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.

Strain-induced martensitic transformation in biomedical Co-Cr-W-Ni alloys

The nucleation,variant selection,and orientation dependence of the strain-induced martensitic transformation(SIMT)process in biomedical Co-Cr-W-Ni alloys were investigated.The experimental results show that theε-hexagonal-close-packed phase was preferentially formed at theΣ3 twin boundaries and high-angle grain boundaries during the tensile process.The theoretical analysis shows that the variant selection of SIMT is governed by Schmid’s law.However,the SIMTedε-phase did not form equally on the two sides of the annealing twins,even though they had the same Schmid factor.This phenomenon is related to the mechanical work developed by the formation of theε-phase.Only the side which has both high Schmid factor and high mechanical work can initiate the SIMT process.A strong<111>fiber texture was formed,and theε-variants tended to appear in grains with orientations close to the<111>and<100>directions during the tensile process.These results can provide theoretical guidance for controlling the SIMT process of Co-Cr-W-Ni alloys to fabricate more reliable stents.

First-principles calculations of Ni–(Co)–Mn–Cu–Ti all-d-metal Heusler alloy on martensitic transformation,mechanical and magnetic properties

The martensitic transformation,mechanical,and magnetic properties of the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) (x=0.125,0.25,0.375,0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5)[(x=0.125,y=0.125,0.25,0.375,0.5) and (x=0.125,0.25,0.375,y=0.625)]alloys were systematically studied by the first-principles calculations.For the formation energy,the martensite is smaller than the austenite,the Ni–(Co)–Mn–Cu–Ti alloys studied in this work can undergo martensitic transformation.The austenite and non-modulated (NM) martensite always present antiferromagnetic state in the Ni_(2)Mn_(1.5-x)Cu_(x)Ti_(0.5) and Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) (y<0.625) alloys.When y=0.625 in the Ni_(2-y)Co_(y)Mn_(1.5-x)Cu_(x)Ti_(0.5) series,the austenite presents ferromagnetic state while the NM martensite shows antiferromagnetic state.Cu doping can decrease the thermal hysteresis and anisotropy of the Ni–(Co)–Mn–Ti alloy.Increasing Mn and decreasing Ti content can improve the shear resistance and normal stress resistance,but reduce the toughness in the Ni–Mn–Cu–Ti alloy.And the ductility of the Co–Cu co-doping alloy is inferior to that of the Ni–Mn–Cu–Ti and Ni–Co–Mn–Ti alloys.The electronic density of states was studied to reveal the essence of the mechanical and magnetic properties.

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.

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.

Effects of Co and Al addition on martensitic transformation and microstructure in ZrCu-based shape memory alloys

The effect of ternary alloying element Al and quaternary alloying element Co on the martensitic transformation of ZrCu-based shape memory alloy was investigated. The results show that the addition of Al and Co in ZrCu alloy decreases both the martensitic transformation temperature and the martensitic transformation temperature hysteresis. Transmission electron microscope (TEM) observations reveal that theCm martensite structure is the preferential formation phase. The intervariant structures in ZrCuAlCo alloy are (021) type I twins, while the dominant substructures inside the martensite variant are the (001) compound twins. With the increase of Co content, tensile fracture strength and strain are improved obviously.

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.

DSC STUDY OF MARTENSITIC TRANSFORMATION KINETICS IN A Cu－Zn－Al－Mn－Ni SHAPE MEMORY ALLOY

The kinetics of the reversible martensitic transformation in a Cu-Zn-Al-Mn-Ni shape memory alloy has been studied by means of differential scanning calorimetry.The apparent activation energy has been calculated and the kinetic equations of positive and adverse martensitic transformation have been established with the variations of temperature and time.

Martensitic Transformation and Magnetic-Field-Induced Strain in Ni-Mn-Ga-RE (RE=Tb, Sm) Alloys

The effects of microamount additions of RE (Tb, Sm) on martensitic transition, the magnetic-field-induced strain and the bending strength of highly textured polycrystalline Ni_(48)Mn_(33)Ga_(19) alloy were investigated. The experimental results show that the addition of RE elements decreases the martensitic transformation temperature and the Curie temperature. But the bending strength of Ni-Mn-Ga-RE (RE=Tb, Sm) alloys increases remarkably because of the grain refinement. As a result, Ni-Mn-Ga-RE alloys will be applied practically with higher reliability and stability due to favorable plasticity and toughness. In addition, the replacement of small amounts Ga by Tb or Sm decreases the magnetic-field-induced strain of the alloys at room temperature.

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.

EFFECT OF RESIDUAL STRESS ON THE MARTENSITIC TRANSFORMATION OF SPUTTER-DEPOSITED SMA THIN FILMS

TiNi thin films were sputter-deposited on circular single-crystal silicon substrates un-der various sputtering parameters. The crystal structure and residual stress of the as-deposited films were determined by X-ray diffraction and substrate-curvature method. The phenomenon of stress-suppressed martensitic transformation was observed. R is considered that the residual stresses in SMA thin films based on circular substrates act as balanced biaxial tensile stresses. The status of equilibrant delays the align-ment of self-accommodated variants and the volume shrinkage during the martensitic transformation.

Martensitic Transformation and Magnetic-Field-Induced Strain in Magnetic Shape Memory Alloy NiMnGa Melt-Spun Ribbon

A magnetic shape memory alloy with nonstoichiometric Ni50Mn27Ga23 was prepared by using melt-spinning technology. The martensitic transformation and the magnetic-field-induced strain (MFIS) of the polycrystalline melt-spun ribbon were investigated. The experimental results showed that the melt-spun ribbons underwent thermal-elastic martensitic transformation and reverse transformation in cooling and heating process and exhibited typical thermo-elastic shape memory effect. However the start temperature for martensitic transformation decreased from 286 K for as-cast alloy to 254 K for as-quenched ribbon and Curie temperature remains approximately constant. A particular internal stress induced by melt-spinning resulted in the formation of a texture structure in the ribbons, which made the ribbons obtain larger martensitic transformation strain and MFIS. The internal stress was released substantially after annealing, which resulted in a decrease of MFIS of the ribbons.

APPLICATION OF COMPUTER GRAPHICS IN MARTENSITIC TRANSFORMATION

This paper discuss the stereographic method of martensitic transformation (MT)in the view of computer graphics (CG) and programs are programmed to realize this method. Habit plane resulting from the programs agrees with that of numerical analysis in terms of the lattice parameters of austenite and martensite and shear mechanism supposed in Fe-22%Ni-0.8%C alloy, so does orientation relationships.

HOT DEFORMATION AND MARTENSITIC TRANSFORMATION BEHAVIORS OF Fe-32%Ni ALLOY

The Hot deformation and martensitic transformation behaviors of Fe-32%Ni alloy was investigated by measurements of electrical resistance and X-ray diffraction. With the increase in strain, the austenite goes through from the work-hardened to the partial dynamcally re-crystallized and then to the completed dynamically re-crystallized. The martensitic transformation characteristics depend on the austenite states. The work-hardening in small strain is helpful to martensitic transformation due to the low dislocation density and little lattice distortion, while the high dislocation density and severe lattice distortion by the increase in strain will hinder the martensitic nucleation. Once dynamic re-crystallization （ DRX ） takes place, the martensitic transformation will be enhanced again, which is related to the heterogeneous dynamic substructures. The growing DRX grain can enhance the martensitic nucleation due to the low dislocation density near its grain boundary.

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.

THERMODYNAMICS OF MARTENSITIC TRANSFORMATION IN Fe-Mn-C AND Fe-Ni-C ALLOYS

A Central Atom Model is introduced and the LFG and Hsu models are modified in order to evaluate the driving force for the martensitic transformation in Fe-Mn-C and Fe-Ni-C al- loys.The results show that the relationship between the driving force and the yield strength of austenite at Ms temperature,σ_(0.2)~γ/M_s,fits Hsu's formula;ΔG~=2.1σ_(0.2)~γ/M_s+907 J/mol.The M_s temperatures of Fe-Mn-C and Fe-Ni-C alloys are also calculated.The calculated results are in good agreement with experimental values.

Martensitic transformation and magnetic properties of aged Ni-Fe-Ga-Ti shape memory alloy

This article reports the effect of ageing on the microstructure, martensitic transformation, magnetic properties, and mechanical properties of Ni51FelsGa27Ti4 shape memory alloy. There are five specimens of this alloy aged at 573 up to 973 K for 3 h per each. This range of ageing temperature greatly affects the microstructure of the alloy. As the ageing temperature increased from 573 up to 973 K, the microstructure of Ni51FelsGa27Ti4 alloy gradually changed from the entirely martensitic matrix at 573 K to the fully austenitic microstructure at 973 K. The volume fraction of precipi- tated Ni3Ti particles increased with the ageing temperature increasing from 573 to 773 K. Further increasing the ageing temperature to 973 K decreased the content of Ni3Ti in the microstructure. The martensitic transformation tempera- ture was decreased steadily by increasing the ageing temperature. The magnetization saturation, remnant magnetization, and coercivity increased with the ageing temperature increasing up to 773 K. A further increase in ageing temperature decreased these raagnetic properties. Moreover, the hardness values were gradually increased at first by increasing the ageing temperature to 773 K, and then dramatically decreased to the lowest value at 973 K.

Effects of Predeformation on the Reverse Martensitic Transformation of TiNi Shape Memory Alloy

DSC was used to study the effects of predeformation on the reverse martensitic transformation of near-equiatomic TiNi alloy. Both the start temperature As and the finish temperature Af of the reverse transformation increased with increasing degree of predeformation, but the algebraic difference between As and Af decreased with increasing predeformation until it reached a minimum value, then remained unchanged with further deformation. Transformation heat also increased with increasing predeformation until it reached a maximum value, then decreased with further predeformation. All the phenomena above were considered to be closely related with the release of elastic strain energy during predeformation.