简介概要

Phase transformation behaviors and mechanical properties of Ti₅₀Ni₄₉Fe₁ alloy with severe plastic deformation

来源期刊：Rare Metals2013年第5期

论文作者：Wen Ma Bin Chen Fu-Shun Liu Qing Xu

文章页码：448 - 452

摘要：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.

稀有金属 (英文版) 2013,32(05),448-452

Phase transformation behaviors and mechanical properties of Ti₅₀Ni₄₉Fe₁ alloy with severe plastic deformation

Division of Science and Technology Information, General Research Institute for Nonferrous Metals

School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics

Metrology and Phsico-Chemical Institute, Capital Aerospace Machinery Company

摘要：

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.

收稿日期：6 August 2013

基金：supported by the National Natural Science Foundation of China (No. 50921003);the Industry, Education and Research Projects of the China Aviation Industrial (No. cxy2012BH04);

Phase transformation behaviors and mechanical properties of Ti₅₀Ni₄₉Fe₁ alloy with severe plastic deformation

Wen Ma Bin Chen Fu-Shun Liu Qing Xu

Division of Science and Technology Information, General Research Institute for Nonferrous Metals

School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics

Metrology and Phsico-Chemical Institute, Capital Aerospace Machinery Company

Abstract：

In this work, transformation behaviors and mechanical properties of cold-rolled shape memory alloy Ti50 Ni49 Fe1 by 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 Ti50 Ni49 Fe1 alloy. 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 % after 30 % SPD, ascribed to the induced defects in cold rolling. Those results indicate that Ti50 Ni49 Fe1 alloy has improved mechanical properties and potential commercial applications after SPD.

Keyword：

TiNiFe alloy; Cold-rolling; Severe plastic deformation (SPD) ; Shape memory effect;

Author： Wen Ma e-mail: mawen19860507@126.com ;

Received： 6 August 2013

1 Introduction

Owing to its outstanding shape memory effect, pseudoelasticity and excellent mechanical properties, Ti Ni-based shape memory alloy is renowned as quite appealing materials and are now being practically used as functional materials for pipe couplings, antennae for satellites, various actuators, medical implants, and guide wires[1, 2].The strength of shape memory alloys is a extremely vital parameter in the practical application.If the applying stress exceeds the strength of the raw materials that can bear, it may lead to the fail of the parts[3].

It is well known that the means of strengthening the binary Ti–Ni alloy can be classified as followings:aging after solution treatment (Ni[50.5 at%practically) [4, 5]and specific thermo-mechanical treatments, i.e., annealing following cold working (Ni\50.5 at%practically) [6], thermal and/or stress cycling[7, 8].SPD is a novel technology used to strengthen raw materials.There are some reports of this method.Valiev et al.[9–11]found that it can obviously improve both tensile strength and elongation of by fining grains employing SPD.Brailovski et al.[12–14]investigated the influence of thermomechanical method on Ti–50.0 at%Ni alloys.They obtained alloy with average grain size of 60–80 nm and advanced recovery strain of 8%and recovery stress of1, 400 MPa applying SPD and post-deformation annealing.However, there are few reports about the influence of SPD on the Ti Ni Fe ternary alloy.This paper investigated the influence of SPD on the Ti Ni Fe alloy, improved both yield stress and shape-memory effects of Ti Ni Fe shape memory alloys, while the Ti Ni Fe alloy is remained relative high elongation.

2 Experimental

Four ingots of quaternary alloys with the nominal compositions of Ti₅₀Ni₄₉Fe₁ (at%) were prepared by induction melting a mixture of 99.7%Ti, 99.7%Ni, and 99.99%Fe in a high frequency vacuum furnace.Ingots were remelted four times and then homogenized in evacuated quartz tubes at 850°C for 24 h.Then ingots were hotrolled into plates with different thicknesses, following with cold-rolling into plates with thickness of 1.1 mm, insuring thickness reduction to be 0%, 15%, 30%, and 50%, marked as CR0%, CR15%, CR30%, and CR50%in this work, respectively.Specimens for measurements were spark cut from the plates.

The phase transformation temperatures were determined by a Q2000 differential scanning calorimetry (DSC) .The temperatures ranged from-90 to 100°C, with a heating/cooling rate of 10°C?min^-1.Phase identification was performed in a Rigaku D/max-r B X-ray diffractometer (XRD) with Cu Ka radiation (k=0.15405 nm, 2h=20°–90°) and a scan speed of 6 (°) ?min^-1.The microstructures of Ti₅₀Ni₄₉Fe₁alloy were performed by a transmission electron microscopy (TEM, FEI Quanta 200F) .Mechanical properties were detected on a MTS model 880 with a strain rate of 5.6 9 10^-4at room temperature (25°C) .Shapememory effect was performed on a tensile test machine (MTS model 880) at-196°C and then pre-strained alloys were heated to 120°C to measure the recovery strain.

3 Results and discussion

3.1 Effect of cold-rolling on phase transformation properties

Figure 1 shows the transformation behaviors of Ti₅₀Ni₄₉Fe₁alloy with CR0%, CR15%, CR30%, and CR50%.It can be seen that Ti₅₀Ni₄₉Fe₁alloy exhibits two exothermic peaks (presenting A?R and R?B19⁰transformations, respectively) and one endothermic peak (presenting M?A transformation) .It is evident that the peaks of both A?R and R?B19⁰transformations are significantly broadened and weakened by SPD.As shown in Fig.1, the alloy with CR15%exhibits A?R, R?B19⁰, and M?A transformations.With more SPD (up to CR30%) , however, R?B19⁰transformation is suppressed, as shown in Fig.1.

Meanwhile, A?R transformation still exists, with widening transformation peaks.This can be ascribed to the large amount of defects (such as dislocation) producing during cold rolling.Defects hinder the movement of the interface between two phases in transformation.In such a case, the transformation peaks get wider and weaker, and vanish eventually.The lattice distorsion of R?B19⁰is larger than that of A?R[15], as a result, the peak of R?B19⁰transformation disappears while that of A?R still remains weak[16].

Fig.1 DSC curves of Ti50Ni49Fe1alloy with different cold-rollings

3.2 XRD analysis

Figure 2 shows the XRD patterns of Ti₅₀Ni₄₉Fe₁alloy after different cold-rollings at room temperature.It can be seen that at room temperature, Ti₅₀Ni₄₉Fe₁alloy with CR0%mainly consists of B2 phase and a small amount of R phase.But after cold rolling, a small amount of martensites (M) appear.The martensitic start transformation temperature (M_s) of Ti₅₀Ni₄₉Fe₁is 18°C, as shown in Fig.1.Some weak peaks are observed at the 2h of 29°, 31°, and 33°, and the intensities of them increase with the increase of SPD, as shown in Fig.2.It is confirmed that these peaks corresponds to martensites’peak according the PDF card.Therefore, although the M_sis lower than room temperature (18°C) , the cold rolling results in stress induced martensite transformation and M phase remains in the alloys with cold rolling at room temperature.

Fig.2 XRD patterns of Ti50Ni49Fe1alloy after different cold-rollings

Meanwhile, it can be seen from XRD patterns in Fig.2that (110) _B2is observed in 42.2°without cold rolling, but shifted to right in 42.6°with cold rolling.The peak of (110) _B2is obviously widened with the increase of cold rolling.The widened peak (110) _B2might be caused by two reasons[15]: (1) lattice defects, resulting from the introduction of much residual stress, influences the XRD patterns; (2) the grains are refined because of SPD after which some grains are broken.According to Debye–Scherrer formula:

where K is constant, 0.89;D is the particle size, nm;B is the integral half width, rad;h is the diffraction degree, (°) ;and k is the wave length of X-ray, nm.It can be calculated from Eq. (1) that after the grains are refined, smaller D will result in the obvious widened peak (larger h) of XRD patterns.

3.3 Effect of cold-rolling on microstructures

Figure 3 shows the TEM images of Ti₅₀Ni₄₉Fe₁alloy after CR0%and CR50%.Comparing with Fig.3a, it is apparent from the Fig.3b that a relatively high density of dislocation is introduced via cold rolling, as shown by the arrow in Fig.3b.The corresponding selected area diffraction pattern (SADP) with a beam direction of (111) _B2is shown in the insert.Diffraction rings are observed.The inner rings corresponds to (110) planes of the B2 phase, with the outer (211) and (220) of the B2 phase, respectively, in which the grains of the cold-rolled alloys are obviously refined.

3.4 Effect of cold-rolling on mechanical properties

Figure 4a shows the stress–strain curves of Ti₅₀Ni₄₉Fe₁alloys with different cold-rolling reductions.For comparison’s sake, the magnitudes of r_b, r_Sand elongation were plotted against thickness reduction in Fig.4b.As illustrated in Fig.4, r_bincreases remarkably with cold-rolling rate increasing, from 618 to 1, 338 MPa with CR0%to CR50%.Also, r_Sincreases with cold rolling increasing.In contrast, the elongation of the alloys exhibits a gradual decrease with cold rolling increasing, as revealed in Fig.4b.Ti₅₀Ni₄₉Fe₁alloy with CR15%exhibits a slight decrease (from 25%of alloy with CR0%to 24%) .Nevertheless, alloys applied more SPD display sharp declining.For example, Ti₅₀Ni₄₉Fe₁alloy with SPD of30%and 50%remain elongation of 16%and 12%, respectively.

This can be ascribed to two following reasons: (1) the high density of dislocation accompanied with severe cold deformation impedes the movement of the particular dislocation; (2) the refined grains result in a mass of grain boundaries which can effectively hinder the slip of dislocations.

For polycrystalline materials, the Hall–Petch equation can describe the relationship between yield strength and grain size[17].Yield strength increases with a decreasing grain size according to the following equation:

where r is the yield stress, and d is the average grain size.Typically, r₀is rationalized as either a frictional stress resisting the motion of gliding dislocations or as an internal back stress.k is the Hall–Petch slope, which is considered to be a measure of the resistance of the grain boundary to slip transfer.

Fig.3 TEM bright field images and (111) B2zone axis selected area electron diffraction (SAED) images of Ti50Ni49Fe1alloy after a CR 0%and b CR 50%

Fig.4 Strain–stress curves of Ti50Ni49Fe1alloys with different cold-rolling reductions a and thickness reduction dependence of rS, rband elongation in the Ti50Ni49Fe1alloys b

Fig.5 Strain dependence of shape-recovery ratio of Ti50Ni49Fe1alloy (curve for alloy with CR 50%being only from 6%to 8%owing to the failure of sample)

At the same time, although the alloy is suffered certain amount of decrement, the Ti₅₀Ni₄₉Fe₁alloy with coldrolling still has a relatively high level elongation of 12%.For shape memory alloys, shape memory effect is another vitally important parameter to evaluate the mechanical properties of these materials.To characterize the influence of cold rolling on the Ti Ni Fe shape memory alloys, the shape memory effect of Ti₅₀Ni₄₉Fe₁alloys with CR0%, CR15%, and CR30%were tested.

As shown in Fig.5, the recovery strain increases with pre-strain level increasing, and the maximum complete recovery strain value of 8.8%is obtained in CR30%alloy.Moreover, the recovery ratio of the hot rolled alloy drastically decreases with pre-strain level increasing.It reduces to*88%with pre-strain of 12%, while that of CR30%alloy maintains relatively high recovery ratio—above 98%.As we know, stress over yield stress (r_S) leads to plastic deformation, which results in irreversible strain of alloys.As seen from Fig.4, r_Sof the cold-rolled alloys are obviously increased.Therefore, higher stress is required to generate plastic deformation.In such a case, the shape memory effects of cold-rolled alloys get enhanced compared with hot-rolled alloy.

4 Conclusion

In this work, shape memory alloy Ti₅₀Ni₄₉Fe₁cold-rolled by SPD (0%, 15%, 30%, 50%) was investigated.There are no martensitic phase transformations when the deformation is up to 30%.And the amount of R phase decreases with the increase of SPD due to the enlargement of temperature interval for R-phase transformation.After coldrolling, the alloy is mainly composed by B2 parent phases, some stress-induced martensitic B19⁰phases occur, and then high density of dislocations are generated, which result in the obvious refinement of grains in the alloys.The yield stress r_bsignificantly increases from 618 MPa with CR0%to 1, 338 MPa with CR50%, respectively.The elongation decreases and remains 12%for 50%SPD.Shape memory effect of 30%SPD increases from 6.5%of CR0%to 8.5%after CR30%.Therefore, the SPD is beneficial for mechanical properties of shape-memory alloy Ti₅₀Ni₄₉Fe₁.