Martensitic Transformation Strain Research Articles

The orientation dependence of transformation strain of Ni–Mn–Ga polycrystalline alloy and its composite with epoxy resin

This study investigated the transformation strains of Ni–Mn–Ga polycrystalline alloy and Ni–Mn–Ga/epoxy resin composites. For the Ni–Mn–Ga polycrystalline alloy, the columnar grains formed along the solidification direction, and the alloy expanded during martensitic transformation in the direction with a transformation strain of ∼500 ppm. For the composite consisting of randomly distributed Ni–Mn–Ga particles, no transformation strain was achieved due to the random distribution of the Ni–Mn–Ga particles. For the composite consisting of properly oriented Ni–Mn–Ga particles, the martensitic transformation strains were found to be orientation-dependent, the shrinking transformation strain-c (parallel to the direction of magnetic field used during curing) up to ∼210 ppm was detected, and the expanding transformation strain-a (vertical to the direction of magnetic field used during curing) was measured to be ∼150 ppm.

Austenite–martensite interface in shape memory alloys

A two-scale phase field simulation is developed for austenite–martensite interface to understand the effects of crystalline symmetry and geometric compatibilities on the reversibility of structural phase transformations in shape memory alloys. It is observed that when the middle eigenvalue of martensite transformation strain is equal to zero, an exact austenite–martensite interface is formed with negligible elastic energy. On the other hand, when the middle eigenvalue is different from 0, an inexact interface between austenite and martensitic twin is formed, and the corresponding elastic energy increases with the increased magnitude of the middle eigenvalue, resulting in substantially higher energy barrier for austenite–martensite transformation, and thus higher thermal hysteresis in shape memory alloys.

Thermoelastic behaviour of martensitic alloy in the vicinity of critical point in the stress–temperature phase diagram

The theoretical phase diagram of the shape memory alloy, which exhibits the first-order martensitic phase transition of the cubic–tetragonal type, has been considered. The thermoelastic behaviour of the ultra-soft Ni–Mn–Ga alloy in the vicinity of the endpoint of the phase transitions line has been modelled. To this end, the strain–temperature and stress–strain dependencies have been computed with the account of the temperature dependence of the elastic modulus of the alloy. Two important features of thermoelastic behaviour of the alloy have been disclosed: (1) even in the case of complete stress-induced martensitic transformation (MT), the MT strain determined from the length of the plateaus at the stress–strain curves is smaller than the ‘spontaneous’ tetragonal distortion of the crystal lattice, which arises on cooling of the alloy and (2) the stress–strain loops may include the plateau-like segment even at temperatures above the critical temperature, which corresponds to the endpoint of the stress–strain phase diagram. These features render the observation of the endpoint of phase transitions line impossible with the help of the stress–strain tests and make preferable the direct structural studies of MTs in the stressed single-crystalline specimens.

Microstructural evolution of Mn-rich antiferromagnetic Mn–Cu alloy under temperature field

This paper reports the microstructural evolution during forward and reverse martensitic transformations as temperature changes in Mn-rich Mn–Cu alloy. The driving force of martensitic transformation and transformation strain in time-dependent Ginzburg–Laudau kinetic equation is considered as functions of temperature. The computer simulation by using a three-dimensional phase-field theory shows that fct (011) twin martensites grow up with decreasing temperature and shrink with increasing temperature under martensitic transformation temperature, Ms. The forward and reverse processes are entirely reversible and show a very small hysteresis. It is proved that the relaxation of stored strain energy during the forward martensitic transformation plays a key role when reverse martensitic transformation takes place. The simulated results are consistent with experiments.

Martensitic transformation and magnetic field-induced strain in Fe–Mn–Ga shape memory alloy

Martensitic and magnetic properties of Fe–Mn–Ga single and polycrystalline alloys were investigated. It was found that Fe–Mn–Ga alloys exhibit martensitic transformation from the paramagnetic L21 Heusler parent phase to the ferromagnetic L10 martensite phase. The martensitic transformation temperatures increased by about 20 K by the application of a magnetic field of 7 T, and a metamagnetic phase transition was observed. In addition, a magnetic field-induced strain of 0.6% associated with magnetic field-induced forward transformation was confirmed in an Fe43Mn28Ga29 single crystal.

Strain characteristics and superelastic response of [formula omitted] single crystals

The intermartensitic transformation, in a two-step complete thermoelastic martensitic transformation in Ni 53.2Mn 22.6Ga 24.2 single crystals, provides a much larger strain than that of the martensitic transformation. With a biasing magnetic field, the intermartensitic transformation strain is inhibited and the martensitic transformation strain is enhanced. Compressive stress–strain characteristics can be affected greatly by a static magnetic field. At low deformation temperature, the irreversible transformation strain induced by the stress becomes reversible, when a static magnetic field is applied. Further, the magnitude of the stress necessary for rearrangement of martensitic variants is dependent on the direction of the biasing magnetic field. Moreover, a well-defined character of the twin-boundary motion, similar to the soliton motion, has been observed upon loading or unloading.

Cyclic deformation behavior of a Ti–26 at.% Nb alloy

The effect of cyclic deformation on superelasticity was investigated in a Ti–26 at.% Nb alloy. Loading and unloading tensile tests with a constant maximum applied strain of 2.5% were carried out until the 500th cycle. The critical stress for inducing the martensitic transformation and superelastic strain decreased, while the accumulated residual strain increased with increasing number of cycles. The increase in the residual strain during cyclic deformation was due mainly to α′′ martensite phase stabilization. Both the residual strain and the residual α′′ martensite phase increased with increasing number of cycles. The stability of superelasticity was improved, i.e. the residual strain decreased and the superelastic strain increased, by intermediate-temperature annealing and/or aging. The specimen annealed at 873 K for 0.6 ks followed by aging at 573 K for 3.6 ks exhibited the most stabilized superelasticity, owing to the combination effect of work hardening and fine ω-phase precipitation.

Giant magnetic-field-induced strain in NiMnGaSi magnetic shape memory alloy prepared by diffusion-reduction

NiMnGaSi alloys were prepared by diffusion-reduction reacting which is a new and simple method. The effect of small mount of Si addition on the martensitic transformation temperature (Tm) and strain was investigated. It was found that the addition of Si caused a stronger magnetic exchange interaction leading to higher Curie temperature and giant magnetic field-induced strain. The typical Ni43.03Mn32.07Ga18.48Si6.43 alloy shows high martensitic transformation temperature (Tm) of 300K and a 0.56% giant strain measure at a magnetic field of 0.7 T.

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0] β and line direction [1 1 1] β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol −1 was obtained, which is in reasonable agreement with experimental results.

High resolution transmission electron microscopy study on the nano‐scale twinning of θ‐NiMn precipitates in an Fe–Ni–Mn maraging alloy

AbstractPrecipitation of nanometer‐sized fct θ‐NiMn intermetallic compound was found to cause age hardening in Fe–10Ni–7Mn (wt%) maraging alloy during isothermal aging at 753 K. High resolution transmission electron microscopy was used to study the substructure of the fct θ‐NiMn precipitates in the alloy after aging for 0.36 and 86.4 ks. Nano‐scale twins on (111) planes were found in the precipitates of intial and later stages of aging. Precipitation of bcc β‐NiMn at 753 K and subsequent martensitic transformation to fct θ‐NiMn during cooling to room temperature was suggested. It was also indicated that precipitation of fct θ‐NiMn at bcc iron matrix gives rise to high coherency strain energy. The (111) twinning is proposed to accommodate the strain energy of either martensitic transformation or coherency strain of precipitation reaction. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of {0 0 1}〈1 1 0〉 texture on superelastic strain of Ti–Nb–Al biomedical shape memory alloys

Anisotropy in recovery strains was examined in a Ti–Nb–Al superelastic β-Ti alloy with a well-developed {0 0 1} β〈1 1 0〉 β texture. An ingot of Ti–24 mol% Nb–3 mol% Al alloy was fabricated by arc melting, homogenized at 1273 K for 7.2 ks, cold-rolled with 99% reduction in thickness and then solution-treated at 873 K for 3.6 ks. X-ray diffraction pole figure and electron backscatter pattern analysis revealed that {0 0 1} β〈1 1 0〉 β texture with grain size of 2 μm was developed in the alloy under the condition of the above thermo-mechanical treatment. Cyclic loading-unloading tensile test was made to evaluate superelastic properties. Tensile specimens were made with various orientation between the rolling direction (RD) and the longitudinal direction. Superelastic strain ( ɛ SE) was measured as the sum of the transformation strain ( ɛ TR) of β–α″ thermoelastic martensitic transformation and pure elastic strain ( ɛ E). It was found that ɛ TR at ϕ = 0° and 90° was almost the same and decreased to a minimum at around ϕ = 45°. On the other hand, it was found that ɛ E increased to a maximum at around ϕ = 45°. These orientation dependences of ɛ TR and ɛ E were in good agreement with the crystallography of the transformation and the anisotropy in Young's modulus in the alloy. ɛ SE was almost a constant regardless of ϕ. It was found that anisotropy in the tensile component of the superelastic strain available along the directions lying within the normal plane can be lowered by the development of the {0 0 1} β〈1 1 0〉 β texture.

Magnetic-Field-Induced Strains of Bonded Ni–Mn–Ga Melt-SpunRibbons

Non-stoichiometric Ni50Mn27Ga23 polycrystallineribbons are prepared by melt-spinning technique. Themagnetic-field-induced strain (MFIS) of Ni–Mn–Ga bulk alloy prepared bybonding the melt-spun ribbons is obtained. The experimental resultsshow that Ni50Mn27Ga23 bonded ribbons exhibit atypical thermal-elastic shape memory effect in the thickness direction.The martensitic transformation strain of bonded ribbons is an expansivestrain of about 0.3% without the magnetic field and a contractivestrain of about -0.46% at the magnetic field of 1 T. The field can notonly enhance the value of the martensitic transformation strain of thebonded ribbons, but can also change the direction of the strain. The bondedribbons alloy presents negative MFIS and obtains a larger value of thestrain though influenced by the adhesive between the ribbons.Therefore, the preparation technique of the Ni–Mn–Ga bulk alloy bybonding melt-spun ribbons is helpful to get rid of the size restrictionof the ribbon and to broaden the applications of the ribbons.

Temperature dependence of transformation strain and magnetic-field-induced strain in Ni 51Mn 24Ga 25 single crystal

The martensitic transformation strain up to 0.6% has been obtained in Ni 51Mn 24Ga 25 single crystalline sample, which is attributed to the effect of preferential orientation of the martensite variants, resulted from the internal stress introduced by the directional solidification. An enhanced transformation strain up to 1.1 % is observed with a bias magnetic field of 1.2 T. There is a large difference of MFIS max ( λ max) from about 600 to 1300 ppm in the temperature range from 192 to 236 K in martensite phase. Based on the shear actuation model, the magnetocrystalline anisotropy energy and the energy for twin variants reorientation as a function of the temperature have been discussed. We proposed that the rate of the decrement of shear modulus ( C′) with respect to the temperature is much larger than that of the magnetocrystalline anisotropy ( K) as a result of the constraint λ max = K/ C′ γ t, which has been considered as the reason for the increasing MFIS max with increasing temperature.

The property of transformation strain in the texturedNi50550.5Mn 2424Ga25525.5 alloy

A large strain of 14% during the martensitic transformation has been observed in the textured Ni50550.5Mn2424Ga25525 .5 polycrystal prepared by vertical zone melting unidirectional solidification. This martensitic transformation strain decreases to 08% at the ninth and tenth thermal cycles. Expansion and shrinkage phenomenon during the martensitic transformation has been obtained, indicating the existence of a preferential orientation of martensitic variants.

Stress State Prediction Applied to Local Composition Changes

A numerical study of quenching (in oil and water) of steel cylinders is carried out to determine the effect of composition changes on the radial profile of the residual stresses. The numerical model takes into account only the martensitic transformation. As a first approach, the composition change has been limited to the carbon content and this has been considered to influence only the M s temperature, the martensitic transformation strain, the yield stress and the strain hardening coefficient (these last two, just for the martensite phase). The composition oscillations representative of micro-segregation bands have been simulated by a sinusoidal fluctuation between 0.35 and 0.45 %C, the average value being 0.4 %C. The sinusoidal composition oscillations have led to very significant fluctuations of the axial residual stress around the calculated value for the constant carbon content (equal to 0.4 %C). The present study clearly shows that the analysis of residual stress measurements in comparison with calculated results must not neglect the disturbing influence of localised composition changes, even when these seem to be not relevant.

Ti-content and annealing temperature dependence of deformation characteristics of Ti XNi (92− X) Cu 8 shape memory alloys

The Ti-content and annealing temperature dependence of the deformation characteristics of Ti X --Ni (92− X) Cu 8 (at.%) ( X=49.0–51.0) alloys was investigated by constant load thermal cycling tests by varying the annealing temperature between 673 and 1273 K. It was found that the stress-induced martensite transformation strain of a 50.0 at.% Ti alloy is larger than those of Ti-poor and Ti-rich alloys due to the precipitates of Ti(Ni 1− X Cu X ) 2 and Ti 2(Ni 1− Y Cu Y ) existing in the Ti-poor and Ti-rich alloys, respectively, acting as obstacles to the growth of preferential martensite variants. By annealing at 1273 K to cause the precipitates of Ti(Ni 1− X Cu X ) 2 being dissolved into the matrix of the Ti-poor alloys, the stress-induced martensite transformation strain exhibits little dependence on Ti-content in the composition range of 49.0–50.0 at.% Ti. The plastic strain, critical stress for slip and recovery strain also show a unique Ti-content and annealing temperature dependence.

The influence of martensite shape, concentration, and phase transformation strain on the deformation behavior of stable dual-phase steels

A continuum model is developed to examine the influence of martensite shape, volume fraction, phase transformation strain, and thermal mismatch on the initial plastic state of the ferrite matrix following phase transformation and on the subsequent stress-strain behavior of the dual-phase steels upon loading. The theory is developed based on a relaxed constraint in the ductile matrix and an energy criterion to define its effective stress. In addition, it also assumes the martensite islands to possess a spheroidal shape and to be randomly oriented and homogenously dispersed in the ferrite matrix. It is found that for a typical water-quenched process from an intercritical temperature of 760 °C, the critical martensite volume fraction needed to induce plastic deformation in the ferrite matrix is very low, typically below 1 pct, regardless of the martensite shape. Thus, when the two-phase system is subjected to an external load, plastic deformation commences immediately, resulting in the widely observed “continuous yielding” behavior in dual-phase steels. The subsequent deformation of the dual-phase system is shown to be rather sensitive to the martensite shape, with the disc-shaped morphology giving rise to a superior overall response (over the spherical type). The stress-strain relations are also dependent upon the magnitude of the prior phase transformation strain. The strength coefficienth and the work-hardening exponentn of the smooth, parabolic-type stress-strain curves of the dual-phase system also increase with increasing martensite content for each selected inclusion shape. Comparison with an exact solution and with one set of experimental data indicates that the theory is generally within a reasonable range of accuracy.

Stress Evolution in Thin film Niti Shape Memory Alloys

ABSTRACTAs a potential micro-actuator material thin film shape memory alloys (SMAs) have drawn attention because of their capability of generating strains of a few percent reversibly. In contrast to SMAs the output strain of traditional actuator material, such as quartz and PZT ferroelectrics, is limited to 100 to 1000 ppm. The authors have investigated the transformation properties and microstructures of thin Ni50Ti50 films on Si. From application point of view, the evaluation of the actuator property should come from direct measurements of the strain output of the film/substrate composite. In the film/substrate composite, the martensitic transformation strains in the SMA thin film are coupled to elastically to the substrate through interface load transfer. Interface properties thus play crucial role on the efficiency of the composite actuators. Since the capability of interface load transfer is determined by the substrate stiffness, experiments were designed to measure the strain output of NiTi/Si composite with varying substrate stiffness. The experiments indicated that the output efficiency of the martensitic transformation strain increases by a factor of 2 when the film/substrate thickness ratio increases from 1/345 to 1/70. The result is favorable for micro-actuator application since much larger film/substrate ratios can be achieved by micro-machining. The absolute value of the transformation strain is about 2 percent which can be improved if the film texture is controlled.Another problem related to the nature of the film/substrate composite is the thermal stress arising from the difference of the thermal expansivities of the film and substrate. The as deposited films were amorphous with a growth tensile stress ranging from 0.6 GPa to 1 GPa, increasing with the substrate stiffness. The initial recrystallization of the amorphous NiS0Ti,0 film results in an increase of the tensile stress by 80 MPa; however upon completion of the recrystallization process the tensile stress was relaxed. Young’s moduli of the recrystallized austenitic NiTi films were calculated from the thermal stresses arising in the composite samples. It was found that the moduli apparently depend on the substrate thickness. This effect is tentatively interpreted to indicate a sensitivity of the mechanical properties of the film on the grain structure of the recrystallized state which in turn might depend on the constraint due to the substrate. TEM studies will clarify the situation.It is concluded that the interface stress restrained the efficiency of the martensitic transformation. Since the NiTi film has a high (interface) yield stress, above 1 GPa, it is proposed that the actuation efficiency of the SMA can be improved by reducing the substrate stiffness while maintaining a high load transfer efficiency. If Si is used as a substrate because of its high thermal diffusivity, the actuator design must weight the mechanical actuator output against its speed.

Morphological aspects of the shear transformation in YBa2Cu3O7-δ: To twin or not to twin?

Abstract The fine structure of YBaCu3O7 is examined: Twin organization, planar defects, and structural phase co-existence are discussed in the context of the tetragonal to orthorhombic transformation. Heterogeneous nucleation of an insulating phase at the twin boundary and sub-grain boundaries is demonstrated. On the basis of the morphological observations, the transformation is classified as martensitic. Considering the role of twin in relieving the martensitic transformation strain, by producing a low strain energy habit plane, the feasibility of the formation of a twinless structure is discussed.

予荷重試験によるTi-Ni合金の応力誘起変態

In this study, the stress dependence of strain-and electrical resistance-temperature relations is investigated by thermal-cycling tests with tensile pre-load for Ti-Ni alloys (49.8 and 50.2 at%Ni) heat-treated at temperature between 743 K and 933 K. The resulting stress (σ)-transformation temperature (T) diagrams are, then, analyzed in detail. The results are summarized as follows.(1) The σ-T diagram in the present study shows similar aspects to that in the tensile test by Miyazaki et al. except for the low stress range in which an analysis with enough accuracy could not be made in tensile test.(2) The σ-T relation of the R→M transformation is not linear and is divided into three parts according to the stress level; (a) low-stress part with large dσ⁄dT, (b) middle-stress part with small dσ⁄dT and (c) high-stress part with relatively large dσ⁄dT. While the R-phase transformation strain increases as the stress becomes larger in the stress range of part (a), it remains constant in the stress range of parts (b) and (c). In the part (c), the R→M transformation takes place before the interaxial angle of the R-phase reaches a maximum. Furthermore, in the stress range of part (c), it is noted that stress dependence of the martensitic transformation strain becomes relatively small, that the residual strain becomes evident at Af point, and that the apparent heat of transformation evaluated from the Clausius-Clapeyron formula increases.(3) The electrical resistance at Mf point increases as stress becomes larger. As for the R→M transformation, the amount of the increase is found to be approximately twice as large as the value estimated from the elongation of the sample during transformation and the apparent increase in the specific resistance which brings residual resistance at Af point.