GENERAL CONDITION FOR AMORPHISATION OF INTERMETALLIC COMPOUNDS

Using simple thermodynamic considerations for the Gibbs free energies of the crystalline and amorphous states a general condition for the solid-state amorphisation of intermetallic compounds by irradiation or mechanical impact was derived. It was obtained that the amorphisation is possible if the maximum elastic energy, ƊHmel, stored during the (chemical) order-disorder transition is larger than ƊHatop, the enthalpy of the topological change of amorphisation. This condition can be rewritten into the form : the amorphisation is not possible if the order-disorder temperature is lower than the melting temperature, TcTm - in contrast to Johnson's condition - the possibility of the amorphisation depends on the relative magnitude of the elastic mismatch energy and the chemical energy of ordering. If their ratio is large enough the amorphisation is possible. Detailed calculations, carried out for a large number of compounds, led to a satisfactory agreement with the experimental observations

On the temperature and stress dependence of transformation strain in single crystalline Cu-Al-Ni shape memory alloys

In the austenite to martensite phase transformation of shape memory alloys the transformation strain, εtr, usually has definite dependence on the external fields while the change in entropy or volume can be considered constant. Under uniaxial loading this dependence of εtr is related to the change of the martensite variant distribution with increasing field parameters. The stress and temperature dependence of the elastic and dissipative energy contributions to the transformation can be correlated with the η dependence of εtr, where η is the volume fraction of the stress induced (single) variant martensite structure. Obviously, this dependence leads to deviation from the linear Clausius-Clapeyron relation too. In this paper we discuss and compare experimental results in single crystalline Cu17.9wt.%Al-2.6wt.%Ni and Cu-11.5wt%-Al5.0wt%Ni alloys obtained from stress-strain and strain-temperature hysteretic loops at different constant temperatures and stresses, respectively. Furthermore, it is shown that the field dependence of εtr is reflected in the field dependence of the dissipative and elastic energy terms too

On relations between the transformation temperatures, stresses, pressures and magnetic fields in shape memory alloys

December 3-5International audienc