G. Guénin

Influence of the recovery and recrystallization processes on the martensitic transformation of cold worked equiatomic Ti–Ni alloy

Abstract The martensitic transformation of Ti–Ni shape memory alloys is very sensitive to thermomechanical treatments (cold work and annealings). In the present paper, a Ti–Ni alloy close to the equiatomic composition has been 40% cold rolled and then submitted to various annealing treatments, each one characterized by temperature and time (Ta, ta). A large range of annealing temperatures (538–778 K) and times (10 s–123 d) has been covered. From differential scanning calorimetry measurements, three zones have been identified with reference to (Ta, ta) values. Zone I, at low temperatures and times, is characterized by badly defined transformations on cooling as well as on heating; it corresponds to a recovery–reverse martensitic transformation stage. Zone II exhibits large changes of the transformation features which occur in two steps on cooling (R phase then martensitic) and in one step on heating; this zone corresponds to the recrystallization and growth of very small stressed grains. Similar transformation behavior can be obtained with different (Ta, ta) annealings, which can be characterized by an activation energy of ≈3.4 eV in the range 698–778 K. Zone III relates to a well defined martensitic transformation taking place in one step with almost no further evolution; it corresponds to the growth of stress-free grains.

Microstructural analysis of the stress-induced ε martensite in a Fe-Mn-Si-Cr-Ni shape memory alloy : Part I. Calculated description of the microstructure

Abstract The γ (f.c.c.)– e (h.c.p.) martensitic transformation is achieved by the introduction of stacking faults on each second compact plane of the f.c.c. structure. These stacking faults are created by the motion of Shockley partial dislocations. Depending on the Burgers vector of these dislocations, the martensite does not require a macroscopic shape change (self-accommodated martensite) or a homogeneous lattice shape change (monopartial martensite). Based on the monopartial nature of the stress-induced martensite, a model describing the martensitic morphology in the Fe–Mn–Si based shape memory alloys is presented. The theoretical results are compared with some observations in a Fe–Mn–Si–Cr–Ni shape memory alloy.

Elaboration and characterization of new low temperature shape memory CuAlBe alloys

Abstract The addition of small concentrations of beryllium to CuAl alloy close to the eutectoidal composition leads to a sharp decrease in the martensitic transformation temperature, while the composition corresponding to the maximum stability of the β phase does not change significantly. The eutectoidal temperature is lowered, but the temperature of β solution treatment remains the same. Moreover, there is no change in the nature of the martensitic transformation (DO 3 lrarr 2; 18R). All these properties are very interesting from the industrial point of view: whatever value M s (“martensite start”) temperature is chosen, the stability of the β phase is approximately the same since the alloy is always close to the eutectoidal composition. This alloy is therefore a good complement of CuAlNi alloy for low transformation temperatures (below room temperature) with approximately the same resistance to ageing. Both one way and two way memory effect (TWME) experiments were carried out on the specimens.

Microstructural analysis of the stress-induced e martensite in a Fe-Mn-Si-Cr-Ni shape memory alloy. Part II : Transformation reversibility

Abstract The shape memory effect exhibited by some Fe–Mn–Si based alloys is related to the γ(f.c.c.)-e(h.c.p.) martensitic transformation. In these alloys, the shape memory effect, incomplete even for low deformation rates, drastically decreases when the deformation rate increases. The evolution of the microstructure of the martensite, and particularly the interaction of martensite plates and grain boundaries, is studied in a Fe–Mn–Si–Cr–Ni alloy to determine the origin of the loss of shape memory. The influence of the back-stress created by the dislocations located at the tip of the martensite bands is pointed out. The reduction of the shape memory is attributed to the accommodation of this back-stress when the deformation rate, i.e. the martensite band width, increases. The same kind of analysis is used to analyse the problem of superelasticity.

Characterization of the stress-induced ε martensite in a Fe–Mn–Si–Cr–Ni shape memory alloy: microstructural observation at different scales, mechanism of formation and growth

Abstract The martensitic transformation induced by traction at room temperature in a Fe–16Mn–9Cr–5Si–4Ni (%mass) has been studied by optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning tunneling microscopy. The samples were previously submitted to a thermomechanical treatment which increases the shape memory properties. The martensitic microstructure and the fine structure of the stacking faults are both studied to clarify the nucleation and growth mechanisms of martensite. The band structure of the martensite is pointed out; these bands correspond to a mixture of thin martensite plates and extremely thin austenitic zones. Inside a grain, the monopartial nature of the martensite has been demonstrated from the elementary plate to all the martensite bands. From all the observations, the pole mechanism appears to be the main mechanism of martensite nucleation.

Microstructural evolution kinetics after plastic deformation of equiatomic Ti–Ni alloy during isothermal annealings

Abstract Ti–Ni shape memory alloys are very sensitive to thermomechanical treatments (cold working and annealing). However, these treatments, already used and described in the literature have not been studied systematically. To identify and to understand the phenomena which occur during these treatments in the equiatomic Ti–Ni alloys, a study was carried out for an extensive range of temperatures (538–778 K) and annealing times (10 s–123 days) after cold working at room temperature. Thanks to the sensitivity of the thermoelectric power which has not yet been used for this kind of alloy, quantitative results for the annealing kinetics have been obtained. Two annealing stages have been observed and characterized with activation energies of 2.7 and 3.6 eV. Transmission electron microscopy allowed to identify these stages with phenomena of ‘recovery’ and ‘recrystallization’ characterized by special features due to interaction with the martensitic transformation itself.

Electro-thermomechanical behaviour of a Ti-45.0Ni-5.0Cu (at.%) alloy during shape memory cycling

Abstract In the present paper, electrical resistance (ER) changes are measured simultaneously with the stress-assisted two-way memory effect (SATWME) in Ti-45.0Ni-5.0Cu (at.%) wires during thermal cycling (max. 15 cycles) for several different stress levels. Interesting qualitative evolutions of the e-ER-T loops during cycling are observed as a function of the applied stress. On cooling, for stresses higher than 175 MPa, a clear deviation of the e-T curves is verified and the reversion of this anomaly is not observed during heating. After some cycles, serrations are frequently observed on the ER-T loops essentially below M f and above A f , indicating an interaction between the formation and reversion of oriented martensite variants with the defects introduced during the thermomechanical cycling. A linear relationship is observed between ER and e for the direct and reverse transformation ranges. The characteristic slope d(Δ R / R )/de is slightly dependent on the applied stress and on the number of thermal cycles.

Martensitic transformation: Phenomenology and the origin of the two-way memory effect

Abstract Primary characteristics of martensitic transformation and their relation to the one-way memory effect and the influence of the stresses are first reviewed. Afterwards, the various training processes are described and analysed from a phenomenological point of view. The physical origin of the two-way memory effect (TWME) is then discussed: it is shown that the structural defects, such as oriented dislocations created during training, are probably responsible for the TWME. The possible mechanisms for the role played by these defects are also reviewed and discussed.

Influence of the mechanical loading history on the stress assisted two way memory effect in a Ti-Ni-Cu alloy

In this work, several tests of thermal cycling under constant load are carried out on Ti-45.0Ni-5.0Cu (at%) shape memory wires. The properties related to the Stress Assisted Two Way Memory Effect (SATWME) of the material are investigated as a function of the mechanical loading history for the same temperature range during cooling and heating. For this reason, two thermomechanical tests have been employed: tests I, where one sample is used for just one constant stress level test and tests II, where only one sample is employed for several constant stress level tests in sequence. The results obtained show that for loads applied below 150 MPa, the transformation temperatures and the thermal hysteresis associated with the transformation of the material are the same during the two tests. However, above 150 MPa in tests II transformation temperatures and thermal hysteresis are respectively higher and smaller than the ones obtained in tests I. On the other hand, transformation temperatures obtained from both tests are in good agreement with the ones measured by DSC and electrical resistance measurements. It is also observed that the SATWME obtained by tests II is smaller than the one measured during tests I. It is shown that these different behaviors are induced by accumulation of plastic strain in the sample during tests II.

Estimation of Internal Stresses in Shape Memory Wires during Thermal Cycling under Constant Load: A Macromechanical Approach:

Untrained shape memory alloys are very sensitive to the introduction of defects associated with the accumulation of plastic strain during thermomechanical cycling. These defects are responsible for the creation of an internal stress field that plays a fundamental role on the macroscopic memory properties (superelasticity and shape memory effects). In this paper a simple physical model based on a macroscopic analysis is developed for the evaluation of the global internal stress field created in Ti-Ni based shape memory wires during thermal cycling under constant load. The results obtained with this macromechanical approach are supported by calorimetric and electrical resistance (ER) measurements performed after failure of the material.