Switching Autowaves in Materials with Dislocations and Martensitic Transformations

Russian Physics Journal - The paper focuses on the investigation of the strain kinetics at the yield point of materials with shear dislocations and martensitic transformations at a microlevel. In...     

冲击诱发NiTi形状记忆合金相变行为研究

NiTi形状记忆合金的高应变动态响应特性在军事、航空等领域具有重要应用.为研究NiTi合金在动态力学诱导下的相变行为,在不同温区不同冲击速率下,通过轻气炮装置对NiTi合金进行了动态加载实验.利用差示扫描量热仪(DSC),综合物性测量系统分析了冲击波残余效应对NiTi合金相变行为的影响.研究发现:受冲击的样品在第一次DSC热循环中观察到了三个马氏体吸热峰,表现为三步逆马氏体相变,而在第二次热循环中其中两个应力诱发马氏体吸热峰因变形恢复消失.形成两个应力诱发马氏体吸热峰的原因可能是晶粒内部与晶界处的相变过程不同步.受冲击后样品DSC放热峰上出现了一小肩峰,表明可能因中间相(R相)的出现而发生了两步相变,结合电阻测量曲线进一步确认R相的存在,且发现奥氏体相向R相转变以及R相向马氏体相转变这两种相变过程在某一温度范围内可同时进行.同时,文中也具体讨论了不同的冲击加载条件对相变过程的影响.     

Ni4Ti3沉淀相对NiTi形状记忆合金相变行为的影响

采用第二近邻修正型嵌入原子势的分子动力学方法,建立了共格沉淀相与半共格沉淀相块状/柱状模型,模拟了温度诱发相变和应力诱发相变,分析了Ni₄Ti₃沉淀相对Ni Ti形状记忆合金相变行为的影响．结果表明,Ni₄Ti₃沉淀相本征应变诱发的弹性应力场对相变中马氏体变体类型、形核位置、分布等有重要影响.在温度诱发相变时,共格沉淀相促进部分马氏体变体的形核生长,能显著提高Ni Ti超弹性形状记忆合金的马氏体相变开始温度;在应力诱发相变时,Ni₄Ti₃沉淀相使马氏体早于无沉淀相区域形核,导致了相变应力降低、抑制了马氏体解孪,减小了应力-应变曲线的滞回环.     

Heterogeneous nucleation of martensite at dislocations and the martensitic-transformation kinetics in shape memory alloys

The martensite phase formation in elastic fields of isolated screw and edge dislocations, as well as in planar clusters of like-sign dislocations and in a two-dimensional network of opposite-sign edge dislocations, is quantitatively analyzed within the theory of smeared martensitic transitions. The heterogeneous nucleation of martensite at dislocations is shown to increase the characteristic temperature of the martensitic transition and its temperature smearing.     

Synergetics of martensitic structures in shape memory crystals

The kinetic mechanism of the formation of spatially inhomogeneous martensitic structures in shape memory alloys is discussed theoretically. In terms of synergetics, the formation of these structures is the result of the self-organization of elementary transforming volumes associated with the motion of transformation dislocations along interphase boundaries. In contrast to the pure thermodynamic approach based on the Ginzburg-Landau theory of phase transformations, the kinetic method allows one to determine the proper physical scale of the phenomenon and to elucidate the effect of structural factors on the transformation parameters.     

A phase field model for the formation and evolution of martensitic laminate microstructure at finite strains

The extraordinary properties of shape-memory alloys stem from the formation and evolution of their complex microstructure. At lower temperatures, this microstructure typically consists of martensitic laminates with coherent twin boundaries. We suggest a variational-based phase field model at finite strains for the formation and dissipative evolution of such two-variant martensitic twinned laminate microstructures. The starting point is a geometric discussion of the link between sharp interface topologies and their regularisation, which is connected to the notion of Γ-convergence. To model the energy storage in the two-phase laminates, we propose an interface energy that is coherence-dependent and a bulk energy that vanishes in the interface region, thus allowing for a clear separation of the two contributions. The dissipation related to phase transformation is modelled by use of a dissipation potential that leads to a Ginzburg–Landau type evolution equation for the phase field. We construct distinct rate-type continuous and finite-step-sized incremental variational principles for the proposed dissipative material and demonstrate its modelling capabilities by means of finite element simulations of laminate formation and evolution in martensitic CuAlNi.     

Relationship between acoustic emission energy and the kinetics of martensite formation in plain carbon steels

The kinetics of the martensitic transformation in Fe-0.80C determined on the basis of dilatometry data is compared to the acoustic emission (AE) energy accompanying the transformation in the same steel reported in a previous study. The discrepancy between the AE energy and the volume fraction of martensite indicates that the mechanism for the generation of AE during the martensitic transformation is not solely dependent on the kinetics and the associated moving interfaces as suggested in previous studies. During the growth of martensite, slip takes place in order to relieve internal stresses, and dislocations are thought to be mainly introduced in the relatively soft austenite matrix. The quantitative analysis in this study demonstrates that the AE energy generated per unit time is a function of both the transformation kinetics and the volume fraction of remaining austenite. This strongly indicates that the moving dislocations associated with the plastic deformation of the austenite surrounding the as-formed martensite are the dominant sources of the generated acoustic waves. This improved AE source model is consistent with the well-accepted mechanism of AE during conventional plastic deformation due to an external load.     

Mechanism for the bidirectional shape memory effect in titanium nickelide crystals

A mechanism for the bidirectional and all-round shape memory effects observed in titanium nickelide crystals is discussed quantitatively by using the theory of diffuse martensitic transformations (DMTs). These effects are associated with an anisotropic distribution of Ti₃Ni₄ particles, which arises in bent crystals subjected to annealing followed by relaxation of coherent microstresses produced by the particles. Using the DMT theory, the influence of the stepwise B2 → R → B19′ phase transition on the magnitude and sign of the radius of curvature of a thin strip of titanium nick-elide is calculated and the conditions are determined under which the bidirectional and all-round shape memory effects occur depending on structural factors and the geometrical parameters of the strip.     

High-temperature shape memory effect and the B2-L10 thermoelastic martensitic transformation in Ni-Mn intermetallics

The properties and structure of the martensitic phase of alloys with a near-stoichiometric equiatomic Ni₅₀Mn₅₀ composition, as well as martensitic transformations in them, are investigated in a wide temperature range by measuring the resistivity and thermal expansion coefficient and applying transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. It is found that Ni₅₀Mn₅₀ and Ni₄₉Mn₅₁ alloys experience the B2 → L1₀ highly reversible thermoelastic martensitic transformation and its related high-temperature deformation of the transformation and shape memory effect. Critical temperatures, volume (ΔV/V = ?1.7%) and linear size effects attributed to the direct and reverse martensitic transformations, and the high-temperature dependences of the martensitic and austenite lattice parameters are determined. It is found that the morphology of tetragonal L1₀ martensitic represents a hierarchy of thin coherent sheets of submicrocrystallites and nanocrystallites with plane near-{111}L1₀ habit boundaries, the crystallites being pairwise twinned according to the {111}〈11 $\bar 2$ 〉 _L10 ∥ {011}〈-1 $\bar 1$ 〉 _B2 twinning shear scheme.     

Surface tension and energy in multivariant martensitic transformations: phase-field theory, simulations, and model of coherent interface

The Ginzburg-Landau theory for multivariant martensitic phase transformations is advanced in three directions: the potential is developed that introduces the surface tension at interfaces; a mixed term in gradient energy is introduced to control the martensite-martensite interface energy independent of that for austenite-martensite; and a noncontradictory expression for variable surface energy is suggested. The problems of surface-induced pretransformation, barrierless multivariant nucleation, and the growth of an embryo in a nanosize sample are solved to elucidate the effect of the above contributions. The obtained results represent an advanced model for coherent interface.     

Diffuse ferroelectric phase transitions in cubically sabilized perovskites

Typical examples of ferroelectrics with diffuse phase transitions (relaxor ferroelectrics), like Pb(Mg_1/3Nb_2/3)O₃ are actually non transforming. The paraelectric phase is fully stablized against a ferroelectric phase transition in this case. A phase transition can be induced, however, by an electric field with appropriate orientation below the temperature of the dielectric constant maximum. The analogy with stress-induced martensitic phase transitions in metallic alloys is pointed out. Pecularities of the properties and of the polarization reversal of such systems are demonstrated. Actual diffuse ferroelectric phase transitions in disordered solid solutions and mixed compounds with a partially stabilized parent phase are compared with athermal martensitic transformations. With particular regard to technical ceramics based on PZT, the influence of interfaces between transformed regions and remnants of the parent phase which have to distinguished from domain walls, and of the reduced stability of the ferroelectric phase on the properties of these systems is discussed. Some effects usually explained solely by domain processes may be understood also from this point of view.

Stabilization of a parent phase against an order-disorder-type phase transition is supposed to be caused by glass-like freezing caused by inelastic cooperative interactions between disordered molecular groups.     

Heterogeneous nucleation of martensite on precipitates and the martensitic-transformation kinetics in shape memory alloys

The formation of martensite in an elastic stress field near a disk-shaped coherent precipitate is discussed in terms of the theory of diffuse martensitic transformations. The heterogeneous martensite nucleation on precipitates is found to increase the characteristic martensitic-transformation temperature, which increases linearly with the volume density of precipitates. The theoretical results are illustrated quantitatively using the example of the B2 → R phase transition in titanium nickelide alloys.     

Martensitic structure of a Cu-Al-Ni alloy after a single cyclic change in the temperature and the action of reactive stresses

The martensitic transformations in a Cu-13.4 wt % Al-4.0 wt % Ni alloy subjected to a single cyclic change in the temperature in the range 293–680 K under conditions of constrained shape-memory deformation are studied by differential scanning calorimetry. These martensitic transformations are found to be closely related to the temperature dependences of the reactive stresses generated in constrained alloy samples during a single heating-cooling cycle. The substantial change in the behavior of these dependences during heating to a temperature above 600 K is caused by the strong effect of the decomposition of the β-phase solid solution on the parameters of the martensitic transformations in this alloy.     

Acoustic emission during thermoelsatic martensitic transformations in titanium nickelide upon a fixed strain

The influence of the strain fixed during a direct martensitic transformation on the acoustic emission in the temperature interval including intervals of martensitic transformations is investigated. The effect of the martensitic phase stabilization is established and its influence on a decrease in the acoustic emission energy during thermal cycling of martensitic transformations upon a fixed strain is demonstrated. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 12–18, March, 2009.     

Analysis of acoustic emission signals originating from bainite and martensite formation

In this paper the characteristics of acoustic signals generated during bainite and martensite formation are studied. The results are discussed in a semi-quantitative manner, since a thorough quantitative analysis of the signals is not feasible because of the limited frequency bandwidth of the system and the effect of internal sample reflections on the signal. The frequency spectra of acoustic emission signals are interpreted using a dislocation source model adopted from acoustic emission studies of plastic deformation. It is assumed that the predominant source of acoustic emission (AE) during displacive transformations is the movement of dislocations, i.e. the slip taking place during growth in order to relieve internal stresses. The results show that the mean frequency of AE signals generated during bainite formation is significantly larger than that of martensitic AE signals. This difference in the spectral density of the AE signals can be attributed to the difference in interface motion of the two transformations, and the consequent different behaviour of the dislocations involved.     

晶体相场方法模拟纳米孪晶结构

应用双模晶体相场模型,模拟共格纳米孪晶结构.结果表明:球状晶粒生长成的共格孪晶片层,在共格面上的原子排列有变形,容易出现位错;条状晶粒凝固生长成的共格孪晶界,比用球状晶粒长大生成的共格孪晶界的原子排列整齐.应用晶体相场模型,可以精确计算纳米孪晶带的厚度,设计和控制带内的原子层数,实现人工操纵纳米共格孪晶片层结构,指导实验研究纳米孪晶结构及其与性能的关系.     

Mechanisms of deformation localization and mechanical twinning under the conditions of phase instability of a crystal in stress fields

A new mechanism of the deformation and reorientation of a crystal has been studied experimentally and theoretically. This mechanism, which is effective in strain localization mechanical twinning bands of metal alloys and intermetallides, is a mechanism of dynamic phase (direct plus reverse martensitic) transformations in fields of high local stresses. The features of the reorientation and the defect substructure in these bands are discussed using electron microscopy data. With models of martensitic transformations based on the concept of cooperative thermal vibrations of extended coherent objects in crystals, the atomic mechanisms of direct plus reverse transformations are analyzed and the reorientation matrices (vectors) and distortion tensors are calculated for some (fcc bcc fcc, bcc hcp bcc) variants of these transformations. The carriers and nature of the above deformation mechanism and the principal physical effects underlying this mechanism are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 28–48, August, 2004.     

Thermally reversible deformation of a TiNi alloy after the end of the thermal and electrical processes induced by the B2 ⟷ R martensitic transformations

The dilatometric measurements of a TiNi-based alloy during thermal cycling through the temperature range of the B2 ? martensitic transformations performed simultaneously with electrical resistance measurements and supplemented with differential scanning calorimetry analysis demonstrate two-stage deformation. At the first stage, thermally reversible forming is accompanied by the thermal effects and the change in the electrical resistance, which are characteristic of these transformations. At the second stage, deformation proceeds in the absence of thermal effects at an unchanged electrical resistance. This behavior is explained using the well-known two stages in the B2 ? transformation.     

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide under isothermal loading

Acoustic emission during thermoelastic martensitic transformations in titanium nickelide is investigated. Phase transitions are initiated by loading the specimen to 200 MPa under isothermal conditions. It is found out that deformation buildup and acoustic emission in the loading-unloading cycles are observed in the first cycle only, during further cycles the acoustic emission is comparable to the background, while the deformation buildup and recovery are not associated with martensitic transformations. It is shown that recovery of the deformation built up during loading occurs due to heating to 600 °C, with the major part of accumulated deformation undergoing recovery already at 250 °C and recovery of its minor part observed at 400 °C. This behavior of acoustic emission and accumulation and recovery of deformation provide evidence of martensitic phase stabilization during cycling of martensitic transformations under conditions of thermo-mechanical cycling. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 89–94, February, 2008.     

Modelling of hysteresis loops taken during the stress- and temperature-induced martensitic transformations

In this article, it has been shown that the process of reconfiguration of the crystal defects system noticeably contributes to the width of the stress–strain and strain–temperature hysteresis loops taken during the stress- and temperature-induced martensitic transformations of the shape memory alloys. It has been demonstrated that the contribution of the defects system to the hysteresis width strongly depends on the alloy temperature and the transformation cycle duration. It has been shown that the hysteresis effect can be observed not only in the course of the first-order phase transition of martensitic type, but also in the course of the gradual deformation of crystal lattice. The obtained results are applicable to the ferroelastic phase transitions in the different crystalline solids.