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 toevaluate 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 ofaustenite at Ms temperature,σ_(0.2)~γ/M_s,fits Hsu's formula;ΔG~=2.1σ_(0.2)~γ/M_s+907J/mol.The M_s temperatures of Fe-Mn-C and Fe-Ni-C alloys are also calculated.Thecalculated results are in good agreement with experimental values.     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

β—Cu基合金马氏体相变热力学

提出对β-Cu基合金热弹性马氏体相变的热力学处理。计算了Cu-Zn,Cu-Al及Cu-Zn-Al合金马氏体相变驱动力,T_0温度及M_s温度,M_s的计算值与实验值符合很好。母相有序降低Cu-Zn和Cu-Zn-Al的M_s,但升高Cu-Al的M_s(T_0)。并简介了两种估算非化学自由能的方法。     

THERMODYNAMICS OF MARTENSITIC TRANSPORMATION IN β-Cu BASE ALLOYS

提出对β-Cu基合金热弹性马氏体相变的热力学处理。计算了Cu-Zn,Cu-Al及Cu-Zn-Al合金马氏体相变驱动力,T_0温度及M_s温度,M_s的计算值与实验值符合很好。母相有序降低Cu-Zn和Cu-Zn-Al的M_s,但升高Cu-Al的M_s(T_0)。并简介了两种估算非化学自由能的方法。     

MARTENSITIC TRANSFORMATION AND THERMAL STABILITY IN Cu-Al-Co AND Cu-Al-Zr ALLOYS

Cu-Al-Co and Cu-Al-Zr alloys were explored with Co or Zr additions in Cu-Al alloys for high temperature shape memory alloys. Samples were quenched after homogenized at 850℃ for 48h. It was found that both Cu-Al-Co and Cu-Al-Zr show AlCus martensitic phase at room temperature and exhibit martensitic transformation temperatures higher than 200℃, showing the potentials for developing as high temperature shape memory alloys. Thermal cycles were performed by DSC instrument on both Cu-Al-Co and Cu-Al-Zr alloys. The results show that Cu-Al-Co loses its martensitic transformation after five thermal cycles, and Cu-Al-Zr exhibits no martensitic transformation in the second thermal cycle.     

DEFORMATION OF CuZnAl SHAPE MEMORY ALLOYS IN MARTENSITIC STATE

Plastic deformation of three CuZnAl shape memory alloys in martensitic state was investi-gated by means of optical and electron metallography,dilatometry and electric resistiviy de-termination.Two of the alloys are in martensitic state at ambient temperature,and theA_f temperature of the third one is far below the ambient temperature.The effect ofdeformation of alloys in martensitic state on the course and temperature of transformation aswell as the hysteresis of transformation cycles has been analyzed and the “effect of the firstcycle” has also been discussed.     

Atom exchange of martensite in Cu- 13Zn- 15Al alloy during non-isothermal aging

1 Introduction Cu-Zn-Al alloys as well as Ni-Ti alloys are known to exhibit the shape memory effect, being associated with a thermoelastic martensite transformation and its reversion[1- 4]. Cu-Zn-Al alloys are less stable than the Ni-Ti alloys above room …     

INFLUENCE OF GRAIN SIZE AND ORDERING OF PARENT PHASE ON M_s IN A CuZnAl ALLOY CONTAINING BORON

The effect of grain size and ordering of the parent phase on the critical points of thermoelasticmartensitic transformation in Cu-25.62 Zn-3.97 Al-0.0018 B(wt-%)shape memory alloyhas been investigated.Based on the thermodynamics of phase transformation,a linear rela-tionship between the starting temperature of martensitic transformation and the reciprocal ofthe square root of grain size is obtained,i.e.M_s temperature rises with increasing grain size.It shows a good agreement with the results of electric resistance measurement.Applying theLandau's theory,a quantitative relationship between M_s and the ordering parameter of theparent phase is set up,which is well confirmed by the results of X-ray diffraction and electricresistance measurement.The activation energy of the ordering process in the parent phase ofthe alloy is calculated to be 46 kJ/mol.     

Phase transition and mechanical properties of Ni30Cu20Mn37＋xGa13-x（x=0–4.5） alloys

Ni30Cu20Mn37＋xGa13-x（x = 0–4.5） alloys were studied with the phase transformation and mechanical properties. With the increase of Mn content, the martensitic transformation temperatures increase and the Curie temperature decreases. Simultaneously, the room temperature microstructure evolves from single phase of austenite to dual phases containing martensite and precipitation. Both the ductility and the strength of the polycrystalline alloys are significantly improved by the precipitation. Coupled magnetostructural transition from weak magnetic martensite to ferromagnetic austenite is obtained in both single-phase and ductile dual-phase alloys.     

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-1...     

Phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys after different heat treatments简

The effects of heat treatments on the phase transformation behavior of Ti49 Ni49.5 Fe1 V0.5 and Ti48 Ni48.5 Fe1 V2.5 alloys were investigated. The results indicate that the alloys subjected to different heat treatments have B2 structure at room temperature. All the specimens exhibit a twostage B2→R→B190martensitic transformation on cooling, but a B190→B2 one-stage reverse martensitic transformation on heating except aged A1 alloy, which undergoes an abnormal two-stage transformation upon heating. The phase transformation temperatures are affected by heat treatments and V content, which can be attributed to the variation of the second-phase particles content in the matrix.     

Influence of Fe Addition on Phase Transformation,Microstructure and Mechanical Property of Equiatomic NiTi Shape Memory Alloy

Based on equiatomic nickel and titanium, three kinds of NiTiFe alloys with a nominal chemical composition of Ni_(49)Ti_(49)Fe_2, Ni_(48)Ti_(48)Fe_4 and Ni_(47)Ti_(47)Fe_6(at.%), respectively, have been designed to investigate the influence of the addition of Fe element on phase transformation, microstructure and mechanical property of equiatomic NiTi shape memory alloy. The microstructures of three kinds of NiTiFe alloys are characterized by the equiaxed grains instead of the dendrites.Consequently, some Ti_2Ni precipitates are found to distribute in the grains interior and at the grain boundaries. The content of Fe element has an important influence on mechanical property of NiTiFe alloy. With increasing content of Fe element,the strength of NiTiFe alloy increases substantially, but the plasticity decreases sharply. It can be concluded that precipitation strengthening and solution strengthening play a significant role in enhancing the strength of NiTiFe alloy. In the case of three NiTiFe alloys, neither martensitic transformation nor reverse transformation can be observed in the range from-150 to 150 ℃. On the one hand, the phase transformation temperature is probably out of the scope of the present experimental temperature. On the other hand, the addition of Fe element probably suppresses first-order martensitic transformation or reverse transformation, and consequently the second-order-like phase transformation from an incommensurate stage to a commensurate stage can probably take place.     

Theory and Modeling of Phase Transformations under Stress in Steel

Thermodynamic prediction of the increment of the formation temperature of proeutectoid ferrite by applied stress is nearly consistent with the experimental data. Kinetics models for ferrite，pearlite and bainite transformations can be shown as modified Johnson-Mehl-Avrami equation in which parameter b(σ) varies with the level of applied stress.The effects of tensile and compressive stresses on enhancement of the ferrite/pearlite and bainite transformations are discussed. The necessity and approach of modification of additivity hypothesis are introduced and the results from modified equation in which some parameters are obtained by regression of two experimental results or taken from TTT and CCT diagrams of a certain steel are superior than that from Scheil‘s equation. Thermodynamic calculation of Ms and nucleation kinetics equations of martensitic transformation under stress are suggested. Modeling of phase transformations under stress in ferrous alloys is briefly described.     

SOME ASPECTS OF PROGRESS AND PERSPECTIVE IN MARTENSITIC TRANSFORMATION

Progress and perspective in martensitic transformations are described,including the definitionand the thermodynamics of martensitic transformatian,the effect of austenite state onmartensitic transformation,kinetics,nucleation and growth the as well as the crystallographyof martensitic transformation.     

Effect of Al addition on properties of TiNiFe shape memory alloys

A little amount of aluminum substituting for Ni was added to Ti50Ni48Fe2 and Ti50Ni47.5Fe2.5 alloys to improve the mechanical properties, especially the yield stress of the TiNiFe alloys. The martensitic transformation temperature and mechanical properties of Ti50Ni48-xFe2Alx and Ti50Ni47.5-xFe2.5Alx （x=0, 0.5, 1） alloys were examined, and it was revealed that 0.5% and 1%（mole fraction） aluminum addition lead to about 10℃ and 60-80℃ martensitic transformation temperature （Ms） decrease, respectively, 1%（mole fraction） aluminum addition enhances remarkably the yield stresses of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 to 560 and 580 MPa, respectively. The systemic microstructure analysis indicates that the second phase Ti2Ni at the grain boundaries plays an important role in improving the mechanical properties of TiNiFe shape memory alloys.     

CHARACTERISTICS OF MARTENSITIC TRANSFORMATION IN DUAL-PHASE STEELS

A theorectical expression for the driving force and M_ point of martensitic transformation hasbeen proposed.The M_ values using this expression are in good agreement withthat obtained experimentally.It was found that the values of M_ and M_ are not only relatedto the carbon content in martensite,but also to the volume fraction of ferrite.     

DAMPING PROPERTIES OF MARTENSITE AND MARTENSITIC TRANSFORMATION IN CuZnAl ALLOYS

The internal friction of alloys in martensite state is believed to be an M/M interface one,which can be explained by an expression deduced from the theory of dislocation internal fric-tion.The internal friction during martensitic transformation consists of two parts,includingthose of the M/M interface and of the phase transformation.The latter is further composedof two portions,the major one produced by reverse martensitic transformation and the otherfrom stress-induced martensite.It was also found that the degradation of damping propertiesof the CuZnAl alloys is related to the dislocation,which is introduced from the exciting pro-cess,and tends to be of stable value after certain excitements.     

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 in （Ni47Ti44）100-xNbx 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.     

Effects of Ni Content and Solution-Aging Treatment on Multi-Stage Transformations of TiNi Shape Memory Alloys

Effects of Ni content and solution-aging treatment on transformation type and transformation temperature of Ti100-xNix (x=33.3-75) alloys were investigated by differential scanning calorimeter (DSC). The results show that one-stage transformation B2→B19′ of the solution-quenched Ti100-xNix alloys occurs. The martensitic transformation temperature TM is constant at x = 40-49, decreases sharply at x = 49-52, increases gradually at x = 52-56, and is constant again at x = 56-70. The alloys after aging at 773 K for 3.6 ks and 36 ks will have the occurrence of one-stage transformation B2→B19′ at x = 40-50.5, and the TM is constant first and then decreases suddenly with increasing of x; when x = 50.5-52, the alloys have the occurrence of two-stage transformation B2→R→B19′ and the TM decreases gradually with increasing of x; when x=53-70, the alloys have the occurrence of three-stage transformation and the TM1 and TM2 all increase first and then keep constant with increasing of x. All alloys after aging at 773 K for 360 ks have the occurrence of one-stage transformation B2→B19′ except for Ti49Ni51 and Ti47Ni53 alloy, which will have the occurrence of two-stage transformation B2→R→B19′ and three-stage transformation, respectively