Materials Science and Engineering A273 – 275 (1999) 366 – 369 The effects of stress on the martensitic transformation in Ni – Al T.R. Finlayson a, *, G.L. Kelly a , T.F. Smith b a Department of Physics, Monash Uniersity, Clayton, Vic. 3168, Australia b Vice -Chancellor’s Department, La Trobe Uniersity, Melbourne, Vic. 3083, Australia Abstract Previous research on a Ni – 37.5at%Al crystal has shown that there are significant differences between the thermal expansion behaviour measured along the three 100 directions. It was believed that an internal, defect strain field, as a result of the crystal production, served to determine its morphology on cooling it below the martensite start temperature (M s ). The current research has focused on the effects of an applied biaxial stress on the thermal strain measured along one of these directions. Measurements taken using capacitance dilatometry show that the applied stress has a dramatic effect on the thermal expansion of the crystal, significantly increasing both the thermal strain detected in the martensite phase and the pretransformation strain. The application of biaxial stress was found to modify the martensite morphology significantly. Extensive cracking was observed on the crystal surface as it was cooled through the martensitic transformation. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Martensite morphology; Pretransformation; Ni – Al; Dilatometry www.elsevier.com/locate/msea 1. Introduction It has been shown that the application of uniaxial compressive [1] and tensile [2] stresses can modify the morphology of the martensitic product in Ni – Al alloys. It has also been shown that the martensitic transforma- tion first occurs on a Ni–Al crystal surface with the bulk transformation requiring a larger driving force [3]. The present study investigates the effect of biaxial compressive stress on the martensite morphology. 2. Method and materials Thermal expansion measurements and microscopic observations have been carried out on a Ni-37.5at%Al single crystal with {100} faces. The crystal was exam- ined in both unstressed and biaxially stressed states. The biaxial stress was supplied by a small, aluminium clamp which was custom machined for the crystal. The applied stress was adjusted by tightening screws holding the clamp together, resulting in a compressive stress of 0.11 0.03 MPa applied in two 100 directions. Ther- mal expansion was measured in the third 100 direc- tion. A capacitance dilatometer, described elsewhere [1], was used to obtain precise thermal expansion data, as the crystal passed through the martensitic transformation. An optical microscope equipped with a temperature- controlled stage, and a field-emission, scanning electron microscope (FE-SEM) capable of cryogenic temperatures 1 , were used to observe the transformation. Unfortunately, the clamp arrangement prevented good contact between the sample and the cooling stage. As a result, the temperature recorded on the micrographs of the clamped sample, which arises from a thermocouple in contact with the base of the sample stage and not the sample itself, is very likely to record a lower tempera- ture than that of the sample. As it was important that the comparative micrographs be taken when the ob- served surface was at the same temperature for both the clamped and the unclamped states, it was decided that the comparative micrographs should be taken when the transformation front in each case had reached the same feature on the crystal surface. The proposed attachment of a thermocouple to the surface of the crystal for * Corresponding author. Tel.: +61-39905-3683; fax: +61-39905- 3637. E-mail address: trevor.finlayson@sci.monash.edu.au (T.R. Fin- layson) 1 Division of Forestry and Forest Products, CSIRO, Clayton, Vic., Australia. 0921-5093/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII:S0921-5093(99)00299-3