The influence of partial cycling on the martensitic transformation kinetics in shape memory alloys

The martensitic transformation kinetics during partial cycling and the so-called “hammer” effect has been carefully characterized in Cu–Al–Ni shape memory alloys. These temperature memory effects were measured by adiabatic calorimetry and analyzed within the frame of a new thermodynamic model. A straightforward study of the nucleation processes explains on quantitative grounds the shift of the reverse transformation to higher temperatures and the presence of a secondary C_p peak associated to these phenomena. The optical observations support the calorimetric results. Finally, the release of the elastic energy in the martensitic state due to the different thermal cycles has been determined.     

Suppression of γ phase in Ni38Co12Mn41Sn9 alloy by melt spinning and its effect on martensitic transformation and magnetic properties

Microstructure, martensitic transformation and magnetic properties of melt-spun Ni₃₈Co₁₂Mn₄₁Sn₉ ribbon were investigated and compared with its bulk counterpart. The formation of second phase (γ phase) was prevented by melt spinning. Ni₃₈Co₁₂Mn₄₁Sn₉ ribbon underwent a thermoelastic martensitic transformation at a little higher temperature than the master alloy. Due to the disappearance of γ phase, the magnetic properties of as-spun ribbon in the plane was close to its bulk counterpart. A magnetic-field-induced reverse martensitic transformation was verified in this ribbon under a relatively low magnetic field of 1.5 T. In addition, the kinetic arrest behavior was also observed even though there still existed a forward martensitic transformation when applying a magnetic field of 8 T.     

Multi-stage martensitic transformations induced by repeated thermal cycling of equiatomic TiNi alloy

Usually a multi-stage martensitic transformation is observed in Ni-rich TiNi alloys after heat treatment at 350–500 °C. It is due to the internal stresses created by the Ni₄Ti₃ participate. In the present work it was found that the multi-stage martensitic transformation appeared in Ti–50.0 at.% Ni alloy after thermal cycles through the temperature range of the phase transitions. Annealed sample undergoing one-stage phase transition was subjected to 32 thermal cycles in the DSC apparatus. The results had shown that three-stage forward martensitic transformation observed after 32 thermal cycle was due to the B2 → R, B2 → B19′ and R → B19′ phase transitions. It was found that the B19′ phase obtained from the B2 phase underwent the reverse transformation at higher temperatures than the B19′ phase obtained from the R phase. After annealing the cycled sample at 400 °C the transformation behavior was similar to the non-thermal cycled alloy. It was concluded that the main reason for the multi-stage phase transition induced by the thermal cycles was the phase hardening.     

Detection of shock-wave-induced internal stresses in Cu-Al-Ni shape memory alloy by means of acoustic technique

1. The present results indicate that the stress-induced β₁→γ₁′ martensitic transformation occurs for an impact duration of 2 × 10⁻⁶ s. This time interval appears to be sufficient also for the subsequent deformation of the γ₁′ martensitic phase to occur.2. A structure memory effect has been found: Cu-Al-Ni austenitic crystals, shock-loaded at room temperature to induce γ₁′–martensite, recall during subsequent temperature-induced martensitic transformation the martensitic variant structure (elastic properties) formed under the shock loading.3. Elastic properties of quenched β₁′ and γ₁′ crystals of the Cu-Al-Ni system are extremely sensitive to the shock-wave loading. Mechanisms of these effects, as well as of the structure memory effect, include the generation of internal stresses due to the high elastic anisotropy of the martensitic phases. These internal stresses either change the distribution of martensitic variants or govern the formation of the martensitic variant structure during the temperature-induced martensitic transformation. The generation of high internal stresses by impact loading of the β₁′ martensitic phase is also detected by several anelastic phenomena.4. In contrast to elastic and anelastic properties, transformation temperatures are insensitive to the impact loading, pointing to the difference of structural elements responsible for the anelastic effects and for the interval and hysteresis of the thermoelastic martensitic transformation.     

Thermodynamic analysis of shape memory phenomena — I. Effect of transformation plasticity on elastic strain energy

The effect of plastic strains accompanying the thermoelastic transformation on shape memory phenomena is evaluated. It is shown that transformation plasticity can be formally included in Eshelby's macroscopic elastic analysis by defining a net transformation strain equal to the difference of the crystallographic transformation strain and the associated plastic strain. A two variant analysis for a thermoelastic martensite transformation is developed which enables calculation of the variant structure along the path of minimum elastic energy. It is shown that plastic strain accompanying the formation of a martensitic plate reduces the elastic energy stored during the forward transformation. The equilibrium variant structure is shown to be determined by both the applied stress and the fraction of martensite. If the plastic strains accompanying the forward and reverse transformations do not cancel, residual elastic stress fields remain after completion of the reverse transformation. The residual elastic strain fields influence subsequent transformation behaviour and provide the driving force for two-way shape memory behaviour.     

Effect of Ni4Ti3 precipitation on martensitic transformation in Ti–Ni

Precipitation of Ni₄Ti₃ plays a critical role in determining the martensitic transformation path and temperature in Ni–Ti shape memory alloys. In this study, the equilibrium shape of a coherent Ni₄Ti₃ precipitate and the concentration and stress fields around it are determined quantitatively using the phase field method. Most recent experimental data on lattice parameters, elastic constants, precipitate–matrix orientation relationship and thermodynamic database are used as model inputs. The effects of the concentration and stress fields on subsequent martensitic transformations are analyzed through interaction energy between a nucleating martensitic particle and the existing microstructure. Results indicate that R-phase formation prior to B19′ phase could be attributed to both direct elastic interaction and stress-induced spatial variation in concentration near Ni₄Ti₃ precipitates. The preferred nucleation sites for the R-phase are close to the broad side of the lenticular-shaped Ni₄Ti₃ precipitates, where tension normal to the habit plane is highest, and Ni concentration is lowest.     

New modelling of the B2 phase and its associated martensitic transformation in the Ti–Ni system

A symmetric two-sublattice model (Ni, Ti, Va)_0.5(Ni, Ti, Va)_0.5 is applied to describe the intermediate B2 compound in order to cope with the order–disorder transition in the Ti–Ni system. Using this model, the ordered B2 and the disordered Ti-rich b.c.c. are described by a single Gibbs free energy function. The B2 phase is the parent phase of the martensitic transformation in the TiNi shape memory alloys (SMAs), and its thermodynamic properties are then reassessed with emphasis on its composition range that is critical for SMAs. The low temperature B19′ phase is also evaluated on the basis of the selected experimental data from the martensitic transformation. Properties related to the transformation are studied in comparison with experimental data. The magnetic contribution is examined for the martensitic transformation. All calculations are in satisfactory agreement with experimental phenomena.     

Stress-induced martensitic transformation in （Ni47Ti44）100-xNbx shape memory alloys with wide hysteresis

The effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of the （Ni47Ti44）100-xNbx （x=3, 9, 15, 20, 30, mole fraction, %） shape memory alloys was investigated in detail by differential scanning calorimetry （DSC） after performing cryogenic tensile tests at a temperature of Ms＋30 ℃. The results show that Nb-content has obvious effect on the process of stress-induced martensitic transformation. It is also observed that the stress-induced martensite is stabilized relative to the thermally-induced martensite （TIM） formed on cooling, and Nb-content in Ni-Ti-Nb alloy has great influence on the reverse transformation start temperature and transformation temperature hysteresis of stress-induced martensite（SIM）. The mechanism of wide transformation temperature hysteresis was fully explained based on the microscopic structure and the distribution of the elastic strain energy of （Ni47Ti44）100-xNbx alloys.     

Site preference,electronic structure and possible martensitic transformation in Heusler alloys Ni2CoZ (Z = Al,Ga, In,Si, Ge,Sn, Sb)

The site preference, electronic structure, magnetic properties and martensitic transformation in Heusler alloys Ni₂CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) have been investigated by first-principles calculations. Different from literature, it is found that these Ni₂CoZ alloys tend to form XA structure (Hg₂CuTi-type), except for Ni₂CoIn, in which L2₁ (Cu₂MnAl-type) structure is preferable. Theoretical calculation reveals that the tetragonal martensitic phase has a lower total energy compared with the cubic austenitic phase, therefore, a structural transition from cubic to tetragonal is likely to happen in these Ni₂CoZ alloys. The largest energy difference is observed in Ni₂CoIn. It is interesting that Ni₂CoSi is paramagnetic in austenitic state, while is ferromagnetic in martensitic state. This leads to a large change in the total moment, which is meaningful for the realization of magnetic field-induced martensitic transformation in this alloy.     

Study of Thermal Scanning Rates on Transformations of Ti-19Nb-9Zr (at.%) by Means of Differential Scanning Calorimetry Analysis

Differential scanning calorimetry (DSC) thermal analysis is a well-accepted technique used to measure the transformation temperatures of shape memory alloy and its thermoelastic transformation energies. In this study, both forward and reverse transformation temperatures of a nickel-free Ti-19Nb-9Zr (at.%) SMA were investigated using DSC technique with different cooling and heating scanning rates in a range of 10 to 100 °C/min. The results showed that the transformation temperature intervals vary substantially with respect to the thermal scanning rates. It is found that the martensitic start (M _s) temperature decreases with decreasing the cooling rates. The optimal scanning rate was found to be 40 °C/min for obtaining the maximum thermoelastic transformation energies stored between the forward and the reverse martensitic transformations. It is believed that the thermoelastic transformation energy increases with the increase in the volume fraction of martensite. Based on these measurements, these thermoelastic transformation energies between the forward and the reverse martensitic transformations were estimated to be ~21 and ~27 J/g, respectively. The appropriate selection of scanning rate for SMA analysis will be discussed.     

Effect of deformation on the stress-induced martensitic transformation in (Ni47Ti44)100−xNbx shape memory alloys with wide hysteresis

Martensite in TiNi-based alloys is reported to be thermally stabilized after a moderate deformation. Hence, this paper investigates the effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of (Ni₄₇Ti₄₄)_100−xNb_x (x=3, 9, 15, 20, 30 at.%) alloys. The stress-induced martensite appears to be stabilized in relation to the thermal-induced martensite that forms on cooling. This observation is confirmed by an increase in the reverse transformation start temperature, during which time the transformation temperature hysteresis reaches about 200°C. Moreover, the Nb content in Ni−Ti−Nb alloy has a great influence on the transformation temperature hysteresis of stress-induced martensite as well as on the process of stress-induced martensitic transformation. The mechanism of wide transformation temperature hysteresis is explained in terms of the microscopic structure of (Ni₄₇Ti₄₄)_100−xNb_x alloys. Furthermore, the temperature interval of the reverse transformation of stress-induced martensite was found to increase slightly as the strain of the high Nb-content alloy increased, though the value was much smaller than that of the thermally induced martensite. Finally, the paper explains the relation between this unique phenomenon and the elastic strain energy.     

Time effect of martensitic transformation in Ni43Co7Mn41Sn9

Effect of thermal arrest on the L2₁-tetragonal martensitic transformation in a NiCoMnSn shape memory alloy was investigated. The phenomenon was studied by interrupted heating/cooling in differential scanning calorimetry analysis. The experimental evidence indicates that the forward martensitic transformation continued to completion during cooling arrest between M_s and M_f. The same behavior was also observed for the reverse transformation on heating. These observations demonstrate that the L2₁-tetragonal martensitic transformation in the Ni₄₃Co₇Mn₄₁Sn₉ alloy is time dependent at the finite cooling rate.     

Step-like and first-order characters of Tb0.2Pr0.8(Fe0.4Co0.6)1.93 with low interstitial carbon content

In this study we report detailed magnetic property of the 4f-3d pseudo-quaternary Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compound by detailed magnetization measurements. Very sharp magnetization jumps across the antiferromagnetic–ferromagnetic transition are observed below 3.0 K, and the number of jump-like transitions increases with decreasing temperature. The time-dependent magnetic relaxation, field sweep rate and cooling field dependence of magnetization jumps resemble the martensitic scenario. The number and occurrence of magnetization jumps are mainly determined by the competitions between the thermal fluctuation energy, elastic energy and Zeeman energy, and the field-induced antiferromagnetic to ferromagnetic phase transition at low temperatures is of first-order in nature.     

P-phase precipitation and its effect on martensitic transformation in (Ni,Pt)Ti shape memory alloys

A new precipitate phase named P-phase has recently been identified in (Ni,Pt)Ti high temperature shape memory alloys. In order to understand the roles played by the fine coherent P-phase precipitates in determining the martensitic transformation temperature (M_s), strength of the B2 matrix phase, dimensional stability and shape memory effect of the alloys, a phase field model of P-phase precipitation is developed. Model inputs, including lattice parameters, precipitate-matrix orientation relationship, elastic constants and free energy data, are obtained from experimental characterization, ab initio calculations and thermodynamic databases. Through computer simulations, the shape and spatial distribution of the P-phase precipitates, as well as the compositional and stress fields around them, are quantitatively determined. On this basis, the elastic interaction energy between the P-phase precipitates and a martenstic nucleus is calculated. It is found that both the chemical non-uniformity and stress field associated with the P-phase precipitates are in favor of the martensitic transformation. Their relative contributions to the increase in M_s temperature are quantified as a function of aging time and the result seems to agree with the experimental measurements. The shape and spatial distribution of the P-phase precipitates predicted by the simulations also agree well with experimental observations.     

Accumulation of Residual Strain in TiNi Alloy During Thermal Cycling

Residual strain accumulation during thermal cycling of the Ti₅₀Ni₅₀ alloy under a constant stress of 200 MPa through the temperature range of complete and incomplete forward martensitic transformation was studied. The temperature range of the forward martensitic transformation during thermal cycling was chosen as 25, 50, 75, or 100% of the M _s-M _f interval measured in the first cycle. It was shown that intensive accumulation of residual strain took place in the last stage of the forward transformation. It was observed that resistivity increased more rapidly with an increase of the fraction of the temperature range of forward martensitic transformation.     

Effect of preliminary plastic deformation on the structure and physicomechanical properties of aging invar alloy N30K10T3

The invar alloy N30K10T3 after water quenching from 1150°C (austenite, γ phase) has the temperature of the start of martensitic transformation M _s ≈ ?80°C and the Curie temperature T _C ≈ 200°C. The effect of aging-induced phase decomposition in a deformed supersaturated solid solution on its hardness HV, electrical conductivity σ, magnetic permeability μ, and linear expansion coefficient β has been studied. It has been shown that cold plastic deformation of the alloy (at 20°C) to 30–50% increases its hardness, virtually does not change the conductivity, and decreases permeability. Aging of the deformed invar results in increasing HV and σ and decreasing μ. At room temperature, the deformed invar has a low linear expansion coefficient; its magnitude grows the faster, the greater the aging temperature T _a. Plastic deformation increases the density of dislocations, which form a banded substructure in austenitic grains. Besides, a metastable martensitic phase has been observed, which undergoes a reverse martensitic transformation into austenite upon heating in the temperature range from 550°C to 650°C. This transformation causes a decrease in the linear expansion coefficient β(T) of the deformed material. In samples aged at T _a = 700°C (after deformation), an athermal aging-induced martensite (α_a phase) appears after cooling them to 20°C. The appearance of the α_a phase is due to an increase in the temperature of the start of the martensitic transformation to above the room temperature caused by aging. In the samples containing the α_a phase, there is observed a decrease in β in the temperature range from 350 to 670°C, which is due to the reverse transformation of the aging-induced martensite into austenite (α_a → γ).     

Transformation Sequence Rule of Martensite Plates and Temperature Memory Effect in Shape Memory Alloys

In thermoelastic martensitic transformation, it is well established that the first martensite plate appearing upon cooling becomes the final one during reverse transformation to austenite upon heating. The results obtained from this work show that the transformation sequence of the martensite appears to be random. Newly formed martensite plates can modify the elastic strain energy level stored in the already existing martensite. Additionally, the elastic strain energy stored in newly formed martensite is not necessarily to be higher than the remaining martensite. The obtained results may assist in understanding phenomena related to partial transformation of shape memory alloys, such as temperature memory effect.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

Phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys after different heat treatments简

The effects of heat treatments on the phase transformation behavior of Ti49 Ni49.5 Fe1 V0.5 and Ti48 Ni48.5 Fe1 V2.5 alloys were investigated. The results indicate that the alloys subjected to different heat treatments have B2 structure at room temperature. All the specimens exhibit a twostage B2→R→B190martensitic transformation on cooling, but a B190→B2 one-stage reverse martensitic transformation on heating except aged A1 alloy, which undergoes an abnormal two-stage transformation upon heating. The phase transformation temperatures are affected by heat treatments and V content, which can be attributed to the variation of the second-phase particles content in the matrix.