Martensitic transformation cycling effects in Cu-Zn-Al shape memory alloys
- PDF / 574,426 Bytes
- 4 Pages / 603.28 x 788 pts Page_size
- 85 Downloads / 214 Views
I.
INTRODUCTION
CERTAIN alloys have the unusual ability to recover an original shape after undergoing what would appear to be, because of the magnitude of the strain involved, permanent deformation. These so-called "shape memory effects" are a direct result of thermoelastic martensitic transformation. In such transformations a balance is struck between the chemical free energy change which accompanies the transformation and the elastic energy built up around the martensite crystallites. The P-->M transformation can be induced by either a decrease in temperature or an increase in stress, with martensite plates forming and growing continuously as temperature decreases and/or as stress increases, and with these plates shrinking away to reform the original parent phase as temperature is increased and/or as stress is decreased. The M--->P reversion is accomplished microstructurally by the exact reverse path of the P-->M transformation. In most shape memory alloys there are several temperature or stress-related crystallographic routes to induce martensite crystallites from the parent phase, whereas for each martensite crystallite there is only one crystallographic route back to the parent phase, namely, the path along which that particular crystallite has been created. Another way to explain this is to point out that martensite crystallites must nucleate within original parent phase grains, but that the converse is not true; the parent phase reforms without nucleation within or from the martensite microstructure, but rather by shrinkage reversion of the numerous martensite crystallites to reform a given grain. Therefore, if the alloy is deformed such as to induce or revise a martensitic microstructure, it may regain its original, prestress shape when temperature and/or stress is changed such as to make the parent phase stable once again. It is implicit in the concept of thermoelastic martensitic transformation that there is no irreversible behavior involved in the PM transformations, i.e., no plastic deformation. However, there is some indirect evidence that thermoelastic behavior is limited by the yield strength of the parent phase 1 and that at least in the first PM cycles the transformation may not be perfectly thermoelastic, z Furthermore, there is direct evidence of the production of dislocation substructural debris as a result of transformation
cycling. 3 In order to clarify and explain these observations, the present work was initiated to study the effects of multiple thermal cycles on the P~--->Mtransformation kinetics of selected Cu-Zn-A1 shape memory alloys and to correlate this with the substructural condition of the microstructure as transformation cycling proceeds.
II.
EXPERIMENTAL
The alloy studied in this work was provided by Delta Metals Research Ltd., Ipswich, Suffolk, England, with a nominal composition of 69.25 wt pct Cu-26.75 wt pct Zn-4.0 wt pct AI and a reputed Ms temperature of about 270 K. This material, originally in the form of 1 cm diameter hot-worked bar, was machined into discs 3 mm in
Data Loading...
