Stress-induced martensitic transformation in (Ni_(47)Ti_(44))(100-x)Nb_x shape memory alloys with wide hysteresis

The effect of deformation via stress-induced martensitic transformation on the reverse transformation behavior of the (Ni47Ti44)100?xNbx (x=3, 9, 15, 20, 30, mole fraction, %) shape memory alloys was investigated in detail by differential scanning calorimetry (DSC) after performing cryogenic tensile tests at a temperature of Ms+30 ℃. The results show that Nb-content has obvious effect on the process of stress-induced martensitic transformation. It is also observed that the stress-induced martensite is stabilized relative to the thermally-induced martensite (TIM) formed on cooling, and Nb-content in Ni-Ti-Nb alloy has great influence on the reverse transformation start temperature and transformation temperature hysteresis of stress-induced martensite(SIM). The mechanism of wide transformation temperature hysteresis was fully explained based on the microscopic structure and the distribution of the elastic strain energy of (Ni47Ti44)100?xNbx alloys.

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.

Pre-existing orthorhombic embryos-induced hexagonal-orthorhombic martensitic transformation in MnNiSi_(1-x)(CoNiGe)_x alloy

The thermal-elastic martensitic transformation from high-temperature Ni_(2)In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely studied in MnMX(M=Ni or Co,and X=Ge or Si)alloys.However,the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear.In this work,the hexagonal-orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi is investigated.One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature(M_(s)).With the the sample cooing to M_(s),the laths turn broader,indicating that the martensitic transformation starts from these pre-existing orthorhombic laths.Microstructure observation suggests that these pre-existing orthorhombic laths do not originate from the hexagonal-orthorhombic martensitic transformation because of the difference between atomic occupations of doping elements in the hexagonal parent and those in the preexisting orthorhombic laths.The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent.Therefore,the orthorhombic martensite can take these pre-existing laths as embryos and grow up.This work implies that the martensitic transformation in MnNiSi_(1-x)(CoNiGe)_(x) alloy is initiated by orthorhombic embryos.

Microstructure and martensitic transformation in quaternary NiTiHfV alloy

The effect of age hardening on the microstructure,martensitic transformation behavior,and shape memory properties of the(Ni_(50)Ti_(30)Hf_(20))_(95)V_(5)alloy was investigated by scanning electron microscopy,transmission electron microscopy,X-ray diffraction,differential scanning calorimetry,microhardness,and bending tests.The results demonstrate a significant influence of V addition on the microstructure of the alloy.V addition leads to the formation of a(Ni,V)_(2)(Ti,Hf)-type Laves phase,which coexists with B19'martensite at room temperature.Aging at 550℃results in precipitation hardening due to the formation of nano-scale orthorhombic H-phase,with the peak hardness observed after 3 h of aging.The alloy at peak hardness state exhibits higher transformation strain and lower unrecovered strain compared to the solution-treated sample.The aged sample achieves a maximum transformation strain of 1.56%under 500 MPa.

Description of martensitic transformation kinetics in Fe-C-X(X = Ni,Cr,Mn,Si) system by a modified model

Controlling the content of athermal martensite and retained austenite is important to improving the mechanical properties of high-strength steels,but a mechanism for the accurate description of martensitic transformation during the cooling process must be addressed.At present,frequently used semi-empirical kinetics models suffer from huge errors at the beginning of transformation,and most of them fail to exhibit the sigmoidal shape characteristic of transformation curves.To describe the martensitic transformation process accurately,based on the Magee model,we introduced the changes in the nucleation activation energy of martensite with temperature,which led to the varying nucleation rates of this model during martensitic transformation.According to the calculation results,the relative error of the modified model for the martensitic transformation kinetics curves of Fe-C-X(X = Ni,Cr,Mn,Si) alloys reached 9.5% compared with those measured via the thermal expansion method.The relative error was approximately reduced by two-thirds compared with that of the Magee model.The incorporation of nucleation activation energy into the kinetics model contributes to the improvement of its precision.

Stress-induced martensite phase transformation of FeMnSiCrNi shape memory alloy subjected to mechanical vibrating polishing

Fe66Mn15Si5Cr9Ni5(wt.%)shape memory alloy(SMA)withγaustenite andεmartensite was subjected to mechanical vibrating polishing and consequently its surface suffered from plastic deformation in the case of compressive stress.Almost completeεmartensite transformation is found to occur in FeMnSiCrNi sample subjected to mechanical vibrating polishing,where stress-induced martensite transformation plays a predominant role.Stressinduced martensite transformation of FeMnSiCrNi SMA is closely related to the orientation of external stress.The complicated compressive stress which results from the mechanical vibrating polishing contributes toεmartensite transformation fromγaustenite of FeMnSiCrNi SMA.Mechanical vibrating polishing has a certain influence on the surface texture ofεmartensite of FeMnSiCrNi SMA,where■<0001>texture appears in the polished FeMnSiCrNi SMA.

Unveiling the cellular microstructure-property relations in martensitic stainless steel via laser powder bed fusion

Laser powder bed fusion(LPBF)is a widely recognized additive manufacturing technology that can fabricate complex components rapidly through layer-by-layer formation.However,there is a paucity of research on the effect of laser scanning speed on the cellular microstructure and mechanical properties of martensitic stainless steel.This study systematically investigated the influence of laser scanning speed on the cellular microstructure and mechanical properties of a developed Fe11Cr8Ni5Co3Mo martensitic stainless steel produced by LPBF.The results show that increasing the laser scanning speed from 400 to 1000 mm/s does not lead to a noticeable change in the phase fraction,but it reduces the average size of the cellular microstructure from 0.60 to 0.35μm.The scanning speeds of 400 and 1000 mm/s both had adverse effects on performances of sample,resulting in inadequate fusion and keyhole defects respectively.The optimal scanning speed for fabricating samples was determined to be 800 mm/s,which obtained the highest room temperature tensile strength and elongation,with the ultimate tensile strength measured at(1088.3±2.0)MPa and the elongation of(16.76±0.10)%.Furthermore,the mechanism of the evolution of surface morphology,defects,and energy input were clarified,and the relationship between cellular microstructure size and mechanical properties was also established.

Grain boundary engineering for enhancing intergranular damage resistance of ferritic/martensitic steel P92

Ferritic/martensitic(F/M)steel is widely used as a structural material in thermal and nuclear power plants.However,it is susceptible to intergranular damage,which is a critical issue,under service conditions.In this study,to improve the resistance to intergranular damage of F/M steel,a thermomechanical process(TMP)was employed to achieve a grain boundary engineering(GBE)microstructure in F/M steel P92.The TMP,including cold-rolling thickness reduction of 6%,9%,and 12%,followed by austenitization at 1323 K for 40 min and tempering at 1053 K for 45 min,was applied to the as-received(AR)P92 steel.The prior austenite grain(PAG)size,prior austenite grain boundary character distribution(GBCD),and connectivity of prior austenite grain boundaries(PAGBs)were investigated.Compared to the AR specimen,the PAG size did not change significantly.The fraction of coincident site lattice boundaries(CSLBs,3≤Σ≤29)and Σ3^(n) boundaries along PAGBs decreased with increasing reduction ratio because the recrystallization fraction increased with increasing reduction ratio.The PAGB connectivity of the 6%deformed specimen slightly deteriorated compared with that of the AR specimen.Moreover,potentiodynamic polarization studies revealed that the intergranular damage resistance of the studied steel could be improved by increasing the fraction of CSLBs along the PAGBs,indicating that the TMP,which involves low deformation,could enhance the intergranular damage resistance.

Isolated Hyperacute T-Waves in West Nile Encephalitis Indicating Atypical Variant of Stress-Induced Cardiomyopathy

Several cardiac outcomes have been reported with West Nile-encephalitis;however, the underlying pathophysiology remains complex. We present a 42-year-old female, with multiple sclerosis, whose neurological symptoms and respiratory decline were finally explained by the diagnosis of West Nile-encephalitis. During her admission, the isolated peaked T-waves indicated the underlying stress-induced cardiomyopathy. The absence of all other causes of hyperacute T-waves, their subsequent resolution with the resolution of infection and improvement in wall motion abnormalities, further supported the association. This case highlights the importance of considering hyperacute T-waves in an approach towards the diagnosis of WNV-encephalitis related atypical variant of stress-induced cardiomyopathy.

Research on Stress-Induced Nucleation ofε-Martensite in Fe-17Mn-10Cr-5Si-4Ni Polycrystalline Alloy

The relation between stacking fault overlap and martensitic nucleation in Fe-17Mn-10Cr-5Si-4Ni alloy was studied.The arrayed structure of dislocations and characteristics ofε-martensite formation under stress were in-situ observed by TEM.The results reveal thatε-martensite nucleates at the top of stacking faults overlapped band,where the ordered arrayed structure of Shockley partial dislocations exists.External stress promotesε-martensitic nucleation ability because of accelerated ordering of Shockley partial dislocations in stacking faults overlapped band.

Superelastic behavior and stabilization of stress-induced martensite in Cu-13.4Al-4.0Ni single crystals

By applying tensile stress along 〈100〉 of β phase, the superelastic behavior and stabilization of stress induced martensite (SIM) of Cu 13.4Al 4.0Ni(mass fraction, %) single crystals were studied. The results show that the pseudo yield stress decreases with the increase of cycling number, and keeping load isothermally has an effect on stabilization of SIM. Previous thermal cycling between ( M s-20 ℃) and ( A f+20 ℃) promotes the superelasticity and the stabilization of SIM as well; the pre thermal cycling also reduces the pseudo yield stress. However, once the stabilization of SIM is produced, it can be destabilized by either the afterwards thermal cooling heating cycling or load and immediately unload cycling in ( A f～ M d). Isothermal treatment in ( A f～ M d) brings restabilization of SIM. The maximum superelastic value from β → β ′ 1(18 R ) is 9% for the studied single crystal. When test temperature is in A f～( A f+50 ℃) and stress is in 0～350 MPa, the superelastic behavior exist. [

A Detailed Observation on Successive Stress-Induced Martensite Transformation in CuAlMnZnZr Alloy Poly crystalline Above A_(f)

The successive stress-induced martensite morphologies and mechanisms in polycrystalline CuAlMnZnZr samples have been examined. By applying stress to the uniform P ] matrix, two or more orientation plates of M18R martensite are stress-induced in a grain. With further increasing stress, one orientation plate depletes the other and coalesces into a single region in some view field. The mechanisms by which these are developed have been ascertained, and include variant-variant coalescence, stress-induced martensite to martensite transformation and the complicated cross-like stress-induced martensite formation.

CHARACTERISTICS OF STRESS-INDUCED TRANSFORMATION AND MICROSTRUCTURE EVOLUTION IN Cu-BASED SMA

The mechanical behavior of shape memory alloys （SMAs） is closely related to the formation and evolution of its microstructures. Through theoretical analysis and experimental observations, it was found that the stress-induced martensitic transformation process of single crystal Cu-based SMA under uniaxial tension condition consisted of three periods： nucleation, mixed nucleation and growth, and merging due to growth. During the nucleation, the stress dropped rapidly and the number of interfaces increased very fast while the phase fraction increased slowly. In the second period, both the stress and the interface number changed slightly but the phase fraction increased dramatically. Finally, the stress and the phase fraction changed slowly while the number of interfaces decreased quickly. Moreover, it was found that the transformation could be of multi-stage： sharp stress drops at several strains and correspondingly, the nucleation and growth process occurred quasi-independently in several parts of the sample.

Constitutive modeling of hot deformation behavior of X20Cr13 martensitic stainless steel with strain effect

Hot deformation behavior ofX20Cr13 martensitic stainless steel was investigated by conducting hot compression tests on Gleeble-1500D thermo-mechanical simulator at the temperature ranging from 1173 to 1423 K and the strain rate ranging from 0.001 to 10 s^-1. The material constants of a and n, activation energy Q and A were calculated as a function of strain by a fifth-order polynomial fit. Constitutive models incorporating deformation temperature, strain rate and strain were developed to model the hot deformation behavior of X20Cr13 martensitic stainless steel based on the Arrhenius equation. The predictable efficiency of the developed constitutive models of X20Cr13 martensitic stainless steel was analyzed by correlation coefficient and average absolute relative error which are 0.996 and 3.22%, respectively.

Effect of annealing temperature on martensitic transformation of Ti_(49.2)Ni_(50.8) alloy processed by equal channel angular pressing

The effect of annealing temperature on the martensitic transformation of a Ti49.2Ni50.8 alloy processed by equal channel angular pressing （ECAP） was investigated by X-ray diffraction （XRD）, transmission electron microscopy （TEM） and differential scanning calorimetry （DSC）. The as-ECAP processed and subsequently annealed Ti49.2Ni50.8 alloys consist of B2 parent phase, Ti4Ni2O phase and B19′ martensite at room temperature. Upon cooling, all samples show B2→R→B19′ two-stage transformation. Upon heating, when the annealing temperature is less than 400℃, the samples show B19′→R→B2 two-stage transformation; when the annealing temperature is higher than 500 ℃, the samples show B19′→B2 single-stage transformation. The B2-R transformation is characterized by wide interval due to the dislocations introduced during ECAP.

Effect of Microstructure Refinement on the Strength and Toughness of Low Alloy Martensitic Steel

Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography （OM）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a HalI-Petch relation with the yield strength. When the prior austenite grain size was refined from 199 μm to 6 μm , the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at 77 K improved more than 8 times, indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength. The fracture surfaces implied that the unit crack path for cleavage fracture is identified as being the packet.

Design of a low-alloy high-strength and high-toughness martensitic steel

To develop a high strength low alloy （HSLA） steel with high strength and high toughness, a series of martensitic steels were studied through alloying with various elements and thermodynamic simulation. The microstructure and mechanical properties of the designed steel were investigated by optical microscopy, scanning electron microscopy, tensile testing and Charpy impact test. The results show that cementite exists between 500℃ and 700℃, M7C3 exits below 720℃, and they are much lower than the austenitizing temperature of the designed steel. Furthermore, the Ti（C,N） precipitate exists until 1280℃, which refines the microstructure and increases the strength and toughness. The optimal alloying components are 0.19% C, 1.19% Si, 2.83% Mn, 1.24% Ni, and 0.049% Ti; the tensile strength and the V notch impact toughness of the designed steel are more than 1500 MPa and 100 J, respectively.

Martensitic transformation of Ti-18Nb(at.%) alloy with zirconium

The addition of 3%～9% Zr on the martensitic transformation of Ti-18Nb(at.%) alloy was investigated. The results of microstructure and X-ray diffraction (XRD) analysis show that the phase constitution of as-quenched Ti-18Nb-9Zr(at.%) alloy consists of the retained matrix and martensite, while that of the other three alloys is single martensite. No trace of athermal phase was found in any of the as-quenched alloys. Unlike the effect of Nb addition on the martensitic transformation start temperature Ms of Ti-18Nb(at.%) alloy, Ms de-creased nonlinearly as increasing the Zr addition from 3% to 9% and Ms decreased much more sharply as increasing the Zr addition. The Ms of as-quenched Ti-18Nb-9Zr alloy was around room temperature. The effect of Zr addition on the phase stabilizing in the Ti-18Nb(at.%) alloy was briefly discussed.

First-principles study on the effect of Hf content on martensitic transformation temperature of TiNiHf alloy

In this paper a first-principles study of the electronic structure and stability of B2 Ti1-xNiHfx （x = 0.2, 0.4, 0.6） and B19′ Ti1-xNiHfx（x = 0, 0.5） alloys is presented. The calculations are performed by the plane-wave pseudopotential method in the framework of the density functional theory with the generalized gradient approximation. This paper calculates the lattice parameters, density of states, charge density, and heats of formation. The results show that the electronic structure and stability of B2 Ti1-xNiHfx change gradually with Hf content. However, Hf content has little effect on the electronic structure and stability of B19′ Ti1-xNiHfx. The mechanism of the effect of Hf content on martensitic transformation temperature of TiNiHf alloys is studied from the electronic structure.

Microstructural evolution and tensile properties of low-carbon steel with martensitic microstructure during warm deforming and annealing

For preparing large-scale nano-grained and ultrafine-grained steel sheets by warm rolling and annealing, the effects of deforming temperature on both the flow stress and the microstructure evolution of 09MnNiD steel with lath martensitic microstructure were studied by warm-compression test and transmission electron microscopy （TEM） observation. Thereafter, the steel with the lath martensitic structure was multi-pass warm-rolled and then annealed. TEM results indicate that nano-grained and ultrafine-grained steel sheets are formed by warm rolling at 400℃ and annealing at 400-600℃. In comparison with the as-warm-rolled specimen, the tensile strength at room temperature changes a little when the rolled samples are annealed below 450℃, and the tensile strength is greatly lowered as the annealing temperature increases to above 550℃.