The effect of annealing treatments and thermomechanical cycling on the transformation behaviors and shape memory effect of Ti48.5Ni48Fe2Nb1.5 shape memory alloys were investigated using electrical resistivity measurement and tensile testing. It is found that the transformation behaviors are influenced considerably by the annealing treatments. Both Ms and As increase with increasing annealing temperature and cooling rate. Martensite stabilization occurs during thermomechanical cycles, thus resulting in lower Ms and recovery ratio. Moreover, thermomechanical cycling leads to pseudo-elasticity. The yield point of stress-induced martensite (σSIM) increases with increasing cycles and pre-formation.
In this work,transformation behaviors and mechanical properties of cold-rolled shape memory alloy Ti50Ni49Fe1by severe plastic deformation(SPD)were intensively investigated.The phase transformation behaviors,phase analysis,and microstructures were characterized by differential scanning calorimetry(DSC),X-ray diffraction(XRD),and transmission electron microscopy(TEM),respectively.Tensile testing was performed to analyze the effect of SPD on the mechanical properties and shape memory of Ti50Ni49Fe1alloy.When the thickness reduction is beyond 30%,the martensitic transformation is suppressed.After cold-rolling,the alloy is mainly composed of B2 parent phases with some stress-induced martensitic B190phases,and high density of dislocations are generated and the grains are obviously refined.The yield stress rb significantly raises from 618 MPa of 0%cold rolling to 1,338 MPa of 50%SPD.Shape-memory effect increases from 6.5%without cold rolling to 8.5%after30%SPD,ascribed to the induced defects in cold rolling.Those results indicate that Ti50Ni49Fe1alloy has improved mechanical properties and potential commercial applications after SPD.
The effects of annealing on the phase transformation behavior and superelasticity of cold-rolled Ti50Ni48Fe2 shape memory alloy were extensively investigated. Curves of temperature dependence of electrical resistivity reveal that both the cold-rolled and annealed specimens exhibit a B2→R→B19'two-stage martensitic transformation upon cooling and a B19'→B2 one-stage transformation upon heating, although the austenitic transformation temperature decreases with the increase of the annealing temperature. Tensile stress–strain curves show the critical stress for stress-induced martensite(rSIM)of Ti50Ni48Fe2 alloys decreases with the increase of annealing temperature due to the decrement of dislocation density caused by the recrystallization. As a result, the rSIM decreases. Upon a cold-rolling and annealing at 623 K for30 min, the Ti50Ni48Fe2 alloy exhibits excellent superelasticity with the maximum recoverable strain of 5.8 % at a loading strain of 7 %. In such a case, a complete superelasticity of 5 % can be obtained in the Ti50Ni48Fe2 alloy after deformation increasing to 15 cycles.
The microstructures and mechanical properties of Ti–47 at%Ni–3 at%Fe shape memory alloy wire under the condition of severe cold-drawing at room temperature and different postdeformation annealing processes were intensively investigated using transmission electron microscope(TEM),X-ray diffraction(XRD),Vickers microhardness tester and electron tensile tester.It is indicated that the structure of the alloy evolves into a predominant amorphous structure with a trace of nanocrystalline B2 phase after the cold-drawing of 76%areal reduction.Postdeformation annealing process exerted significant influence on the microstructure and mechanical properties.Crystallization occurs when the cold-drawn wire was annealed at 300℃ for 30 min.The ultimate tensile strength and ductility as well as the superelasticity of the wire are improved significantly by cold-drawing plus postdeformation annealing.
The phase stability and hardness of Ti–30Zr–M(M=Nb,V,Cr,Mo,Fe,Ni and Al)shape memory alloys were investigated by structural observations,d-electron alloy design and microhardness tests.Optical microscopy and X-ray diffraction results show that Nb,V,Cr,Mo and Fe are allβ-stability elements in Ti–30Zr–M alloys with the effect enhanced by the presence of Zr.The composition of the least stableβ-phase alloy values of Nb,V,Cr,Mo and Fe in Ti–30Zr alloys are 12%,9%,5%,3%and 2%,respectively,indicating an increasing sequence of theβ-stability ability.Ni and Al showα-stability or moderate effects and the addition of Ni induces the appearance of(Ti,Zr)_(2)Ni phase in Ti–30Zr–5/15Ni alloys.Based on the d-electron alloy design method,the bond order of Ti and alloying atoms(Bo)and the metal d-orbital energy level(Md)are calculated to build the Bo–Md diagram of Ti–30Zr–M alloys associated with the phase structure identification.The microhardness of Ti–30Zr alloy is increased by addition of a small amount of the alloying element,except for Nb.There is a drop of the microhardness of each Ti–30Zr–M alloys whenβphase appears by the addition of more alloying elements.
The effect of the orientation on the magnetostriction in Fe 81 Ga 19 alloy has been investigated experimentally and theoretically.The Fe 81 Ga 19 [001] and [110] oriented crystals were prepared and the magnetostriction was measured under different pre-stress.The saturation magnetostriction of the [001] oriented crystal increases from 170×10-6 to 330×10-6 under the pre-stress from 0 to 50 MPa.The [110] oriented crystal has a saturation magnetostriction from 20×10-6 to 140×10-6 with the compressive pre-stress from 0 to 40 MPa.The magnetostriction of [001] and [110] oriented crystals has been simulated based on the phenomenological theory.The domain rotation path has been determined and the resultant magnetostriction calculated under different pre-stress.The experimental and simulated results both show that the [001] oriented crystal exhibits better magnetostriction than [110] oriented crystal.The enhancement of the saturation magnetostriction by the compressive pre-stress in the [110] oriented crystal is higher than that in the [001] oriented crystal.
The effect of austenite aging at 823 K on the microstructures and martensitic transformation behavior of Co 46 Ni 27 Ga 27 alloy has been investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and differential scanning calorimeter (DSC). The microstructure observation results show that the unaged Co 46 Ni 27 Ga 27 alloy is composed of the tetragonal nonmodulated martensite phase and face-centered cubic γ phase. It is found that a new nanosized fcc phase precipitates in the process of austenite aging, leading to the formation of metastable age-affected martensite around the precipitates with composition inhomogeneity. Two-stage reverse martensitic transformation occurs in the samples aged for 2 and 24 h due to the composition difference between the age-affected martensite and the original martensite. For the Co 46 Ni 27 Ga 27 alloy aged for 120 h, no reverse transformation can be detected due to the disappearance of the metastable age-affected martensite and the small latent heat of the original martensite. The martensitic transformation temperatures of the Co 46 Ni 27 Ga 27 alloy decrease with an increase in aging time.
The first-order phase transition in Gd5Si2Ge2 is sensitive to both magnetic field and pressure.It may indicate that the influences of the magnetic field and the pressure on the phase transition are virtually equivalent.Moreover,theoretical analyses reveal that the total entropy change is almost definite at a certain Curie temperature no matter whether the applied external field is a magnetic field or a pressure.The entropy change curve can be broadened dramatically under pressure,and the refrigerant capacity is improved from 284.7 J/kg to 447.0 J/kg.
