The tension-compression asymmetry of martensite phase transformation in a metastable Fe40Co20Cr20Mn10Ni10 high-entropy alloy

The microstructural evolution of a metastable face centered cubic(FCC)Fe40Co20Cr20Mn10Ni10high-entropy alloy(HEA)under both tension and compression is systemically investigated.The results show much higher level of martensite phase transformation from FCC structure to hexagonal closed packed(HCP)structure under compression than tension,indicating a distinct tension-compression asymmetry.The compressive tests underwent higher true stresses,which further provided stronger driving forces to trigger the phase transformation than those in tensile tests.Except for the martensite phase transformation,dislocation planar slip prevails in both tension and compression,along with the occasional formation of mechanical twins.Dislocation slip dominates the whole tensile deformation,while both dislocation motions and martensite phase transformation play critical roles in the compressive deformation.The martensite phase transformation is preferred to nucleate at grain or subgrain boundaries due to a medium stacking fault energy(SFE)of^20 m J m^-2.The formation of HCP phase via partial dislocation emission from low-angle grain boundaries offers additional pathways for martensite phase transformation.Our study thus remarkably benefits the understanding of the deformation mechanisms of metastable HEAs.

Effect of strain-induced martensitic phase transformation on the formability of nitrogen-alloyed metastable austenitic stainless steels

Strain-induced martensitic phase transformation and its influence on the forrnability of newly developed nitrogen-alloyed metastable austenitic stainless steels were systematically investigated. Yield strength for the as- received steels bearing low nickel content was around 300 MPa and their elongation ratios varied from 55.2% to 61.7%. Erichsen numbers of these samples differed from 13.82 to 14.57 mm. Although its Cu content was lower than that of other samples, steel D2 exhibited better plasticity and formability, which was attributed to ~/--,c~＇ martensitic phase transformation. EBSD, XRD, and magnetism tests showed that increases in deformation ratio gradually increased the α＇ martensite phase of a sample ,thereby contributing to its strain and inducing the optimal transformation-induced plasticity effect. An Meeo/5o temperature of around 20 ℃, which is close to the deformation temperature,provided the austenite with adequate stability and gradually transformed it into martensite, thereby endowing lean ASS with better formability.

Martensitic transformation in Cu-doped NiMnGa magnetic shape memory alloys

This paper studies the martensitic transformation in the Cu-doped NiMnGa alloys. The orthorhombic martensite transforms to L21 cubic austenite by Cu substituting for Ni in the Nis0-xCuxMn31Ga19 （x=2 10） alloys, the martensitic transformation temperature decreases significantly with the rate of 40 K per Cu atom addition. The variation of the Fermi sphere radius （kF） is applied to evaluate the change of the martensitic transformation temperature. The increase of kF leads to the increase of the martensitic transformation temperature.

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.

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）.

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.

Effect of cold rolling on the microstructural, magnetic, mechanical, and corrosion properties of AISI 316L austenitic stainless steel

This study has evaluated the effect of different levels of cold rolling（from 0 to 50%）on the microstructural,magnetic,and mechanical properties and the corrosion behavior of 316L austenitic stainless steel in Na Cl（1 mol/L）＋H_2SO_4（0.5 mol/L）solution.Microstructural examinations using optical microscopy revealed the development of a morphological texture from coaxial to elongated grains during the cold-rolling process.Phase analysis carried out on the basis of X-ray diffraction confirmed the formation of the ferromagneticα′-martensite phase under the stresses applied during cold rolling.This finding is in agreement with magnetic measurements using a vibrating sample magnetometer.Mechanical properties determined by tensile and Vickers microhardness tests demonstrated an upward trend in the hardness-to-yield strength ratio with increasing cold-rolling percentage,representing a reduction in the material’s work-hardening ability.Uniform and localized corrosion parameters were estimated via potentiodynamic polarization corrosion tests and electrochemical impedance spectroscopy.In contrast to the uniform corrosion,wherein the corrosion current density increased with increasing cold-working degree because of the high density of microstructural defects,the passive potential range and breakdown potential increased by cold working,showing greater resistance to pit nucleation.Although pits were formed,the cold-rolled material repassivation tendency decreased because of the broader hysteresis anodic loop,as confirmed experimentally by observation of the microscopic features after electrochemical cyclic polarization evaluations.

In situ scanning electron microscopy observation of deformation and fracture behavior of Ti-3Zr-2Sn-3Mo-25Nb alloy

The plastic deformation and fracture processes of Ti-3Zr-2Sn-3Mo-25Nb alloy for surgical implants were directly observed by in situ scanning electron microscopy(SEM).The effect of phase transformations on deformation-induced martensitic transformation accompanying the cyclic tensile fracture processes was investigated.The results reveal that the metastable alpha martensites(α″) promotes deformation-induced martensitic transformation to ductile fracture,whereas the omega(ω) and alpha(α) phases drastically prevent slip dislocation and deformation-induced martensitic transformation to brittle fracture.

Stress-Induced Martensitic Transformation of Zr_(50)Cu_(25)Ni_(10)Co_(15) Nanocrystals Embedded in an Amorphous Matrix

The stress induced martensitic phase transformation of spherical ZrCu nanocrystals embedded in an amorphous matrix was studied in this paper. Microstructural observations revealed that the martensitic transformation of the nanocrystal was hindered by the surrounding amorphous coating. The existence of two-step transformation from the austenite phase（B2） to the base structure martensite（B19＇） and finally to the most stable superstructure martensite（Cm） was also demonstrated. The Cm martensite with（021） type I twinning symmetrically accommodation was surrounded by the B19＇ martensite with dislocation morphologies.

Formation of face-centered cubic titanium in laser surface re-melted commercially pure titanium plate

Micron-scale face-centered cubic titanium phase（named as δ phase） were noticed in the re-melted zone of laser surface re-melted commercially pure titanium plate.The morphology,sub-structure,orientation and distribution of δ phase were investigated by scanning electron microscopy,electron back-scattered diffraction and transmission electron microscopy.Three kind formation processes of δ phase were put forward based on the investigation.The first one is α＇→δ transformation which takes place in single α＇grains and leads to the orientation relationship {001}δ//{0001}α＇〈 110 〉 δ//〈 112^-0 〉α＇.The second one is β→α＇＋ δ transformation which takes place at α＇/α＇interfaces and leads to the orientation relationship{001}δ//{11^-0}β〈110〉 δ//〈111〉β.The third one is another kind of β→α＇＋ δ transformation that takes place at α＇/α＇interfaces and leads to the orientation relationship{11^-1}δ//{11^-0}β〈 110 〉 δ//〈 111 〈 β.It is believed that the transformations of δ phase are stress assistant ones and in the present investigation,the phase transformation stress of β→α＇transformation acts as the assistant driving force for the formation of δ phase.

Oxygen addition for improving the strength and plasticity of TiZr-based amorphous alloy composites

The addition of a small amount of oxygen improves the mechanical properties,especially plasticity,of Ti_(45.7)Zr_(33)Ni_(3)Cu_(5.8)Be_(12.5) amorphous alloy composites(AACs)at room temperature(298 K).Compared to the plasticity of AACs without added O(5%),the plasticity of the composites with 0.73 at.%O(nominal composition)was much higher(11%).Even at O content higher than 0.73 at.%,the AACs exhibited good plasticity.The highest plasticity of∼12.3%was observed with 2.87 at.%O.Two distinct mechanisms are proposed to explain the enhanced plasticity of the AACs.At low O content,although deformation-induced phase transformation was suppressed,a substantial amount of α" martensite was formed.The microstructural features of α" martensite,such as thinner laths and homogeneous distribution,induced the formation of multiple shear bands in the amorphous matrix.At high O content,deformation-induced phase transformation was seriously suppressed.A dispersed nanoωphase was formed during rapid solidification in AACs with O content higher than 1.45 at.%.This resulted in a weakening in the anisotropy ofβdendrites and led to their homogenous deformation.Furthermore,multiple shear bands were formed in the amorphous matrix.Apart from plasticity,the strength of the AACs also increased with an increase in the O content.This phenomenon was explained in terms of three mechanisms,viz.the solid-solution-strengthening effect of O,fine-grain strengthening of β dendrites,and secondary phase strengthening by the nano ω phase.

Microstructural evolution in 35CrMnSi steel during multi-cyclic quenching and partitioning heat treatment

A novel method,i.e.multi-cyclic quenching and partitioning(M-Q-P)heat treatment,is used to tailor the content of retained austenite(RA)in commercial steels.For 35CrMnSi steel,5 times of Q-P heat treatment can increase the content of RA from 8vol.%to 17 vol.%.As a result,the ultimate elongation of the steel is improved from 17.4%after the typical Q-P heat treatment to 27.1%after 5 times of Q-P treatment.Meanwhile,the improved combination of strength and ductility for steels by typical Q-P heat treatment is retained by the multi-cyclic Q-P heat treatment.It is shown that the content of RA in some specific steels,and furthermore their mechanical properties,can be regulated through the M-Q-P.

Degeneration and rejuvenation of shape memory effect associated with the precipitation of coherent nano-particles in a Co-Ni-Si shape memory alloy

In a solution treated Co-20Ni-6Si shape memory alloy,coherent nano-particles were precipitated after annealing at 873 K for 1 min,but the shape memory effect almost vanished.It is attributed to that the coherent nano-particles not only suppressed the stress-induced face-centered cubic to close-packed hexagonal martensite transformation but also damaged the crystallographic reversibility of reverse martensite transformation.After further annealing at 1073 K for 1 min,the shape memory effect was rejuvenated owing to the dissolution of nano-particles.Besides,the recovery strain significantly increased to5.1%from the solution treatment of 3.1%after annealing at 1073 K for 1 min.

Effect of Pd content on crystallization and shape memory properties of Ti-Ni-Pd thin films

The Pd content dependence of the crystallization process of Ti–Ni–(19.1–35.3)Pd(at.%)thin films fabricated by a sputter-deposition method was investigated.Ti–Ni–(19.1–26.1)Pd(at.%)as-deposited thin films were found to be amorphous,whereas Ti–Ni–(29.1–35.3)Pd(at.%)thin films were crystalline in the as-deposited condition.Both the crystallization temperature and activation energy for the crystallization of the amorphous thin films decrease with increasing Pd content.The shape memory effect was confirmed in the in situ crystallized thin film.The finer grain size in the in situ crystallized thin film results in a higher critical stress for slip and a smaller recovery strain when compared with the thin film crystallized by post annealing.