J. Pons

Premartensitic phenomena and other phase transformations in Ni–Mn–Ga alloys studied by dynamical mechanical analysis and electron diffraction

Abstract Ni–Mn–Ga ferromagnetic ordered shape memory alloys are known to exhibit phonon softening and soft mode condensation into a premartensitic phase prior to the martensitic transformation itself. In the present work, this unique behaviour of Ni–Mn–Ga system has been studied as a function of electron concentration (i.e., alloy composition) and a region on the phase transformations diagram which corresponds to the stability of the intermediate phase has been determined, being completely in the ferromagnetic zone. The results were mainly obtained by means of dynamical mechanical analysis and transmission electron microscopy. The two types of lattice instability which might occur in the parent phase driving it to either the soft mode condensed intermediate phase or to the martensitic phase are discussed in this work, together with precursor phenomena and the intermartensitic transformation observed in alloys with the highest electron concentration.

INTERNAL FRICTION ASSOCIATED WITH THE STRUCTURAL PHASE TRANSFORMATIONS IN Ni-Mn-Ga ALLOYS

Abstract A double peak in the temperature dependence of internal friction (IF) and elastic modulus in some off-stoichiometric Ni 2 MnGa shape memory alloys has been observed both on cooling and heating between the parent and martensite phases. Transmission electron microscopy (TEM) observations have shown that these anomalies are related to structural transformations from the parent cubic phase (P) to an intermediate cubic modulated phase (I) and from the I phase to the martensitic one (M). As for the IF results, the I to M transformation has the characteristics of a first-order phase transition, whereas the P to I transformation shows some distinctive features, such as no temperature-rate dependence.

Stabilisation of martensite by applying compressive stress in Cu-Al-Ni single crystals

Abstract Compression tests have been carried out for a Cu-Al-Ni single crystal at temperatures well above the martensitic transformation, near the transformation or below it (in martensitic state). The composition was selected in order to obtain either β-β′ or β-γ′ thermal martensitic transformations, after suitable thermal treatments. The characteristics of the martensitic transformation and structural changes after the compression tests have been studied by means of calorimetry (DSC) and TEM. The obtained results show that when a compressive stress is applied on quenched samples (TTA treatment, β-β′ thermal transformation) a β-(β′)-γ′ transformation or a β′-γ′ one are stress-induced, depending whether the initial state is the parent or the martensitic phase. For aged samples (TTB treatment, β-γ′ thermal transformation) the application of stress brings about the β-γ′ transformation or γ′ re-orientation, depending on the initial state. In all the cases a notable martensite stabilisation is observed only when the stress–strain loop is not closed, that means when a permanent strain remains in the material after unloading. A direct relationship between the applied deformation when stressing the sample and the degree of stabilisation has been obtained for different strain values (between 5% and 12%) and for each set of samples (TTA and TTB). At the same time, the evolution of the characteristics of the martensitic phases with the degree of deformation has been studied. The stress induced stabilisation mechanism is related to the presence of non-twinned γ′ martensite which makes difficult the retransformation to the parent phase.