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Scientific report 1992-1994

1.3 Shape memory alloys

staff: J. Van Humbeeck, L. Delaey, L. Froyen, P. Wollants
researchers: R. Stalmans, I. Hurtado, Woo Yang-Jang, P. Ratchev, Gu Qing, Chen Qing Fu, R. Petrov, S. Kustov
students: G. Davignon, D. Smits, W. Michielsen

As a consequence of a martensitic transformation several alloy systems can show the shape memory effect, superelasticity and a high damping capacity. The role of processing and thermomechanical treatments on the martensitic transformations in the most industrialised systems, such as Cu-Zn-Al, Cu-Al-Ni and NiTi alloys, is a first research topic in this domain. Another one is the thermodynamic modelling of the martensitic transformation in relation to the one- and two-way memory effect. Finally in the search for shape memory alloys with high transformation temperatures (> 100 deg.C), three alloy systems were investigated: Cu-Al-Ni, Fe-Mn-Si-Cr-Mn and Cu-Zr.

The mechanical properties of Cu-Al-Ni alloys, alloyed with Ti and Mn in order to improve their machinability and grain refinement, were investigated as function of the hot rolling parameters (reduction and temperature in the -phase). The larger the reduction ratio and the lower the deformation temperature, the better are the mechanical properties at room temperature. This finding could be related to a small and homogeneous grain size. The presence of Ti and Mn leads to the formation of precipitates from nm to m size. The distribution and concentration of the precipitates depend on the thermomechanical history of the material. They control not only the mechanical properties but also the transformation characteristics and especially the damping capacity and the E-modulus of both the martensite and the -phase. The latter, however, not necessarily to the same extent. This creates possibilities of large modulus changes by martensitic transformations, which creates opportunities for their use in smart materials. The origin of this phenomenon is related to the interaction of the dislocations in the -phase and interface dislocations in the martensite phase with the different types of precipitates [IJ93 RAT], [IJ93 RAT2], [IJ94 DEL], [PR93 HUR], [Ph.D.-12], [MT93-5], [MT94-12].

In the case of Fe-Mn-Si-Cr-Ni alloys (As 150 deg.C) the conditions for a maximal one-way shape memory effect could be established by a training cycle in which the amount of prestrain, the cycling temperature and the holding time at the maximum temperature are the most important parameters. It was also shown that the recovery stress remains high (order of 200 MPa), even after cooling to room temperature, which opens interesting opportunities for the use of these alloys as coupling devices [IJ94 GU], [PR93 FED], [PR94 GU], [PR94 GU2], [PR94 HUM2], [Ph.D.-11].

The Cu50Zr50 system has been investigated, especially for its very high transformation temperatures. This alloy system has a very wide transformation hysteresis, As 350 deg.C and Ms 100 deg.C, but shows a memory effect, though small, since the material is very brittle. Thermal cycling, however, creates significant shifts of the transformation temperatures: As increases while Ms is depressed. Once the forward martensite transformation is depressed to below -10 deg.C, an R-phase type like transformation with very small hysteresis (5 deg.C) occurs. In order to improve the mechanical properties and the cycling stability Zr and/or Cu were replaced by a few percent of Ti or Ni. Some of the results are promising [IJ94 KOV].

An attempt was made to produce NiTi alloys by mechanical alloying of pure Ni and Ti. It appeared that the relative sizes of both powders are very important to allow the necessary diffusion to obtain the NiTi phase. So far no pure NiTi-B2 phase could be obtained but interesting results were obtained concerning the production of elements with different porosities, depending on the thermal treatments and the previous mechanical alloying conditions [MT93-4].

Thermally- and stress-induced thermoelastic martensitic transformations have been analysed in the reference frame of equilibrium thermodynamics. It has been explained that due to frictional work and the storage of elastic energy these transformations, from a thermodynamic point of view are irreversible. In the light of these considerations some actual points of discussion have been analysed [IJ93 WOL].

Another thermodynamic study was based on experimental results on polycrystalline Cu-Zn-Al alloys. A new thermodynamic model has been formulated that predicts a work output during the cooling of a well trained two-way shape memory element, and allows a quantification of the two-way memory strain and the recovery stresses during constrained heating. It was experimentally proven and quantified. Based on this model the two-way memory effect and recovery stresses are now investigated in single crystals of Cu-Al-Ni alloys. It appears that it is very difficult to induce the two-way memory effect. First results have shown that martensite stabilisation due to the stresses used by training occurs rather dramatically. It can only be avoided by systematic overheating of the sample [PR93 CHA], [PR93 HUM], [PR93 STA], [PR94 HUM], [Ph.D.-1].

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Copyright ©1995 Dept. MTM, K.U.Leuven . All rights reserved.
Authored by: Hans C. Arents
First created: Mar  6, 1995
Last modified: May 24, 1995