简介概要

Microstructure and properties of Cu-2.8Ni-0.6Si alloy

来源期刊：Rare Metals2013年第3期

论文作者：Xiang-Peng Xiao Bai-Qing Xiong Guo-Jie Huang Lei Cheng Li-Jun Peng

文章页码：228 - 233

摘要：The phase transformation behavior and heat treatment response of Cu-2.8Ni-0.6Si （wt%） alloy subjected to different heat treatments were studied by X-ray diffraction, transmission electron microscopy observation, and measurement of hardness and electrical conductivity. The variation of hardness and electrical conductivity of the alloy was measured as a function of aging time. On aging at the temperature below TR （500-550°C） in Cu-2.8Ni-0.6Si alloy, the transformation undergoes spinodal decomposition, DO22 ordering, and d-Ni2Si phase. On aging at the temperature above TR （500-550 °C）, the transformation products were precipitations of d-Ni2Si. The free energy versus composition curves were employed to explain the microstructure observations.

Rare Metals 2013,32(03),228-233+2

Microstructure and properties of Cu-2.8Ni-0.6Si alloy

Xiang-Peng Xiao Bai-Qing Xiong Guo-Jie Huang Lei Cheng Li-Jun Peng Qi-Ming Liang

State Key Laboratory for Fabrication and Processing of Nonferrous Metals, General Research Institute for Nonferrous Metals

China Nonferrous Metals Processing Technology, Albetter Albronze Co., Ltd.

作者简介：Xiang-Peng Xiao e-mail:xiao_xiangpeng@126.com;Bai-Qing Xiong e-mail:xiongbq@grinm.com;

收稿日期：3 July 2012

基金：financially supported by National"863"Foundation of China (No. 2006AA03Z522);Science and Technology of Beijing (No. 10231103);

Microstructure and properties of Cu-2.8Ni-0.6Si alloy

Abstract：

The phase transformation behavior and heat treatment response of Cu-2.8Ni-0.6Si (wt%) alloy subjected to different heat treatments were studied by X-ray diffraction, transmission electron microscopy observation, and measurement of hardness and electrical conductivity. The variation of hardness and electrical conductivity of the alloy was measured as a function of aging time. On aging at the temperature below TR (500-550°C) in Cu-2.8Ni-0.6Si alloy, the transformation undergoes spinodal decomposition, DO22 ordering, and d-Ni2Si phase. On aging at the temperature above TR (500-550 °C), the transformation products were precipitations of d-Ni2Si. The free energy versus composition curves were employed to explain the microstructure observations.

Keyword：

Microstructure; Hardness; Electrical conductivity; Spinodal decomposition; DO22 ordering;

Received： 3 July 2012

1 Introduction

Cu–Ni–Si-based alloys are widely used for the production of high-strength,electrically conductive parts such as connectors and lead frames[1–3].The high strength is caused by the nanoscale precipitates in the alloy during aging[4].Meanwhile,the purity of the copper matrix,the size and number of the precipitates,and the precipitating nucleation location have key effects on the electrical conductivity of the alloy[5–7].The aging hardening in Cu–Ni–Si alloy has been accepted by researchers.Age hardening in this system was first investigated by Corson[8]and the precipitating phase responsible for the hardening was identified as dNi₂Si based on quasi-binary section of the Cu–Ni–Si ternary diagram.Fujiwara and Kamio[9]investigated the effect of the alloy composition and the hardness changed during the precipitation process and increased with increasing an alloying element during isothermal aging.Lockyer and Nobel[10]attributed the strengthening to high-degree coherency between precipitations and matrix across the habit plane,and transmission electron microscopy(TEM)of thin foils shows associated strain contrast effects that persist up to and beyond peak hardening.

The aim of the present study was to distinguish between these various theories by characterizing the microstructure changes occurring during the stages of the aged Cu–Ni–Si alloy and correlating the microstructure with measured mechanical properties.The characteristics of the precipitate have been established by X-ray diffraction(XRD)and TEM and correlated with hardness and electrical conductivity of the alloy.

2 Experimental

The Cu–2.8Ni–0.6Si(wt%)alloy ingot was fabricated using a medium-frequency induction furnace.The Cu and Ni blocks were first melted in the furnace.Then,intermetallic Cu–Si alloy of the required quantities was added to the molten bath.The melting and casting operations were carried out in an N₂atmosphere to prevent the alloy from oxidizing.After surface defect was removed,the ingot was hot-rolled at 920°C for 1.5 h,reducing the ingots thickness from 35 to 7 mm.The resultant strip was solution treated at900°C for 1.5 h followed by water quenching.The alloy sheets were aged at 450,500,and 550°C for different times and then air-cooled to room temperature(25°C).Vicker’s hardness tests were carried out on 430SVD testing machine under a 10 kg load and holding for 30 s.Electrical conductivity was carried out on 7501 eddy current conductivity meter.At least six measurements were taken for each data point in both cases(i.e.,hardness and electrical conductivity measurements).An X-ray diffractometer,D8 Focus Bruker,was applied to analyze phase transformation of materials on aging for various times,with the scanning speed of10(°)?min^-1and work voltage and current of 35 k V and25 m A,respectively,using Cu as target and Ni as filter.The TEM samples were prepared by a conventional electropolishing method using an electrolyte of 25 vol%HNO₃and 75 vol%CH₃OH at-30°C.JEM 2100 La B6 operating at 200 k V was employed to carry out most of the electron microscopy.

3 Results

3.1 Mechanical properties

Curves show the relationship between hardness and time for Cu–Ni–Si alloy aged at temperatures of 450,450,and500°C as given in Fig.1a.Figure 1a indicates the effects of time in artificial aging process on hardness of the alloy.It is found that hardness of the aged alloys increases with increasing time of aging first and then decreases after peak hardness reaches at each temperature level.With the increase of aging temperature,the time of the peak hardness decreases.Such as 450,500,and 550°C aging reach its maximum hardness at HV211,HV210,and HV208,respectively,whereas the corresponding times were 16,4,and 0.5 h.This is because secondary phase precipitating process is more easily activated at high temperature than at low temperature during the primary stage of artificial aging and the precipitates at this period are coherent with the base lattice and then led to lattice distortion,which is a key factor to get peak hardness of the alloy[11].

The curves of the variation of electrical conductivity with increasing aging time at various temperatures are shown in Fig.1b.Figure 1b indicates that the electrical conductivity of Cu–Ni–Si alloy increases with prolonging of the aging time and temperature.The key factors of effects on conductivity of solid solutions are total volume and size of the solute particle,but elements and grain boundary also substantially affect the conductivity[12].At the initial stage of aging,the electrical conductivity of alloy sharply increases and tends to be stable.Furthermore,the increase in electrical conductivity at the initial stage is faster with increasing aging temperature.On aging at 450°C,the electrical conductivity is 28.6%IACS after 0.5 h and 44%IACS after 16 h.However,the electrical conductivity is33.3%IACS with aging at 500°C for 0.5 h and 47.5%IACS after 16 h.At 550°C,the electrical conductivity is37.3%IACS for 0.5 h and 46.4%IACS after 16 h.It is the precipitation of Ni₂Si from the supersaturates solid solution that results in the initial sharp increase of electrical conductivity.The solute atoms diffused through the vacancies The longer the aging time,the less the supersaturated vacancies and thus the slower the precipitation process will be.The precipitates reduce the contents of solute atoms in the matrix and result in a continuous increase in electrical conductivity during the aging,which increases the electrical conductivity of the Cu–Ni–Si alloy[13,14].

Fig.1 Effect of aging temperature on hardness a and electrical conductivity b of Cu–Ni–Si alloy

Fig.2 XRD of Cu–Ni–Si alloy aged at 500°C for various time periods

3.2 Phase structure

X-ray diffraction patterns are taken for all samples aged at500°C for 0.5–16 h.Figure 2 shows the XRD patterns of Cu–2.8Ni–0.6Si alloy aged for various time periods.After aging for 0.5,1,and 2 h,the(200)diffraction peaks broaden,which is a characteristic of spinodal decomposition.The reason for the broadening lies in the formation of poor and rich solute regions in the matrix during the spinodal decomposition.The lattice value in solute-rich regions is less than the average value,whereas in solutepoor regions,it is larger.Then,around the main diffraction peak,especially the base peak,there will be found broadened peaks that have similar crystal plane distance with the main diffraction peak[15].

3.3 Microstructure observation

Specimens aged at 500°C for 0.5 h were observed to have a modulated structure that produced satellite reflections near(200)diffraction spots(Fig.3).Aged for 0.5 h because spinodal decomposition happened in the supersaturated solid solution,in the subsequent process of aging the affinity of Ni to Si was greater,so the process of the ordering was stronger and the superlattice diffraction peak appeared.Figure 4 shows the microstructure evolution of phase transformation of the Cu–Ni–Si sample aged at500°C for 1 h.The corresponding select area diffraction patterns(SADP)are presented in Fig.4b.It seems that at500°C,the(Ni,Si)-rich phase of the spinodal modulated structure becomes ordered,but it remains in the skeleton of a three-dimensional modulated structure.The superlattice diffraction spots between(000)_Cuand(311)_Cuin Fig.4b(at the position of(1/2 10)_Cu)indicate that the ordering occurs at an early stage of the aging process.From Fig.4b,it can be distinguished that the type of the ordering is DO₂₂ordering because the typical(1/2 10)_Cuis unique to DO₂₂ordering[16].However,the LI₂ordering cannot be clarified in this alloy because all the LI₂superlattice reflections overlap with some of the DO₂₂superlattice reflections.A few precipitates also can be seen in Fig.4a.The precipitates can be identified as d-Ni₂Si.The d-Ni₂Si phase adopted a disk-like morphology to reduce the elastic strain energy;this can be understood by noting the lattice mismatch between the d-Ni₂Si phase and the disordered f.c.c.matrix.The lattice parameter of the f.c.c.matrix is a₀=0.3615 nm;the lattice parameters for the d-Ni₂Si are a=0.703 nm,b=0.499 nm,and c=0.372 nm[17].Based on the above estimations,therefore,the mismatch between precipitate and matrix is of the order of 1%in the b and c directions,which lie in habit plane,and as such,the interface between the precipitate and the habit plane is highly coherent.In a direction,there is no obvious match between the periodicity of the precipitate and the matrix in the parallel(110)_Cudirection,neither would lattice correspondence in this direction be expected.To reduce the boundary energy,the d-Ni₂Si phase grew in the b and c directions and minimized its size in the direction resulting in disk morphology.From the analysis in Fig.4a the crystal orientations between the copper-based matrix and d-Ni₂Si precipitates are determined as[-112]_Cu//[32-4]_d,(110)_Cu//(2-11)_d.

Fig.3 a Microstructures of Cu–2.8Ni–0.6Si alloy aged at 500°C for 0.5 h,b SADP of[001]Cuzone axis

Fig.4 a Microstructures of Cu–2.8Ni–0.6Si alloy aged at 500°C for 1 h,b SADP of[-112]Cuzone axis

When increasing the holding aging time further to 4 h the amount of the second-phase particles within the matrix of the precipitates increases.Figure 5a–d are the microstructures of Cu–2.8Ni–0.6Si alloy aged at 500°C for 4 h and its SADP,respectively.From Fig.5a,c,we can find that the average precipitates diameter is 20 nm.According to the analysis of SADP in Fig.5b,d,the kinds of precipitate can be expressed as d-Ni₂Si,and the orientation relationship between the precipitates and Cu matrix can be expressed as[001]_Cu//[-110]_d,(010)_Cu//(001)_d;[-112]_Cu//[32-4]_d,(110)_Cu//(2-11)_d.

Fig.5 a and c TEM images of Cu–2.8Ni–0.6Si alloy aged at 500°C for 4 h,b SADP of a,and d SADP of c

Fig.6 Schematic free energy versus composition plot with free energy

4 Discussion

The change of microstructure depends on the factors of aging time,the concentration of the alloy,and the prior cold deformation.From the XRD and TEM analyses,the Cu–2.8Ni–0.6Si alloy decomposition of supersaturated solid solution by organizational change exists at a critical aging temperature T_R(500–550°C).Above this,a discontinuous precipitation distribution in matrix with disklike d-Ni₂Si particle takes place both at the grain boundaries and in the intragranular regions.During aging below T_R,the supersaturated solid solution gets decomposed by the spinodal mechanism into periodic regions that are(Ni,Si)rich and(Ni,Si)poor,indicating that ordering takes place.Thus,the processing aging treatment results in a metastable phase with the DO₂₂structure.The d-Ni₂Si phase with an orthorhombic structure appears during further aging.

The changes of microstructure can be explained by means of a set of free energy versus composition curves(Fig.6).Below the T_Raging temperature,the composition C₀first undergoes spinodal decomposition,giving rise to a modulated alignment of(Ni,Si)-rich and(Ni,Si)-poor regions.As the composition of the(Ni,Si)-rich region reaches the point X,it undergoes an ordering transformation to the DO₂₂structure;with prolonged aging time,the(Ni,Si)regions are transformed from DO₂₂ordering to d-Ni₂Si phase,which leads to the free energy being lowered[18].

When the aging temperature was above T_R(500–550°C),the abilities of solute atom diffusion improve,whereas the alloy decomposition driving force decreases.Thus,the(Ni,Si)-rich regions are no longer the main controlling factor of the solid solution decomposition;phase transition driving force of the size is the main factor determining the decomposition of the solid solution.Because the d-Ni₂Si precipitates have minimum free energy,the phase transformation has not experienced a series of metastable processes,direct formation of stable d-Ni₂Si phase though the aging temperature reached above T_R.

5 Conclusion

At early stages of aging,the hardness and electrical conductivity rapidly increase.Then,the hardness decreases slowly after reaching the peak,whereas the conductivity continues to rise.During the aging temperature below T_R(500–550°C)in a Cu–2.8Ni–0.6Si alloy,there are three transformation products:modulated structure resulting from spinodal decomposition,DO22 ordering structure nucleating from the modulated structure,and d-Ni₂Si phase with disk-like structure appearing in(Ni,Si)-rich regions On the aging at the temperature above T_R(500–550°C)the transformation products were precipitations of d-Ni₂Si Crystal orientations between the copper-based matrix and the d-Ni₂Si are determined as[001]_Cu//[-110]_d,(010)_Cu//(001)_d;[-112]_Cu//[32-4]_d,(110)_Cu//(2-11)_d.

Acknowledgments This Project was financially supported by National‘‘863’’Foundation of China(No.2006AA03Z522)and Science and Technology of Beijing(No.10231103).