简介概要

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

来源期刊：Rare Metals2013年第5期

论文作者：Xiao-Yun Song Yan Li Fei Zhang

文章页码：475 - 479

摘要：NiTi–Al-based alloys are promising high-temperature structural materials for aerospace and astronautics applications.A new NiTi–Al-based alloy Ni–43Ti–4Al–2Nb–2Hf（at%）was processed via isothermal forging.The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope（SEM）,X-ray diffraction（XRD）,and tensile tests.Results show that the microstructure of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy consists of NiTi matrix,Ti2Ni phase,and Hf-rich phase.The simultaneous addition of Nb and Hf,which have strong affinities for Ti sites,promotes the precipitation of Hf-rich phases along the grain boundaries.The tensile strengths of Ni–43Ti–4Al–2Nb–2Hf alloy are dramatically increased compared with the ternary Ni–46Ti–4Al alloy.At room temperature and 650°C,the yield stress of Ni–43Ti–4Al–2Nb–2Hf alloy reaches 1,070 and 610 MPa,respectively,which are 30%and 150%higher than those of Ni–46Ti–4Al alloy.The improved tensile property results from the solid solution strengthening by Nb and Hf,as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.

稀有金属 (英文版) 2013,32(05),475-479

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

State Key Laboratory of Nonferrous Metals & Processes, General Research Institute for Nonferrous Metals

Key Laboratory of Aerospace Materials and Performance (Ministry of Education), Beijing University of Aeronautics and Astronautics

摘要：

NiTi–Al-based alloys are promising high-temperature structural materials for aerospace and astronautics applications.A new NiTi–Al-based alloy Ni–43Ti–4Al–2Nb–2Hf (at%) was processed via isothermal forging.The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope (SEM) , X-ray diffraction (XRD) , and tensile tests.Results show that the microstructure of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy consists of NiTi matrix, Ti2Ni phase, and Hf-rich phase.The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries.The tensile strengths of Ni–43Ti–4Al–2Nb–2Hf alloy are dramatically increased compared with the ternary Ni–46Ti–4Al alloy.At room temperature and 650°C, the yield stress of Ni–43Ti–4Al–2Nb–2Hf alloy reaches 1, 070 and 610 MPa, respectively, which are 30%and 150%higher than those of Ni–46Ti–4Al alloy.The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.

收稿日期：12 August 2013

基金：supported by the National Natural Science Foundation of China (No. 51201016);

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

Xiao-Yun Song Yan Li Fei Zhang

State Key Laboratory of Nonferrous Metals & Processes, General Research Institute for Nonferrous Metals

Key Laboratory of Aerospace Materials and Performance (Ministry of Education) , Beijing University of Aeronautics and Astronautics

Abstract：

NiTi–Al-based alloys are promising high-temperature structural materials for aerospace and astronautics applications. A new NiTi–Al-based alloy Ni–43Ti–4Al– 2Nb–2Hf (at%) was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope (SEM) , X-ray diffraction (XRD) , and tensile tests. Results show that the microstructure of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy consists of NiTi matrix, Ti2 Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni–43Ti–4Al–2Nb–2Hf alloy are dramatically increased compared with the ternary Ni–46Ti–4Al alloy. At room temperature and 650 °C, the yield stress of Ni–43Ti–4Al– 2Nb–2Hf alloy reaches 1, 070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni–46Ti– 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2 Ni and Hf-rich phases.

Keyword：

Intermetallics; NiTi–Al alloy; Isothermal forging; Tensile property; Microstructure;

Author： Xiao-Yun Song e-mail: songxiaoyun82@126.com ;

Received： 12 August 2013

1 Introduction

Near-equiatomic Ni Ti binary alloys, known as shape memory alloys, have good tensile strength and ductility at room temperature.However, strength and oxidation resistance of Ni Ti alloys decrease sharply at temperature above500°C, limiting their application at higher temperature[1, 2].Recently, the strength of Ni Ti alloys at ambient and elevated temperature was improved by the addition of Al through solid solution strengthening and precipitation hardening of the Ti₂Ni and Ni₂Ti Al phases[3, 4].For instance, the compressive yield strength of Ni–50Ti–6Al alloy at 600°C exceeds 650 MPa.Therefore, Ni Ti–Albased alloys attracted attention as potential high-temperature structural materials because of their low density, high specific strength, and oxidation resistance.

Previous studies examined the effects of adding alloying elements such as Nb[5], Mo[6], Zr[7], and Hf[8]on microstructures and mechanical properties.These elements, which are classified as solid solution strengthening elements, reportedly enhance Ni Ti–Al-based alloys.For example, the compressive yield stress of Ni–42Ti–4Al–4Hf alloy at 650°C reaches*700 MPa.In addition, Nb effectively improves the oxidation resistance of Ni Ti–Al alloys, with the addition of 2 at%fulfilling the required oxidation resistance at 800°C[5, 9].The Ni/Ti ratio also influences the mechanical properties and oxidation resistance of Ni Ti–Al-based alloys[10, 11].

As with other structural materials, the high-temperature strength of Ni Ti–Al-based alloys is a tradeoff with their RT ductility.Zheng et al.prepared Ni Ti–Al-based alloys using directional solidification so as to obtain better property[12, 13].Isothermal forging is another valid way for improving the mechanical properties of intermetallics.Few studies reported on the tensile properties of isothermally forged Ni Ti–Al-based alloys.In the present work, a Ni–43Ti–4Al–2Nb–2Hf alloy was designed based on the Ni–46Ti–4Al alloy and prepared via isothermal forging.The microstructure and tensile behavior of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy were discussed in detail.

2 Experimental

Nominal compositions of Ni–46Ti–4Al and Ni–43Ti–4Al–2Nb–2Hf (at%) alloys were prepared using high purity Ni (99.98 wt%) , Ti (99.8 wt%) , Al (99.99 wt%) , Nb (99.87 wt%) , and Hf (99.9 wt%) in a vacuum induction melting furnace on a water-cooled copper hearth and cast into a graphite mold.The ingots were repeatedly melted four times to insure homogenization.U80 mm 9 120 mm workpiece was cut from the ingot and isothermally forged at 950°C until its height decreased by60%.Samples for microstructure observations, X-ray diffraction (XRD) , and tensile tests were prepared from the as-forged alloy via electrical discharge machining.

XRD analysis was conducted for phase identification on an X-ray diffractometer (Rigaku D/max 2200PC) using Cu Ka radiation.The microstructures were observed on a Cambridge3400 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) .The tensile tests were performed in open air using an MTS880 testing machine with an initial strain rate of 4 9 10^-4s^-1from room temperature to 800°C.Round specimens with 5 mm diameters and25 mm gauge lengths were used.The fracture surface morphologies were observed on a Cambridge 3400 SEM.

3 Results and discussion

3.1 Microstructures

The back-scattered electron micrographs of the Ni–46Ti–4Al and Ni–43Ti–4Al–2Nb–2Hf alloys prepared via isothermal forging are presented in Fig.1.It can be seen that the grains of the two as-forged alloys are elongated perpendicular to the forging direction, with a grain size of*50–100 lm.As shown in Fig.1a, the ternary Ni–46Ti–4Al alloy is composed of dual phases, i.e., grey matrix and small amount of black second phases along the grain boundaries.According to our previous study[8], the matrix is primary Ni Ti, and the black phase is Ti₂Ni produced by the peritectic reaction during the Ni Ti alloy solidification, with Al dissolved in the two phases.In Ni Ti–Al-based alloys, the existence of Ti₂Ni phase can strengthen the Ni Ti matrix, but reduce room-temperature ductility because of its hardness and high brittleness[3].

With the addition of 2 at%Nb and 2 at%Hf, as shown in Fig.1b, the microstructure consists of four regions, namely, a grey matrix (A) , a light grey matrix (B) , a black phase (C) , and a white phase (D) .The composition of each region is presented in Table 1, and the XRD pattern is shown in Fig.2.Based on the XRD and EDS results, both regions A and B in Fig.1b are Ni Ti phases, while the latter phase contains more Nb and Hf elements than the former.As refractory elements, owing to their quite low diffusion rate, Nb and Hf are segregated along the grain boundaries during solidification.Region C in Fig.1b is Ti₂Ni phase with some Al, Nb, and Hf dissolved.Region D in Fig.1b is a Hf-rich phase and located primarily on the grain boundaries accompanied by the Ti₂Ni phase.It is found that the solubility of Hf in Ni Ti is more than 2 at%.There is no Hf-rich phase observed in Ni–44Ti–4Al–2Hf alloy with the same Hf content[8].When added to Ni Ti alloy, Hf and Nb both primarily replace Ti atoms[14, 15].It is therefore suggested that the simultaneous addition of Nb and Hf decreases the solubility of the two elements in the Ni Ti matrix and leads to the precipitation of Hf-rich phase.

Table 1 EDS result of phase compositions of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy (at%) 下载原图

Table 1 EDS result of phase compositions of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy (at%)

Fig.1 SEM images of Ni–46Ti–4Al a and Ni–43Ti–4Al–2Nb–2Hf b alloys prepared via isothermal forging

Fig.2 XRD pattern of as-forged Ni–43Ti–4Al–2Nb–2Hf alloy at room temperature

3.2 Tensile properties

The tensile properties of Ni–46Ti–4Al and Ni–43Ti–4Al–2Nb–2Hf alloys at room temperature (25°C) and at elevated temperature, including yield stress (YS) , ultimate tensile stress (UTS) , and elongation are shown in Fig.3.At room temperature, the yield and ultimate tensile strengths of Ni–46Ti–4Al alloy are 800 and 1, 090 MPa, respectively.With Nb and Hf addition, the yield and ultimate tensile strengths of Ni–43Ti–4Al–2Nb–2Hf alloy increase to 1, 070 and 1, 285 MPa, respectively.Similar to other Ni Ti–Al-based alloys, YS and UTS of the two alloys decrease markedly with the increase of temperature.At800°C, an obvious strain hardening does not happen which can be seen in the tensile stress–strain curve.That is, the YS is close to the UTS.This result is consistent with that on the Ni–42Ti–4Al–4Zr alloy[7]and is probably related to dynamic recrystallization during the coarsening of the particles at 800°C[3].The YS of the Ni–43Ti–4Al–2Nb–2Hf alloy at 650 and 800°C are 610 and 295 MPa, respectively, which are considerably higher than those of the Ni–46Ti–4Al alloy.Therefore, the addition of Nb and Hf dramatically improves the strengths of Ni Ti–Al alloys at room and elevated temperatures.However, as shown in Fig.3c, the ductility of Ni Ti–Al alloy is sharply deteriorated by adding Nb and Hf.For Ni–43Ti–4Al–2Nb–2Hf alloy, the room-temperature tensile tests are stopped before the alloy apparently yields.As temperature rising up to650°C, the elongation markedly increases to 15%.Thus, the brittle to ductile temperature (BDTT) of Ni–43Ti–4Al–2Nb–2Hf alloy is below 650°C.

Fig.3 Tensile properties of Ni–46Ti–4Al and Ni–43Ti–4Al–2Nb–2Hf alloys at different temperatures:a yield stress, b ultimate tensile stress, and c elongation

Fig.4 Fracture surface morphologies of Ni–43Ti–4Al–2Nb–2Hf alloy after tensile tests at a room temperature and b 650°C

Figure 4 shows the fracture morphologies of the Ni–43Ti–4Al–2Nb–2Hf alloy after the tensile test at room temperature and 650°C.A cleavage fracture is clearly observed on the surface of the room-temperature tension specimen, indicating brittle deformation (Fig.4a) .At650°C, numerous equiaxial dimples are visible, indicating ductile fracture (Fig.4b) .These results are consistent with the tensile curves, which suggest that the BDTT of the Ni–43Ti–4Al–2Nb–2Hf alloy is less than 650°C.

Based on the above analysis, adding Nb and Hf significantly improves the strengths of Ni Ti–Al alloy at room and high temperatures.As shown in Fig.1b, the microstructure of the Ni–43Ti–4Al–2Nb–2Hf alloy consists of Ni Ti matrix, Ti₂Ni phase and Hf-rich phase.Generally, the strength of multiphase alloys depends on the content, strength, and distribution of each phase.Therefore, two factors contribute to the improvement of strength.Firstly, from Table 1, refractory elements Nb and Hf are dissolved in the Ni Ti matrix and the Ti₂Ni phase.In Ni Ti–Al–Nb and Ni Ti–Al–Hf alloys, Nb and Hf are served as strengthening elements in solid solutions, and increase the tensile strength at room temperature and elevated temperatures[5, 8].Therefore, the solid solution strengthening by Nb and Hf greatly enhances the tested alloy.Meanwhile, as refractory elements, Nb and Hf can decrease the diffusion rate at elevated temperatures, which will further improve the hightemperature strength of the alloy.Secondly, compared with the Ni–46Ti–4Al alloy, the addition of Nb and Hf accelerates the precipitation of the Hf-rich phase, which is stronger than Ni Ti matrix.Thus, the Hf-rich phase can result in dispersion hardening effect.

Given the importance of the strength to weight ratio of structural materials, the specific yield strength of Ni–43Ti–4Al–2Nb–2Hf alloy was calculated to be 166 MPa? (g?cm^-3) ^-1at room temperature and 94 MPa? (g?cm^-3) ^-1at 650°C.These values are comparable or even higher than those of GH4169, a widely used nickel-based superalloy.Accordingly, the Ni–43Ti–4Al–2Nb–2Hf alloy has potential applications at medium temperatures.However, the strength of these alloy declines sharply at 800°C, which hinders its widespread application.Meng et al.[3]suggested that the softening of Ti₂Ni at 800°C is the major reason for the lower strength of the Ni Ti–Al-based alloys.This phenomenon is due to the low melting point of Ti₂Ni (982°C) .Pan et al.[16]reported that the Ti₂Ni phase softens because of cracks that originate from the intercellular regions and grain boundaries.Hence, strategies such as heat treatment should be used to control Ti₂Ni softening or to introduce a high-strength phase.It has been reported that in Ni–42Ti–4Al–4Hf alloy the Ti₂Ni phase dissolves via solid solution treatment at 1, 100°C.During aging at temperatures of 600–800°C, numerous fine Ni₂Ti Al (Heusler, L2₁) intermetallic phase particles precipitate within the Ni Ti matrix and retain coherent or semi-coherent to the matrix.This makes the Ni Ti–Al-based alloys form a dual-phase structure that is analogous to the classical c/c⁰system of nickel-based superalloys[17].According to the above-mentioned phenomena, different heat treatments were conducted on the asforged Ni–43Ti–4Al–2Nb–2Hf alloy.Figure 5 shows the SEM images of the alloy after solution treatment at 1, 100°C.Compared with the as-forged microstructure in Fig.1b, Ti₂Ni phases disappear after solution treatment.Thus, it is demonstrated that brittle Ti₂Ni phases can be removed in the tested alloy.This will be discussed in detail later.

Fig.5 SEM image of as-forged Ni–43Ti–4A–2Nb–2Hf alloy after solution treated at 1, 100°C

4 Conclusion

In this study, Ni–46Ti–4Al and Ni–43Ti–4Al–2Nb–2Hf alloys were prepared via isothermal forging.The microstructure and tensile properties at room and high temperatures were investigated.Results show that the as-forged Ni–46Ti–4Al alloy is composed of Ni Ti matrix and small amount of Ti₂Ni phases.For the Ni–43Ti–4Al–2Nb–2Hf alloy, with 2 at%Nb and 2 at%Hf addition, a Hf-rich phase is precipitated along the grain boundary accompanied with Ti₂Ni phase.This is due to the simultaneous addition of Nb and Hf elements which tend to occupy Ti sites in Ni Ti.

Compared with Ni–46Ti–4Al alloy, the tensile strengths of Ni–43Ti–4Al–2Nb–2Hf alloy at room and high temperatures dramatically increase by the addition of Nb and Hf.At 650°C, the YS of 610 MPa and specific yield strength of 94 MPa? (g?cm^-3) ^-1are achieved.The increase of strength is attributed to the solid solution strengthening by Nb and Hf, as well as dispersoid hardening of the Ti₂Ni and Hf-rich phases.