DOI : 10.1051/jp4 : 20031035 Stress-strain-Temperature behaviour for martensitic transformation in Ni-Mn-Ga single crystal compressed along <001> and <110> axes J. Pons, V. A. Chernenko\ E. Cesari and V. A. L'VOV2 Departament de Fisica, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain 1 Institute of Magnetism, Vemadsky Str. 36-b, 03142 Kiev, Ukraine 2 Taras Shevchenko University, 03127 Kiev, Ukraine Abstract. The strain recovery upon heating pre-deformed martensite as well as the stress-strain behaviour in compression of parent phase of single crystalline Ni49 4Mn27 7Ga22 9 alloy transforming martensitically at T = 274 K has been studied as a function of temperature and different orientation of compression axes. The constructed critical stress to induce the martensite transformation vs temperature (Oc-7) phase diagrams can be represented by straight lines with slopes of-2. 5 and-9. 4 MPa/K in <001> and <110> directions, respectively, which are in a good agreement with the theoretically estimated values. 1. INTRODUCTION The transformation behaviour and type of martensite in atomic and ferromagnetic ordered shape memory Ni-Mn-Ga alloys is dependent on element content [1, 2] and extemal fields [3, 4]. Stress-induced martensitic transformations (MT's) exhibited by these alloys under uniaxial loading show interesting features. Indeed, a sequence of stress-induced MT's under compression along <110> [4-7] or tension along <100> [5] was found for several Ni-Mn-Ga alloys with thermal induced MT near 300 K. These alloys belong to the Group Il in accordance with the classification scheme of Ref. [1]. The effect of temperature on the orientation dependent stress-induced MT's in these alloys was not enough explored before, although the theoretical aspect of this problem has been considered in detail [8, 9]. In the present work, the strain recovery behaviour of pre-strained martensite as well as the single or multi-stage superelasticity effect as a function of orientation and temperature has been determined. In contrast to previous works [4, 6], the Oc-T phase diagrams upon compressing along different crystallographic directions have been obtained using the same Mn-enriched Ni-Mn-Ga single crystal of the Group Il The results for the dcrcldT slopes are compared to theoretical estimations obtained through a Clausius- Clapeyron type equation. 2. EXPERIMENTAL PROCEDURE A single crystal with composition Ni494Mn2uGan9 was grown by the Bridgman method from an induction melted and cast ingot. The crystal was annealed at 1170 K for 27h in argon atmosphere followed by slow cooling. The orientation of the crystal was determined by X-ray diffraction (back reflection Laue method) with uncertainty of ± 2°. Differential scanning calorimetry (DSC) and temperature ramps of low field ac magnetic permeability were used to determine the characteristic temperatures and latent heat of MT as well as Curie temperature, Tc. The thermal induced shape change of pre-strained specimens was studied by dilatometry (Perkin Elmer TMA-7). TEM selected area