85 atomic percent Ni shape memory alloys (SMAs). The stabilizations of the R-phase transformations occurred with two separate transformation peaks were observed in DSC study in prolonged aged conditions. In conventional ageing temperature ageing at 300 degree C, R-phase transformations exhibit an increase in A 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 arrow right B19' transformation temperature initially; this result is very peculiar for this ageing treatment. The energy required for the nucleation of this phase Delta HB19' arrow right R on heating cycle was 2.24 J/g, which was much lower than the reverse transformation (on cooling) Delta Hb19' arrow right 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 arrow right B2 in heating. The occurrence of the R-phase transformation is attributed to precipitation-induced inhomogeneity of the matrix, both in terms of composition and internal stress fields.