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.

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.

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 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 Alloying Elements on Microstructure,Martensitic Transformation and Mechanical Properties of Ni-Mn Based Alloys

The microstructural features, shape memory behavior and mechanical properties of Ni-Mn based alloys were investigated by differential scanning calorimetry （DSC）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and thermal cycling test under various stresses. The transformation temperatures shifted toward lower temperatures when adding a third element into the Ni-Mn system. The addition of 10 at. pct Fe increased considerably the mechanical properties exhibiting still high transformation temperatures. However, it was found that in NiMn40Fe10 alloy the martensitic transformation is not thermoelastic in nature. The mechanism of this transformation and the crystallography of Ni-Mn（50-x）-Fex （x=5, 7, 10, 20 at. pct） alloys are presented.

Analysis of phase transformation from austenite to martensite in NiTi alloy strips under uniaxial tension

Phase transformation from austenite to martensite in NiTi alloy strips under the uniaxial tension has been observed in experiments and numerically simulated as a localized deformation. This work presents an analysis using the theory of phase transformation. The jump of deformation gradient across the interface between two phases and the Maxwell relation are considered. Governing equations for the phase transformation are derived. The analysis is reduced to finding the minimum value of the loading at which the governing equations have a unique, real and physically acceptable solution. The equations are solved numerically and it is verified that the unique solution exists definitely. The Maxwell stress, the stresses and strains inside both austenite and martensite phases, and the transformation-front orientation angle are determined to be in reasonably good agreement with experimental observations.

Effect of Titanium on the Martensitic Transformation Temperature of Cu-14Al-4Ni Shape Memory Alloy Rapidly Solidified

The present research aimed to analyze the influence that different contents of titanium(x=0.5,0.6 and 0.7 wt.%)have on the martensitic transformation temperature of a Cu-14Al-4Ni(wt.%)SMA(shape memory alloy).The Cu-14Al-4Ni-xTi samples were casted in an arc-melting furnace and rapidly solidified.All samples underwent heat treatment in a tubular furnace at a temperature of 1,100°C for 30 min and water quenched at 25°C.Subsequently,samples were analyzed by SEM(scanning electron microscopy)with EDS(energy dispersive spectroscopy),XRD(X-ray diffraction)and DSC(differential scanning calorimeter).SEM images and XRD patterns showed that the presence of titanium modified the alloy’s microstructure,induced the formation of three titanium rich phases called“X”phase(CuNi2Ti,Cu3Ti and AlCu2Ti)and reduced the presence of the brittle phaseγ2(Cu9Al4)for samples with 0.6 and 0.7 wt.%Ti.The titanium added to the copper based SMA also functioned as a refiner,reducing GS(grain size)up to approximately 80%with the increase of Ti content.DSC results exhibited low enthalpy levels,hysteresis,as well as low start martensitic transformation temperatures.

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.

Influence of cooling rate on phase transformation and microstructure of Ti-50.9%Ni shape memory alloy

Heat treatment of Ti-50.9%Ni （mole fraction） alloy was studied by differential scanning calorimetry, X-ray diffraction, scanning electron microscopey and energy dispersive X-ray analysis to investigate the influence of cooling rate on transformation behavior and microstructures of NiTi shape memory alloy. The experimental results show that three-stage phase transformation can be induced at a very low cooling rate such as cooling in furnace. The cooling rate also has a great influence on the phase transformation temperatures. Both martensitic start transformation temperature （Ms） and martensitic finish transformation temperature （Mf） decrease with the decrease of the cooling rate, and decreasing the cooling rate contributes to enhancing the M→A austenite transformation temperature. The phase transformation hysteresis （Af-Mf） increases with the decrease of the cooling rate. Heat treatment is unable to eliminate the textures formed in hot working of NiTi sample, but can weaken the intensity of them. The cooling rate has little influence on the grain size.

Effects of Sm on Phase Transformation in Ni-Mn-Ga Alloys

The effects of small amount additions of Sm on the martensitic transition and magnetic phase transition of polycrystalline Ni-Mn-Ga alloys were investigated. The experimental results show that the Sm doped alloys also undergo a thermal-elastic martensitic transformation and reverse transformation during cooling and heating process and the addition of Sm decreases the martensitic transformation temperature and Curie temperature in different degree respectively. Ni-Mn-Ga alloys of adding Sm still possess Heusler structure, but their crystal lattice parameters are modified slightly. The addition of a proper amount of Sm does not basically decrease T_c of the alloy when avoiding the appearance of second phase. In addition, the doped alloys have favorable toughness because of grain refinement of Sm.

STUDY ON THE MAGNETISM AND PHASE TRANSFORMATIONIN Fe-Mn-Si-C SHAPE MEMORY ALLOYS

The hyperfine interactions of two shape memory alloys have been studied by Mossbauer effect measurement at various temperatures. The Mossbauer spectra exhibit a mag-netic change from antiferro magnetic state to paramagnetic state when the temperuture rises. The Fe-Mn-Si alloys have a small hyperfine field and silicon element increases the hyperfine field and magnetic susceptibility. Thermo-induced γ→ ε trunsforma-tions are suppressed by Neel transition and by increasing carbon content, whereas stress induced γ→ ε transformation occurs in both alloys. Antiferromagnetic spin order can suppress thermo-induced γ→εtransformations efficiently, but cannot sup-press stress induced γ → ε transformation.

Phase transformation behaviors and shape memory effects of TiNiFeAl shape memory alloys

Measurements of electrical resistivity, X-ray diffraction, and tensile test at room temperature and ？196°C were performed to investigate the effects of Al addition substituting Ni on the phase transformation behaviors, the mechanical properties, and the shape memory effects of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys. It is found that 1at% Al addition dramatically decreases the martensitic start transformation temperature and expands the transformation temperature range of R-phase for TiNiFeAl alloys. The results of tensile test indicate that 1at% Al improves the yield strength of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys by 40% and 64%, but de- creases the plasticity to 11% and 12% from 26% and 27% respectively. Moreover, excellent shape memory effect of 6.6% and 7.5% were found in Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys, which results from the stress-induced martensite transformation from the R-phase.

MARTENSITIC AND R PHASE TRANSFOR MATIONS IN NiTiand NiTiHf

The ongoing collaborative research between Auburn University and Johnson Controls was described. A Ti rich binary NiTi alloy and two ternary alloys having the compositions Ni 49 Ti 51- x Hf x with 1% and 3%(more fraction) Hf were investigated. The influence of Hf content, cold working(CW) and heat treatment temperature (HT) on the martensitic and R phase transformations was thermally analyzed using differential scanning calorimetry (DSC). The crystal structures of the alloys were examined by X ray diffraction. Results showed that at low HT, the martensitic transformation temperature (TT) decreases with increasing CW for all the alloys studied. In this alloy, for a given CW, the martensite TT increases with increasing HT above 350 ℃. This phenomenon is enhanced by the presence of Hf. Different mechanisms can be responsible for the observed behavior. These mechanisms may involve interaction between phase transformations, dislocations and internal stresses. The effect of Hf substitution is likely due to the atomic size difference between Ti and Hf. A secondary phase,identified as Ti 2Ni in binary NiTi and (TiHf) 2Ni in the two ternary alloys, was observed. The influence of cold work, heat treatment and thermal cycling on the R phase (R) transformation in three alloys was also thermally analyzed. Results showed that such transformation depends on composition as well as CW and HT conditions in a complex manner. Effort was made to identify the microstructure and the factors responsible for the observed behavior in the R phase transformation.

PHASE TRANSFORMATIONS IN RAPIDLY SOLIDIFIED TiNi SHAPE MEMORY ALLOYS

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Transformation behavior and shape memory effect of Ti_(50-x)Ni_(48)Fe_2Nb_x alloys by aging treatment

The influence of aging treatment on transformation behavior and shape memory of the Ti 50_x Ni_(48) Fe_2 Nb_x(x=0,0.6,0.8,1.0,and 1.2) alloys was investigated using differential scanning calorimeter(DSC),mechanical drawing machine,and microhardness tester in this paper.It is indicated that the aging treatment has a significant effect on the phase transformation temperatures(M_s,M_f,M_p,A_s,A_f,and A_p) and microhardness of the samples.The phase transformation temperatures are found to decrease initially with the increasing aging temperature from 300 to 500 ℃ and increase with further increase of the aging temperature.The aging treatment at intermediate temperature between 400 and 500 ℃ results in an improved shape memory effect.In addition,the highest microhardness value is also obtained.

Dissolution Behavior of Delta Ferrites in Martensitic Heat-resistant Steel for Ultra Supercritical Units Blades

The dissolution behavior of delta ferrites in martensitic heat-resistant steel was studied.And the reason why the dissolution rate of delta ferrites decreased with dissolution time was discussed.The experimental results show that the chemical compositions of delta ferrites negligibly change with dissolution time.The decrease of dissolution rate of delta ferrites with dissolution time should be attributed to the change of shape and distribution of delta ferrites.The shape of delta ferrites tends to transfer from polygon to sphere with dissolution time,causing the decrease of specific surface area of delta ferrites.The distribution position of delta ferrites tends to transfer from boundaries of austenite grains to interior of austenite grains with dissolution time,decreasing the diffusion coefficient of alloy atoms.Both them decrease the dissolution rate of delta ferrites.

TRANSFORMATION OF RETAINED AUSTENITE IN CARBURIZED CASE DURING FATIGUE CRACK GROWTH

The morphology of the retained austenite in the carburized case of 20CrNiMo steel and its transformation during fatigue crack propagation through the case were investigated by using X-ray and TEM analysis.In the carburized case both film and block shaped retained austenite were found.Due to the crystallographic orientation relationship at the interface,the fatigue crack is inclined to pass through the block shaped retained austenite and thereby stim- ulates its strain-induced martensitie transformation.During the process of the fatigue frac- ture,most of the retained austenite structures on the crack path are transformed into the martensite,and the untranaformed parts on the fracture surface remain less than 6%.The transformation of the retained austenite,which is restrieted mainly within the plastic zone,oc- curs only during the proeess of fracture,and is independent of the magnitudes of the external stress,stress ratio and cyclic number.The volume expansion accompanying the transforma- tion creates an additional residual displacement of about 0.44μm on fracture surfaces,which is equivalent to the magnitude of the plasticity-induced residual displacement.The phase transformation induced fatigue crack closure is believed to be an important factor affecting the fatigue crack behaviors in the high carbon laver of the carburized case.

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）.

Anomalous Phase Transformation from Martensite to Austenite in Fe-13%Cr-4%Ni-Mo Martensitic Stainless Steel

The thermal stability of the reversed austenite in a low carbon Fe-13%Cr-4%Ni-Mo（in wt pct） martensitic stainless steel was investigated by X-ray diffraction（XRD） and superconducting quantum interference device（SQUID） magnetometer.Different amount of the reversed austenite was obtained by controlling tempered temperature.It was found that the amount of the reversed austenite increased after keeping the specimens at liquid nitrogen temperature for 72 h.The mechanism of the anomalous phase transformation from martensite to austenite was discussed.