Effects of superimposed electric field and porosity on the hydrostatic pressure-induced rhombohedral to orthorhombic martensitic phase transformation in PZT 95/5 ceramics

The hydrostatic pressure-induced martensitic transformation from the ferroelectric rhombohedral to antiferroelectric orthorhombic phase in PZT 95/5 ceramics has been studied using neutron diffraction. The transition to the orthorhombic phase initiates at a pressure of 260 MPa and is almost complete at 290 MPa. This stress range is much narrower than that observed in uniaxial loading, which starts at 200 MPa and is incomplete even at 400 MPa. The narrower stress range observed under hydrostatic loading is attributed to a lack of internal stress developed during the transformation. By contrast, the work required to start the transformation is approximately the same under both types of loading. The transformation progresses more gradually with increasing pressure when a static electric field is applied to a specimen in a pre-poled state. Tests carried out on porous specimens, having a relative density of approximately 90%, demonstrated that the transformation occurred over a narrow pressure range but with a lower transformation pressure of approximately 220 MPa.     

Effect of hydrostatic pressure on martensitic transformation in a ferromagnetic shape memory alloy Ni2MnGa

Ni₂MnGa transforms in the cooling process from the parent (P-) phase to the intermediate (I-) phase and then to the martensite (M-) phase. Under a uniaxial stress, a new phase (X-phase) appears, and the P → I transformation distinctively separates to the P → X, and the X → I transformations. In the present study, we have studied the effect of hydrostatic pressure on these transformation temperatures. As a result, we found that the distinct separation described above does not occur under a hydrostatic pressure of up to 0.9 GPa. The P → X and the I → M transformation temperatures increase with increasing hydrostatic pressure, and the ratio is 13.1 K/GPa and 16.2 K/GPa, respectively. The change in molar volume and/or thermal expansion coefficient associated with the transformation is estimated from these values.     

Effect of magnetic field on martensitic transformation in iron-nickel alloys with different size of austenite grains

The effect of the size of austenite grains on the temperature of the beginning of martensitic transformation during cooling, on the amount of the α-phase, and on the value of the critical field that causes the appearance of the first portions of martensite is studied for the case of treatment of iron-nickel alloys in a magnetic field.     

Role of magnetism on the martensitic transformation in Ni-Mn-based magnetic shape memory alloys

The effect of magnetism on the martensitic structural transformation has been analyzed through the evolution of the transformation temperatures of several Ni-Mn-Ga and Ni-Mn-In alloys subjected to high-temperature quenching and post-quench annealing thermal treatments. It is found that the atomic order variations associated with the thermal treatments affect the structural transformation in different ways depending on the character of the magnetic ordering in the austenitic and the martensitic phases. In particular, regardless of composition, the variation in the atomic order affects the martensitic transformation temperature only in those alloys in which at least one of the structural phases show magnetic order at the transformation temperature, whereas those transformations taking place between paramagnetic phases remain unaffected. The observed behaviors are explained in terms of the effect of the magnetic exchange coupling variations on the free energy difference between austenite and martensite. The results confirm the key role of magnetism in the martensitic transformation.     

Effect of magnetic fields on isothermal martensitic transformation in alloy N24G4

The effect of constant magnetic fields on isothermal martensitic transformation and structure of alloy Fe-14%Ni-4%Mn is studied. A C-diagram of the isothermal martensitic transformation and volume kinetic diagrams in a stationary magnetic field are plotted. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 3–8, May, 2007.     

The effect of electronic and magnetic valences on the martensitic transformation of CoNiGa shape memory alloys

Martensite start temperatures (M_s) and magnetic properties of several Co–Ni–Ga shape memory alloys (SMAs) with B2 austenite structure were investigated as a function of composition. It was found that, in addition to the well-known effect of the valence electron concentration (e/a ratio) in increasing M_s, the magnetic character of the alloy also plays a decisive role in determining M_s. These results were further corroborated through electronic structure calculations based on density functional theory. Experiments and calculations suggest that, for a given composition of Ga, the higher e/a ratios result in the higher M_s. Moreover, at a constant e/a ratio, the higher the magnetic valence number (Z_m) of the alloy is, the lower the M_s becomes. It was concluded that Z_m can be used as an indicator for the compositional trends in M_s in the present ferromagnetic SMAs in addition to the e/a ratio. Statistical analysis of the experimental results suggests that an increase in e/a of 0.1 yields an increase in M_s of about 190 K, while a change in Z_m of +0.1 results in a decrease in M_s of about 160 K. The calculations also confirmed that ferromagnetism may ultimately result in entropic and/or energetic stabilization of the austenite (cubic) phase, yielding lower M_s. These findings seem to be valid for other ferromagnetic SMA systems, where different M_s temperatures were reported for a given e/a ratio.     

The suppression and recovery of martensitic transformation in a Ni-Co-Mn-In magnetic shape memory alloy

The intrinsic mechanism of the martensitic transformation (MT) suppression observed in Ni-Co-Mn-In alloys fabricated under non-equilibrium conditions still remains mysterious. Here, we used the undercooling technique to obtain a solidified microstructure in non-equilibrium state, subsequently leading to MT suppression even further cooling to 10 K. It was found that primary dendrite-like In-depleted precipitates occurred during solidification under a large undercooling. After a prolonged annealing, the MT interestingly appeared again due to the dissolution of the precipitates and the recovery of equilibrium chemical composition in the matrix.     

Three-dimensional phase field model and simulation of martensitic transformation in multilayer systems under applied stresses

The phase field microelasticity theory is used to formulate a three-dimensional phase field model of a multivariant martensitic transformation under external load. The model is based on the exact solution of the elasticity problem in the homogeneous modulus approximation. The transformation-induced coherency strain and applied stress are explicitly taken into account. Computer simulations are performed for a generic cubic→tetragonal martensitic transformation in a multilayer system consisting of alternating active and inert layers. The development of the martensitic transformation through nucleation, growth and coarsening of orientation variants is simulated at different levels of the applied stress. The simulated martensitic structure has a complex polytwinned morphology. The simulation predicted such effects as the formation of texture and the stress-induced transformation that are in a general agreement with the experimental observations. The simulation produced realistic stress–strain hysteresis loops, which, in principle, can be used for the formulation of the constitutive equations of the macroscopic mechanics for the active system.     

Dependence of the martensitic transformation and magnetic transition on the atomic order in Ni-Mn-In metamagnetic shape memory alloys

The analysis of atomic order and its influence on the magnetic and structural properties of Ni-Mn-In metamagnetic shape memory alloys has been performed. The effect of the different thermal treatments on the magnetic and structural transformation temperatures, as well as on the thermodynamics of the martensitic transformation, has been made by calorimetric measurements. The evolution of the degree of long-range atomic order with temperature has been determined by neutron diffraction experiments, thus confirming the effect of thermal treatments on the atomic order. Calorimetric and structural results allow thermal treatments to be directly related to atomic order, and to allow the effect of the atomic order on the martensitic and magnetic transformations in Ni-Mn-In alloys to be quantified. The thermodynamics of the martensitic transformation depends on the atomic order as indicated out by its influence on the transformation entropy. In addition, a correlation between the transformation entropy and changes in the magnetic-field-induced transformation temperatures has been found through the evolution of the atomic order.     

热/强磁场耦合时效对富镍Ti-Ni形状记忆合金相变行为的影响

采用示差扫描量热仪(DSC)和透射电子显微镜(TEM),研究热/强磁场耦合时效对Ti-50.6%Ni(摩尔分数)形状记忆合金中Ti3Ni4相析出及相变行为的影响。结果表明:经过500℃、2.5 h热/强磁场耦合时效处理后,Ti-50.6%Ni合金呈现典型的三阶段相变,即晶界附近的B2—R转变、晶内的B2—B19′转变和晶界附近的R—B19′转变;随着磁感应强度的增加,B2—R相变峰和B2—B19′相变峰均向低温区漂移,而R—B19′相变峰位基本保持不变。TEM观察显示,热/强磁场耦合时效后,晶界附近的Ti3Ni4析出相数量明显减少,表明热/强磁场耦合时效可能阻碍了晶粒内部的Ni向晶界周围迁移,抑制了Ti3Ni4相的形核。     

EBSD技术在磁场下等温珠光体相变研究中的应用

讨论了背散射电子衍射(EBSD)技术在获得有关Fe-2Si-3Mn(at%)铸钢磁诱发珠光体(MIP)相变的多相组织信息方面的实际应用。结果表明,面扫描中电子背散射衍射花样(EBSDP)的标定率与MIP的转变量关系极大,转变量越大则标定率越高,面扫描分析数据能更好地反映MIP的分布、取向和结构信息,相变初期形成的珠光体MIP存在微织构<100>//ND;相变早期形成的珠光体量较少,区域扫描中EBSDP标定率低,晶体学取向分析可用各个珠光体(P)团内铁素体片的点分析结果的集合来定性描述,有望获得磁场处理后多晶样品表面的微织构信息。对实际样品的分析还表明:同一P团可由多形核机制形成;相变的初期阶段在原奥氏体晶界上的不连续析出物为铁素体。     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

冷却速度对超高强马氏体钢的马氏体相变起始温度和硬度的影响

采用热膨胀法测量了马氏体试验钢在20~70 ℃/s冷却速度下的马氏体相变起始温度(Ms点温度),并研究了其对试验钢硬度的影响。结果表明,冷却速度的提高可增加奥氏体强度、提高奥氏体的热稳定性,进而降低马氏体相变起始温度。同时,更高的冷却速度容易造成晶格畸变,增大位错密度,还会提高过冷度,细化有效晶粒尺寸,使试验钢的硬度提高。