Neutron diffraction experiments on isothermal pseudoelastic phase transformations in Cu-13.1Al-
4.0Ni (wt.%) single crystal shape memory alloys require a "stop-start" approach at different levels of
strain during the austenite (A) to martensite (M) forward transformation and the M to A reverse
transformation to collect diffraction data [1]. This stop-start nature of the tests has uncovered creep-like
phenomena where there is stress relaxation when the forward transformation is interrupted (at
constant strain) and stress recovery when the reverse transformation is interrupted (at constant
strain). This material response has been confirmed by independent tests on a table-top
thermomechanical tensile test machine. We also report results on strain recovery when the forward
transformation is interrupted at constant stress. This type of behavior has been previously reported
for Nickel-Titanium(NiTi) tubes [2] and NiTi polycrystalline wires [3]. Certain notable differences
between the creep-like behavior and classical creep response of metallic alloys will be highlighted;
due to these differences, we refer to the results reported here as "pseudo-creep".
Copper-Aluminum-Nickel (Cu-Al-Ni) single crystal shape memory alloy (SMA) wires show great potential in actuator applications due to their high stress-free transformation temperatures and superior mechanical stability compared to common Nickel-Titanium SMAs. In this paper, Cu-13.3%Al-4%Ni (wt %) single crystal wires with stress-free transformation temperatures in the range of 80° C to 120° C were subjected to stress cycling tests at ambient temperatures up to 100° C at low deformation rates. Stress/strain curves up to 9% and 3% strain in the range of the transformation temperatures point to the possibility of phase transformation by detwinning. However, the residual overall strain after unloading decreased significantly at 60° C for both 3% and 9% strains. Accumulation of plastic deformation was observed for subsequent cycles.
The experimental results of a preliminary study on stress-strain behavior of Cu-13.3% Al- 4% Ni (by wt.) single crystal shape memory alloy grown along the [001] direction at high temperatures are given. An Instron testing machine with a high temperature environmental chamber has been used to study the quasi-static stress-strain response of 1.5 mm diameter Cu-Al-Ni single crystal wires at different ambient temperatures in the range 100 - 160°C. Local strain measurements using a highly sensitive extensometer are compared with overall strain measurements computed from the net displacements between grips. The effect of stress cycles on overall strain on full loading, after unloading and after heating in between stress cycles has been discussed.
This paper reports experimental results of an airfoil-based flap actuator that is actuated using high temperature Nickel-Titanium (NiTi) polycrystal and Copper-Aluminium-Nickel (CuAlNi) single crystal wires with a nominal diameter of 1.5 mm. The stress-free transformation temperatures of the commercially available NiTi wires are Mf = 53°C, Ms = 70°C , As = 95°C , Af = 110°C whereas those for the CuAlNi wires are Mf = 80°C ,Ms = 100.5°C, As = 104.5°C , Af = 117°C. Due to a significantly low electrical resistivity of the CuAlNi, the commonly used joule heating approach for thermal actuation is shelved for a heating coil approach. Uniaxial stress measurements, trailing edge flap deflections and temperature measurements are recorded during a typical heating and cooling cycle using a load cell in line with the SMA wire, a LVDT at the trailing edge tip and a thermocouple on the wire (outside the heating coil). It is seen that actuation by the CuAlNi (with a prestrain = 5.5%) leads to about a 50% higher tip deflection and about a 67% lower cooling time after actuation as compared to the corresponding values for NiTi (with a prestrain = 5.6%). The larger tip deflection is attributed to a higher strain recovery for the CuAlNi as compared to the NiTi during phase transformation whereas the lower actuation time is attributed, in part, to the narrow hysteresis in the stress-free transformation temperatures of the CuAlNi (~ 37°C) as compared to the NiTi (~ 57°C).
Electrical resistivity and Strain recovery measurements have been used for the study of the stability of R-phase in NiTi shape memory alloy upon thermal cycling under a constant tensile stress of 100 MPa. Two samples are chosen for the study of which one sample heat-treated at 560°C exhibits a pure martensitic phase at ambient temperature and the other heat-treated at 380°C consists of a mixture of R-phase and martensitic phase at ambient temperature. In the case of the sample heat-treated at 380°C, under the applied stress of 100 MPa, heating part of the cycle also shows the presence of R-phase unlike the case of stress free condition where only cooling part exhibits the R-phase. For the sample heat-treated at 560°C, the thermal cycling under tensile stress of 100 MPa initiates R-phase in the first cycle itself whereas under stress free condition it requires about 15 cycles to initiate R-phase. The sample heat-treated at 560°C exhibits more recoverable strain in the initial cycles than that of the sample heat-treated at 380°C, but after large number of thermal cycles of the order of 1000 the recoverable strain in both samples is found to be almost the same. The sample heat-treated at 380°C is found to be more stable against plastic deformation with thermal cycling and hence can be preferred over the sample heat-treated at 560°C for the two-way applications of SMA.
R<-->A phase transformation in NiTi alloy is associated with a very small recoverable strain of the order of 0.5% compared to that of 6-8% recoverable strain of M<-->A transformation. The stability of R<-->A transformation against large number of thermal cycles is an attractive feature for an application of SMA. A spring of NiTi is designed and fabricated, using 40% prior cold worked nitinol wire of dia 1mm, with appropriate heat-treatment such that martensitic transformation takes place through two stage A-->R-->M. The transformation behaviors of this wire are studied using DSC and electrical resistivity measurements. Further, the spring has been subjected to thermo-mechanical treatment for getting a two-way memory, viz:, while heating the spring gets contracted and while cooling it gets elongated. This spring has been used as an thermal actuator and temperature variation is confined between 50-65 degree(s)C to utilize only R<-->A transformation. Linear stroke of 4cm by the spring is used to rotate a platform carrying a load of 2 kg. The efficiency and reliability of the spring is tested over a million thermal cycles.
Transformation characteristics of near equiatomic, prior cold worked Nitinol have been studied through thermomechanical analysis and electrical resistivity measurements, using TMA-50 and a four-probe setup, respectively. The dilatometric and electrical resistivity curves are obtained for the samples heat-treated between 300 and 600 degrees C. Examination of the dilation curves show that, in the martensitic phase there is positive thermal expansion where as, during M yields A transformation there is also uniaxial contraction till Af. In the austenitic phase there is positive thermal expansion and these thermal expansion values agree with the published values for respective phases. While cooling, at Ms uniaxial expansion starts and this continues till Mf is reached. In the present work the R-phase and associated hysteresis has also been investigated. On cooling from A-phase, uniaxial expansion is found to start from Rs and it stops at Rf. The transformation temperatures determined in this method agree very closely with those values obtained using electrical resistivity probe. Hysteresis area is found to be smaller in the A $ARLR M transformation. The area under hysteresis loop associated with R-phase is found to be a constant against thermal cycling. Certain applications like clamps and splints require a large hysteresis loop, while some solid sate actuators require relatively smaller hysteresis loops. An attempt is made to explain R-phase transformation in terms of thermo-mechanical data.
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