Acoustic Emission in Martensitic Transformations in Alloys under Mechanical Loading

In the alloys undergoing thermoelastic martensitic transformations under mechanical loading, accumulation and recovery of deformation is observed. The acoustic emission accompanying the martensitic transformations is a reflection of peculiar features of deformation-induced behavior of an alloy in the course of its transformation. The anomalous acoustic effect is correlated with the buildup of reversible deformation but is not so with that of the irreversible deformation.     

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.     

Microplastic deformation of polycrystals during cyclic loading

A model of microplastic deformation of polycrystals during zero-start cyclic loading with tensions lower than the yield strength is proposed according to which during cycling, thermally activated movement of dislocations occurs under conditions of stress relaxation. Based on this model and the statistical theory of polycrystalline microdeformation, the accumulation of microplastic deformation is theoretically described as a function of the number of loading cycles and the stress amplitudes. It is theoretically proved that in the cycling process the microplastic deformation that accumulates over one cycle decreases as the number of cycles increases; up to the macroscopic elastic limit it is independent of the stress amplitude, and then sharply increases. Agreement of the theory with experimental data for spring alloys is observed in the density of mobile dislocations, which decreases during cycling.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 29–34, March, 1990.     

Behavior of the nickel-titanium alloys with the shape memory effect under conditions of shock wave loading

The behavior of the Ti_51.1Ni_48.9 and Ti_49.4Ni_50.6 alloys with shape memory effects has been studied under submicrosecond shock wave loading in the temperature range from −80 to 160°C, which includes both the regions of the stable state of the specimens in the austenite and martensite phases and the regions of thermoelastic martensitic transformations. The grain size of the studied alloys varies from initial values 15–30 to 0.05–0.30 μm. The dependences of the dynamic elastic limit on the temperature and on the elemental composition are similar to the dependences of the yield stress of these alloys under low strain rate loading. The rarefaction shock wave formation as a consequence of the pseudoelastic behavior of the alloy during a reversible martensitic transformation has been revealed. A decrease in the grain size leads to an increase in the dynamic elastic limit and decreases the temperatures of martensitic transformations.     

High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling

Creep tests were performed on MC2 single crystal superalloy at 950°C/200?MPa and 1150°C/80?MPa under isothermal and thermal cycling conditions with a tensile axis along the [0?0?1] direction. It was found that the thermal cycles strongly affect the creep behavior at 1150°C but not at 950°C. This was related to the repetitive precipitation and dissolution of small γ′ rafts at the higher temperature, as revealed by quantitative characterization of the γ/γ′ microstructure. The dislocation microstructure exhibits similar trends in all the tested conditions, with a very high activity of a[1?0?0]-type dislocations climbing through the rafts. Such climbing dislocations constitute a recovery process for the deformation active system. It appears that the density of a[1?0?0] dislocations, and not their climb velocity or diffusion rate, is the key parameter for the control of creep rate. The thermal cycles, which imply the creation and subsequent dissolution of rafts, provided new dislocations, which explains the acceleration of creep observed under such conditions.     

Acoustic emission from periodically perturbed systems: SOC and predictability

The statistics of acoustic emission (AE) from as diverse processes as volcanic activity (Stromboli, Italy) and martensitic transformations driven by thermal cycles, is shown to verify the paradigm of self-organized criticality. However, catastrophic event predictability both in laboratory (the onset of martensitic transformations) and in on-site applications (volcanic seisms and explosions) through the analysis of historical AE series, is not ruled out as long as the emitting samples are subjected to (quasi)periodic low-frequency/large-scale dynamics.     

Neutron-irradiation-induced shape memory effect in a TiNi alloy

The recovery of inelastic strains in Ti-Ni alloy samples irradiated in a nuclear reactor under isothermal conditions was studied. Before irradiation, the cylindrical samples were compressed to a residual strain of 3–6% in the martenstici state at room temperature. The samples were irradiated at a temperature of 45°C, which does not exceed the temperature of the onset of the reverse martensitic transformation A _S . Irradiation with a fastneutron fluence of 5 × 10²⁰ cm^?2 is established to result in the recovery of the residual strain. The value of the recoverable strain is comparable to that observed under the conditions of the shape memory effect on heating of the deformed alloy and even somewhat exceeds it. The obtained data show that neutron irradiation can induce the shape-memory effect in the TiNi alloy. This is due to a decrease in the temperatures of the martensitic transformations under irradiation.     

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.     

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.     

Studying the dynamics of microdefect growth in carbon fiber reinforced plastics under mechanical loading by means of ultrasonic microscopy

Strength characteristics of carbon fiber reinforced plastics (CFRPs) are investigated by nondestructive means as microstructural changes in a material’s bulk under external mechanical loads. CFRP microstructure is studied experimentally via pulsed ultrasonic microscopy at the level of mechanical deformation resulting in degradation of a material’s properties. The process of composite deformation is studied by means of stepped stretching. Acoustic emissions are used to identify the stage preceding final destruction (the accumulation of microcracks, fibers breaking, and delamination) as an indicator of a material’s degradation. Pulse acoustic microscopy is used to observe the accumulation of microcracks in individual layers of a material. To study the behavior of a CFRP microstructure upon mechanical loading, tensile stress was applied to samples with cross-ply packing of fibers (0°, 90°) and (45°, ?45°). It is shown that the brittle fracturing of reinforcing fibers is typical of CFRPs with fiber orientation (0°, 90°), and is accompanied by growing areas of stress concentration and a rise in of acoustic emission activity, with a subsequent increase in the signal energy and the formation of extensive interlaminar delamination. Acoustic emission shows a low level of activity for CFRP samples with fiber orientation (45°, ?45°), which is accompanied by the formation of structural microdefects that are clearly visible in acoustic images.     

Effect of preliminary deformation on the fine structure of a TiNi-based alloy in the premartensitic region

Results are given of an in situ X-ray diffraction study of the crystal-lattice state of a TiNi-based alloy during deformation under the effect of an external stress in the pretransition region preceding the B2-R-B19′ martensitic transformations. The pretransition state preceding martensitic transformations in the alloy under study was found to manifest itself in a specific behavior of the lattice parameter and thermal expansion coefficient in the B2 phase. The magnitude of the thermal expansion coefficient in the B2 phase nonmonotonically depends on the amount of deformation and the applied stress.     

Influence of charge rate on the cycling degradation of LiFePO<Subscript>4</Subscript>/mesocarbon microbead batteries under low temperature

The lithium-ion batteries show extremely poor cycling performance at low temperature. The main degradation mechanism is not clear. To address the fading mechanism, the cycling degradation of commercial LiFePO₄/mesocarbon microbead (MCMB) batteries under various charge rate (1/10C, 1/3C, 1/2C, and 1C) at ?10 °C is systematically investigated using nondestructive tests combining with post-mortem analysis. The low-temperature charging under high charge rates of 1/3C, 1/2C, and 1C results in severe lithium plating, which leads to extremely serious capacity loss. In contrast, no lithium plating occurred under low charge rate of 1/10C. The lithium plating on the anode surface leads to consumption of active lithium ions and electrolyte, which causes the capacity decay and increases ohmic resistance (R _b) with cycling number under high charge rates. The lithium plating on the anode surface is partially reversible, which brings about the capacity recovery of batteries after 80 cycles at 25 °C. The above results are proved by the followed post-mortem measurements. The evolution of the surface morphologies of MCMB electrodes upon cycling shows that a layer composed of rod-like lithium is formed on the anode surface.     

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.     

Quasi-equlibrium description of acoustic emission in martensitic transformations

Acoustic emission is used to study martensitic transformations in metal systems within the framework of the quasiequilibrium theory. In this approach, the dissipative part in the balance equation of driving forces for the matertensitic transformation is represented as the sum of contributions from heat dissipation and “nonchemical” energy dissipation due to acoustic emission. The acoustic contribution is defined as dynamic relaxation which in its turn is related to the transformation kinetics. Examples of events responsible for the production of acoustic radiation are the “microexplosive” nucleation-collapse of martensite crystals, single acts of their stick-slip displacement, and plastic relaxation of elastic transformation stresses. Siberian Physicotechnical Institute at Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 72–79, October, 1999.     

Deformation-and fracture-induced acoustic emission in rocks

The informative parameters and the behavior of the acoustic emission that accompanies the deformation and fracture of rock samples under various conditions of mechanical loading, thermal action, and humidification are considered. The main directions of the development of models for the acoustic emission in geological materials are analyzed.     

内应力对Mn2NiGa铁磁形状记忆合金的结构、相变和磁性能的影响

通过施加压应力的方法,在铁磁形状记忆合金Mn₂NiGa中引入残留内应力,研究了内应力对 Mn₂NiGa材料的结构、相变和磁性能的影响.研究发现,加压过程使材料发生了塑性形变,在材料内部引入了大量的位错缺陷.卸载后保留的位错缺陷在材料中造成了残留的内应力,导致了马氏体相变温度大幅度提高, 使原本室温下的母相转变成了马氏体相.测量到导致样品转变成马氏体的阈值压应力为1.0 GPa.加压形成的马氏体中的残留内应力将矫顽力从低于50 Oe提高到350 Oe.残留内应力在730 K的热处理中由于位错缺陷的消失而得以消除,样品实现了马氏体逆相变.如此高的逆相变温度使得 Mn₂NiGa马氏体的居里温度测量成为可能,获得了530K的数值.     

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.     

In-situ strain localization analysis in low density transformation-twinning induced plasticity steel using digital image correlation

The effect of deformation temperature on the strain localization has been evaluated by an adapted digital image correlation (DIC) technique during tensile deformation. The progress of strain localization was traced by the corresponding strain maps. The electron backscatter diffraction analysis and tint etching technique were utilized to determine the impact of martensitic transformation and deformation twinning on the strain localization in both elastic and plastic regimes. In elastic regime the narrow strain bands which are aligned perpendicular to the tension direction were observed in temperature range of 25 to 180 °C due to the stress-assisted epsilon martensite. The strain bands were disappeared by increasing the temperature to 300 °C and reappeared at 400 °C due to the stress-assisted deformation twinning. In plastic regime strain localization continued at 25 °C and 180 °C due to the strain-induced alfa-martensite and deformation twinning, respectively. The intensity of plastic strain localization was increased by increasing the strain due to the enhancement of martensite and twin volume fraction. The plastic strain showed more homogeneity at 300 °C due to the lack of both strain-induced martensite and deformation twinning.     

High-strength and ductile glassy-crystal Ni–Cu–Zr–Ti composite exhibiting stress-induced martensitic transformation

We present a Ni-based crystal-glassy composite material having superior strength paired with a considerable ductility of 15%. The formation of a metastable crystalline phase in a glassy matrix during solidification has been proven capable of promoting a strain-induced martensitic transformation leading to enhanced plasticity under compression at room temperature. Underlying mechanisms of plastic deformation are discussed in terms of the interplay between dislocation slip in the crystalline phase and shear deformation in the glassy matrix. We suppose that the strain-induced martensitic inclusions serve as strong barriers for shear band propagation, promoting shear band branching and multiple shear band formation, thus extending the ductility and preventing a premature brittle fracture. The acoustic emission technique has been employed to clarify the kinetics of transformation and stages of plastic deformation.     

In-situ synchrotron X-ray diffraction study of stress-induced phase transformation in Ti50.1Ni40.8Cu9.1 thin films

Stress-induced martensitic transformation of as-sputtered and post-annealed Ti_50.1Ni_40.8Cu_9.1 thin films was investigated using in-situ synchrotron X-ray diffraction (S-XRD) technique. For the as-deposited film, in-situ S-XRD analysis showed a martensitic transformation from parent phase to martensite during initial loading, followed by reorientation of martensite variants via detwinning. This detwinning process induced a strong 〈0 2 0〉 fiber texture along the loading direction and a strong 〈0 0 2〉 fiber texture perpendicular to the loading direction. For the 650 °C annealed film, there is only elastic deformation, followed by a martensitic transformation during deformation.