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Effect of ageing on martensitic transformation has been investigated in a Cu-Al-Ni alloy. Single crystalline specimens were annealed for 30 min at temperatures T"" = 175 to 350°C. Changes in the martensitic transformation were followed by calorimetry (DSC) and optical microscopy.Detailed observations of structural changes due to annealing were made by TEM. The most remarkable structural feature in the as-quenched Pi phase are prismatic dislocation loops which have been formed by coalescence of excess quenched-in vacancies. Most probably, some off-stoichiometric Al atoms segregated to the plane of the loops during quenching. The first coherent precipitates of the equilibrium y 2 phase are observed in the vicinity of the dislocation loops after ageing at 200°C, and their size and density raise with increasing T m . A linear increase of the transformation temperatures is observed for T^ = 200 -300°C. This shift can be most consequently explained by depletion of the matrix of Al atoms and annealing out of excess quenched-in vacancies. Moreover, stresses and concentration changes in the vicinity of the y 2 precipitates could facilitate the nucleation of martensite. The DSC data as well as optical observations indicate that yi' martensite is induced in samples annealed at T m up to 300°Cj whereas a different type of martensite suddenly appears in the samples aged above this temperature.

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Tensile tests were performed in situ in a transmission electron microscope to investigate the twinning mechanism in non-modulated NiÀMnÀGa martensite. The reorientation of the twin variants occurs via twinning dislocations. Their generation and movement were followed; the glide plane and Burgers vector were verified. Individual twinning dislocations were visualized.NiÀMnÀGa is an interesting and widely investigated shape memory alloy showing remarkable properties, such as magnetoresistance, 1 magnetocaloric behavior, 2 thermally induced shape memory effect, 3 superelasticity 4 and, particularly, magnetic-field-induced-strain. 5 In alloys that transform upon cooling into modulated martensites, the martensitic twin structure is coupled with magnetic domains in such a way that the magnetic field can reorient martensitic twin variants, which results in very large strains. 6 The magnetic-fieldinduced-strain (magneto-plasticity) in NiÀMnÀGa is directly related to the reorientation of one martensite variant to another twin variant. 5 The mobility of the twin boundaries plays a decisive role in these processes. A microscopic model of the reversible motion of the twin boundary in the ferromagnetic shape memory alloys was proposed by Müllner et al.,7,8 with their concept being based on the movement of twinning dislocations. 9 Although the magnetoplasticity in NiÀMnÀGa has been described and discussed thoroughly in the literature, detailed investigations of the twin variant reorientation by transmission electron microscopy (TEM) are very rare. 10À12 The in situ TEM straining method used in the present study is highly suitable for this purpose as it enables the direct observation of the stress-induced processes. The experiments were intended to be carried out on modulated as well as nonmodulated martensites. However, they could be successfully performed only on the non-modulated samples. The results were partly published in Refs. 10 and 11. The samples of 5M martensite were too brittle for tensile tests inside the electron microscope, and only preliminary results were obtained for 7M martensite. The non-modulated tetragonal NiÀMnÀGa martensite exhibits superelasticity and shape memory effect while the magnetic-field-induced-strain was reported only 0.17%. 13 Nevertheless, we believe that an understanding of the stress induced twin variant reorientation in the non-modulated martensite will provide insight into the magnetically induced reorientation processes in the modulated martensites as well.The experiments were performed on a Ni 52:4 Mn 27:3 Ga 20:3 alloy, with M s ¼ 398 K, A f ¼ 408 K, T c ¼ 380:2 K. The structure of the alloy at room temperature was tetragonal martensite, a ¼ 0:549 nm, c ¼ 0:661 nm. The samples, 1:67 mm  5:5 mm  0:07 mm, were prepared from a plate with [001] normal in the austenite state, and thinned in the central part in a double jet polisher (30% solution of HNO 3 in methanol, À40 ○ C) to form a hole. The samples were strained in tension at room temperature in a JEM 1200EX microscope equipped wit...