Akin, Erhan (2010-08). Effect of Aging Heat Treatments on Ni52Ti48 Shape Memory Alloy. Master's Thesis.
Ni-rich NiTi shape memory alloys (SMAs) are capable of attaining a wide range
of transformation temperatures depending on the heat treatment conditions and superior
thermo-mechanical cycling stability, which are desired for repeated solid-state actuation.
High Ni-content Ni-rich SMAs have very low transformation temperatures in a
solutionized condition due to the high Ni-content of the matrix. Slow cooling (furnacecooling)
from solutionizing temperature and additional aging heat treatments result in the
formation of Ni-rich precipitates such as Ni4Ti3, Ni3Ti2 and Ni3Ti and increase
transformation temperatures above ambient by depleting excess Ni from the matrix.
However, the precipitates do not undergo a martensitic phase transformation and they
decrease the transformation strain by reducing the volume fraction of the material capable
of transforming. Meanwhile, recent preliminary work shows that Ni3Ti precipitates
dominate fatigue failure.
The objectives of the present study are: (1) to eliminate Ni3Ti but still have Ni4Ti3
precipitates, which are responsible for the dimensional stability and increase
transformation temperatures, (2) to investigate the effect of heat treatments on the
transformation strain, and (3) to select single variant Ni4Ti3 precipitates through
constrained aging for the formation of oriented internal stress and eventually obtain twoway
shame memory effect (TWSME) and enhanced dimensional stability. Based on these
objectives, the effect of aging heat treatment on transformation temperatures,
microstructural evolution, and shape memory behavior were investigated for a Ni52Ti48
shape memory alloy (SMA) by using differential scanning calorimetry (DSC), optical
microscopy, scanning electron microscopy (SEM), and thermo-mechanical testing,
including isobaric heating-cooling experiments under various stress levels.
It was observed that solutionizing at 900 degree C for 24 hours eliminated Ni3Ti type
precipitates, but additional aging heat treatments are needed to form Ni4Ti3 precipitates to
increase transformation temperatures. Furnace-cooling and additional aging heat
treatment results in the multi-stage martensitic transformation due to chemical and stress
inhomogeneities in the microstructure. Aging of the controlled furnace-cooled material at
400 degree C for 48 hours resulted in the highest transformation temperatures among all
processing conditions investigated due to the combination of Ni3Ti precipitates and 27 percent
volume fraction of the Ni4Ti3 precipitates, which led to the depletion of Ni from the
transforming matrix. However, since overaging results in losing coherency of the
precipitates, dimensional stability during isobaric thermal cycling was negatively