Effects of high-pressure heat treatment on the solid-state phase transformation and microstructures of Cu_(61.13)Zn_(33.94)Al_(4.93) alloys

The phase transformation activation energy of the Cu61.13Zn33.94A14.93 alloys, which were treated at 4 GPa and 700 ℃ for 15 minutes, was calculated by means of differential scanning calorimetry curves obtained at various heating and cooling rates. Then, the effects of high-pressure heat treatments on the solid-state phase transformation and the microstructures of Cu61.13Zn33.94A14.93 alloys were investigated. The results show that high-pressure heat treatments can refine the grains and can change the preferred orientation from （111） to （200） of α phase. Compared with the as-cast alloy, the sample with high-pressure heat treatment has finer grains, lower β＇→β and/β→β＇ transformation temperature and activation energy. Furthermore, we found that high cooling rate favours the formation of fine needle-like α phase in the range of 5-20℃/min.

Effect of high Pressure on the Solid-State Phase Transformation Microstructure of Cu-Zn Alloy

The solid-state phase transformation microstructure of Cu-Zn alloy under different high pressure were investigated by means of SEM and XRD. The results show that the α phase with smaller grain size, different shape and random distribution appears in the Cu-Zn alloy, when the solid-state phase transformation generation in the Cu-Zn alloy under 25～750 ℃ and 3～6 GPa high pressure, and the volume fraction of transformation phase decreases with increasing pressure, under high pressure (6 GPa), the changes of microstructure of Cu-Zn alloy is not obvious. In addition, the effect of high pressure on the solid-state phase transformation microstructure of Cu-Zn alloy was discussed.

SOLID-STATE PHASE TRANSFORMATION OF WELDED METAL IN CONTROL OF WELDING DISTORTION PROCESS

Based on the tests of a build-up welding at plate edge (BWPE) and amulti-layer build-up welding on plate (MBWP), the article studies on the solid-state phasetransformations which affect welding distortion process and on the influence rule of transformationstarting temperature (TST) of welded metal to the welding residual distortion. The weldingdistortion can be decreased or controlled by the transformation volume expansion caused bysolid-state phase transformation of welded metal during the cooling. The test results of BWPE showthat when TST is at about 190 deg C, the bending distortion of welded specimen is the smallest, andits displacements at free end are decreased to 58 percent and 67 percent compared with those ofconventional welding electrodes A102 and E5015, which TST are less than room temperature and equalto 758 deg C respectively. The test results of MBWP show that when TST were at 100 approx 250 deg C.the welded specimen would appear reversible bending distortion compared with those of A102 andE5015. The maximum deflection value of reversible bending distortion in 8 mm thick plate is -2.94 mmat about 170 deg C of TST. The test results provide a valuable method to decrease or to controlwelding residual distortion.

Cooling rate effects on the structure and transformation behavior of Cu-Zn-Al shape memory alloys

Different fragments of a hot-rolled and homogenized Cu–Zn–Al shape memory alloy（SMA） were subjected to thermal cycling by means of a differential scanning calorimetric（DSC） device. During thermal cycling, heating was performed at the same constant rate of increasing temperature while cooling was carried out at different rates of decreasing temperature. For each cooling rate, the temperature decreased in the same thermal interval. During each cooling stage, an exothermic peak（maximum） was observed on the DSC thermogram. This peak was associated with forward martensitic transformation. The DSC thermograms were analyzed with PROTEUS software： the critical martensitic transformation start（Ms） and finish（Mf） temperatures were determined by means of integral and tangent methods, and the dissipated heat was evaluated by the area between the corresponding maximum plot and a sigmoid baseline. The effects of the increase in cooling rate, assessed from a calorimetric viewpoint, consisted in the augmentation of the exothermic peak and the delay of direct martensitic transformation. The latter had the tendency to move to lower critical transformation temperatures. The martensite plates changed in morphology by becoming more oriented and by an augmenting in surface relief, which corresponded with the increase in cooling rate as observed by scanning electron microscopy（SEM） and atomic force microscopy（AFM）.

Effect of heat treatment on the transformation behavior and temperature memory effect in TiNiCu wires

The effect of heat treatment on the phase transformation behavior of TiNiCu shape memory alloy wires and the temperature memory effect in this alloy were investigated by the resistance method. These results showed that with increasing annealing temperature and annealing time, the phase transformation temperatures of TiNiCu wires were shifted to higher temperatures in the heating and cooling process. It was also found that incomplete thermal cycles, upon heating the TiNiCu wires, which were arrested at a temperature between the start and finish temperatures of the reverse martensite transformation, could induce a kinetic stop in the next complete thermal cycle. The kinetic stop temperature was closely related to the previous arrested temperature. This phenomenon was defined as the temperature memory effect. The result of this study was consistent with the previous report on the phenomenon obtained using the differential scanning calorimetry method, indicating that temperature memory effect was a common phenomenon in shape memory alloys.

SNAP-ACTION PHASE TRANSFORMATION IN TWO-WAY SHAPE MEMORY Ni-Ti ALLOY

The snap-action behavior of a Ni-Ti alloy disc which is controlled by combination of a nonlinear stress field and temperature has been studied.After treatment for two-way shape memory,all shape memory strain of snap-action finishes abruptly at a certain temperature within an interval of less than 1 ms.The results of resistance measurement and in-situ X-ray diffraction indicate that the snap-action strain is mainly resulted from the snap-action β (?)R transformation.

EFFECT OF HEAT TREATMENTS ON THE MICROSTRUCTURE AND TRANSFORMATION CHARACTERISTICS OF TiNiPd SHAPE MEMORY ALLOY THIN FILMS

Microstructure and phase transformation behaviors of the film annealed at different temperatures were studied by X-ray diffractometry （XRD）, transmission electron microscopy and differential scanning calorimeter （DSC）. Also tensile tests were examined. For increasing annealed temperature, multiple phase transformations, transformations via a B19-phase or direct martensite/austenite transformtion are observed. The TiNiPd thin film annealed at 750℃ had relatively uniform martensite/austenite transformtion and shape memory effect. Martensite/austenite transformtion was also found in strain-temperature curves. Subsequent annealing at 450℃ had minor effect on transformation temperatures of Ti-Ni-Pd thin films but resulted in more uniform transformation and improved shape memory effect.

Magnetocaloric and barocaloric effects in a Gd_5Si_2Ge_2 compound

The first-order phase transition in GdsSi2Ge2 is sensitive to both magnetic field and pressure. It may indicate that the influences of the magnetic field and the pressure on the phase transition are virtually equivalent. Moreover, theoretical analyses reveal that the total entropy change is almost definite at a certain Curie temperature no matter whether the applied external field is a magnetic field or a pressure. The entropy change curve can be broadened dramatically under pressure, and the refrigerant capacity is improved from 284.7 J/kg to 447.0 J/kg.

Magnetic properties and magnetocaloric effects in HoPd intermetallic

A large reversible magnetocaloric effect accompanied by a second order magnetic phase transition from PM to FM is observed in the Ho Pd compound. Under the magnetic field change of 0–5 T, the magnetic entropy change-ΔS max M and the refrigerant capacity RC for the compound are evaluated to be 20 J/（kg·K） and 342 J/kg, respectively. In particular,large-ΔS max M（11.3 J/（kg·K）） and RC（142 J/kg） are achieved under a low magnetic field change of 0–2 T with no thermal hysteresis and magnetic hysteresis loss. The large reversible magnetocaloric effect（both the large-ΔS M and the high RC）indicates that Ho Pd is a promising material for magnetic refrigeration at low temperature.

Magnetic properties and magnetocaloric effects in Er_(1-x)Gd_xCoAl intermetallic compounds

The magnetism and magnetocaloric effect in Er1-xGdxCoAl（x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 1） were investigated. The Er1-xGdxCoAl compounds were synthesized by arc melting. With the increasing Gd content, the N′eel temperature（T N）linearly increases from 14 K to 102 K, while the magnetic entropy change（-？S M） tends to decrease nonmonotonously.Under the field change from 0 T to 5 T, the-？S M of the compounds with x = 0.2–1 are stable around 10 J/kg·K, then a cooling platform between 20 K and 100 K can be formed by combining these compounds. For x = 0.6, 0.8, 1.0, the compounds undergo two successive magnetic transitions, one antiferromagnetism to ferromagnetism and the other ferromagnetism to paramagnetism, with increasing temperature. The two continuous magnetic transitions in this series are advantageous to broaden the temperature span of half-peak width（δT） in the-？S M–T curve and improve the refrigeration capacity.

Effects of an ultra-high magnetic field up to 25 T on the phase transformations of undercooled Co-B eutectic alloy

While there have been multiple recent reports in the literature focusing on the effects of magnetic field on the phase transformation behaviors,the research conducted with an ultra-high magnetic field greater than 20 T is still preliminary.In the current study,the structure evolution of Co-B alloys are experimentally studied with undercooling.The effects of a 25 T magnetic field on the solidification behavior and the subsequent solid-state phase transformation behavior have been investigated.The 25 T magnetic field is confirmed to have little effect on the homogeneous nucleation,but have some influence on the heterogeneous nucleation of Co_(3) B and Co_(23)B6 phases by modifying the wetting angleθ.The decomposition of Co_(23)B6 phase in the subsequent cooling process can be effectively suppressed by applying the 25 T magnetic field.The present work might be helpful for not only theoretically understanding the influence of ultra-high magnetic field on the phase transformation behaviors but a potential technology of field-manipulation of magnetic materials.

Application of phase-field modeling in solid-state phase transformation of steels

Solid-state phase transformation is usually associated with excellent mechanical properties in steel materials.A deep understanding of the formation and evolution of phase structure is essential to tailor their service performance.As a powerful tool for capturing the evolution of complex microstructures,phase-field simulation quantitatively calculates the phase structures evolution without explicit assumptions about transient microstructures.With the development of advanced numerical technology and computing ability,phase-field methods have been successfully applied to solid-state phase transformation in steels and greatly support the research and development of advanced steel materials.The phase-field simulations of solid-phase transformation in steels were summarized,and the future development was proposed.

Solid-state phase transformation of TC11 titanium during unstable thermal cycling in laser melting deposition process

Thermal cycling procedure during laser additive manufacturing (LAM) process causes the appearance of bright and dark patterns on the etched surface of TC11 alloy components. The formation mechanisms of these patterns and the solid-state transformation related to LAM process are systematically investigated with the predication of temperature fields using the finite element software ABAQUS. The results indicate that by increasing subsequent thermal cycles, the peak temperatures for every cycle decrease. When peak temperatures are above Tβ(phase transition temperature of β phase), which is 1010℃ in TC11 alloy, no pattern is observed. Meanwhile, a decrease in peak temperature leads to appearance of an ultrafine basket-weave α+β microstructure (dark contrast) with gradually increased amount of α colonies in the alloy. A special bimodal microstructure with ‘fork-like'α lamella appears in the layer when the peak temperatures of thermal cycles firstly fall into α+β dual-phase region. And this special bimodal microstructure gives a bright contrast and only appears at the region where the peak temperatures are below 970℃, leaving the rest region with a dark contrast. With the continuous increase in thermal cycles in α+β dual-phase region,α lamella gradually coarsens. After five thermal cycles in α+β two-phase region, no further changes in microstructure are observed, and the morphologies of α lamella in dark and bright regions are almost the same but with different amounts of α phase.

Influence of recovery heating rate on shape memory effect in up-quenched Cu-Al-Mn alloy

The effect of recovery heating rate on shape memory effect of the up-quenched Cu-8.88Al-10.27Mn（mass fraction, %） alloy was investigated by optical microscopy, electron transmission microscopy（TEM） and electrical resistivity measurement. It is found that the shape recovery rate decreases as the heating rate decreases. It can reach 75% when the heating rate is 20 ℃/min, while it is only 8% when the heating rate is 1 ℃/min. In situ microstructure observation indicates that the dependence of shape memory effect on recovery heating rate is caused by the stabilization of twinned martensite induced by deformation. The analysis of electrical resistivity shows that the stabilization of twinned martensite may be ascribed to formation of compound defects of vacancies and dislocations at the boundaries of twinned martensite during the slow heating. The compound defects prevent the reverse transformation of twinned martensite.

Martensitic transformation,magnetocaloric effect and phase transition strain in Ni50Mn36-xGexSn14 Heusler alloys

In present work,the effect of Ge substitution for Mn on crystal structure and martensitic transformation was carefully investigated in magnetic shape memory Ni_(50)Mn _(36-x)Ge_(x)Sn_(14)(x=0,1) alloys.From X-ray diffraction(XRD) patterns,it can be found that each sample possesses cubic austenitic structure(L21) at room temperature and the main peak(220) shifts to higher degree with Ge doping,indicating that the cell volume of austenitic phase shrinks.With Ge content increasing,martensitic transformation(MT),temperature shifts to higher temperature region and the difference of magnetization between martensitic and austenitic phases(AM) also increases.In addition,the magnetocaloric effect(MCE) and phase transition strain(ΔL/L) were investigated in Ni50Mn35-GeSn14alloy.The maximal magnetic entropy change(ΔSm) associated with martensitic transition is 3.9 J·kg^(-1)-K^(-1)with applied magnetic field change of 5 T and the maximal ΔL/L reaches 0.18% in this alloy.

Microstructure,compression property and shape memory effect of Ru-Nb high temperature shape memory alloy

The microstructure,phase transformation,compression property and strain recovery characteristics of equiatomic Ru-Nb high temperature shape memory alloy were investigated by means of optical microscope,X-ray diffraction(XRD),differential scanning calorimetry(DSC),compression tests and transmission electron microscopy(TEM).When cooling the alloy specimen from high temperature to room temperature,β(parent phase)→β’(interphase)→β″(martensite) two step phase transformation occurs.The microstructure at room temperature shows regularly arranged band morphology with the monoclinic crystal structure.The twinning relationship between the martensite bands was determined to be(101) Type I.Reorientation and of the martensite bands inside the variant and dislocation were found during compression at room temperature.The maximum complete recovery strain is about 1.5%.

Influences of La and Ce doping on giant magnetocaloric effect of EuTiO

Giant reversible magnetocaloric effects and magnetic properties in Euo.9Ro.lTiO3 （R = La, Ce） are investigated. The antiferromagnetic ordering of pure EuTiO3 can significantly change to be ferromagnetic as substitution of La （x = 0.1） and Ce （x = 0.1） ions for Eu2＋ ions. The values of -ASM and RC are evaluated to be 10.8 J/（kg.K） and 51.8 J/kg for Euo.gCeo.lTiO3 and 11 J/（kg.K） and 39.3 J/kg for Euo.9Lao.lTiO3 at a magnetic field change of I0 kOe, respectively. The large low-field enhancements of --ASM and RC can be attributed to magnetic phase transition. The giant reversible MCE and large RC suggest that Euo.9Ro.ITiO3 （R = La, Ce） compounds could be promising materials in low temperature and low magnetic field refrigerants.

Temperature Effect on Early Oxidation Behavior of NiCoCrAlY Coatings: Microstructure and Phase Transformation

Initial oxidation behavior of NiCoCrAlY coating prepared by arc-ion plating has been studied in air at 900, 1000 and 1100 ℃. The results showed that phase transformation from transient θ-Al_(2)O_(3) to α-Al_(2)O_(3) was highly related to the temperature and oxidation time. The oxide scale in the initial stage was mainly composed of θ-Al_(2)O_(3) at 900 ℃. Instead, more amount of α-Al_(2)O_(3) emerged out with increasing oxidation temperature. The elemental distribution after oxidation confirmed that faster chromium diffusion to the oxide scale played an important role in the speedy transformation from θ-Al_(2)O_(3) to α-Al_(2)O_(3). Y segregation at scale/coating interface resulted in less cavity formation and hence improved the oxide scale adherence.

Giant low-field magnetocaloric effect in EuTi(1-x)NbxO3(x = 0.05, 0.1, 0.15, and 0.2) compounds

The magnetic properties and magnetocaloric effect(MCE)of EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds are investigated.Owing to electronic doping,parts of Ti ions are replaced by Nb ions,the lattice constant increases and a small number of Ti^(4+)(3d^0)ions change into Ti^(3+)(3d^1).It is the ferromagnetism state that is dominant in the derivative balance.The values of the maximum magnetic entropy change(-?SM^(max))are 10.3 J/kg·K,9.6 J/kg·K,13.1 J/kg·K,and 11.9 J/kg·K for EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds and the values of refrigeration capacity are 36,33,86,and 80 J/kg as magnetic field changes in a range of 0 T–1 T.The EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds with giant reversible MCE are considered as a good candidate for magnetic refrigerant working at lowtemperature and low-field.