稀有金属(英文版) 2019,38(11),1062-1069

Coarsening behavior of(Ni,Co)₂Si particles in Cu-Ni-Co-Si alloy during aging treatment

Xiang-Peng Xiao Hai Xu Jin-Shui Chen Jun-Feng Wang Jiao Lu Jiao-Bo Zhang Li-Jun Peng

Institute of Engineering Research,Jiangxi University of Science and Technology

School of Materials Science and Engineering,Jiangxi University of Science and Technology

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

作者简介：*Xiang-Peng Xiao e-mail:252616369@126.com;

收稿日期：20 September 2017

基金：financially supported by the National Natural Science Foundation of China(Nos.51561008 and 51461017);Jiangxi Yorth Major Natural Science Foundation(Nos.20171ACB21044 and 20161BBE50030);

Coarsening behavior of(Ni,Co)₂Si particles in Cu-Ni-Co-Si alloy during aging treatment

Xiang-Peng Xiao Hai Xu Jin-Shui Chen Jun-Feng Wang Jiao Lu Jiao-Bo Zhang Li-Jun Peng

Institute of Engineering Research,Jiangxi University of Science and Technology

School of Materials Science and Engineering,Jiangxi University of Science and Technology

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

Abstract：

The coarsening behavior of(Ni,Co)₂Si particles in Cu-Ni-Co-Si alloy was investigated by experimental observations and coarsening kinetics calculations when aged at 450,500,550 and 600℃ for different durations.The results show that the critical particle radius for coherence mismatch is found to be 10.3 nm,and particles larger than 25 nm are generally semi-coherent.The relationship of(Ni,Co)₂Si particles size and aging time follows Lifshitz,Slyosov and Wagner(LSW) theory.The particle size distributions fit well to the LSW theoretical distribution.The activation energy for(Ni,Co)₂Si coarsening is accurately determined to be(216.21 ± 5.18)kJ mol^-1 when considering the effect of temperature on the solution concentrations in matrix.The coarsening of(Ni,Co)₂Si particles in Cu-Ni-Co-Si alloy is controlled by diffusion of Ni,Co and Si in Cu matrix.The growth of particles for long durations suggests that vacancies can be trapped within the structure for long time despite their mobility.

Keyword：

Coarsening behavior; (Ni,Co)₂Si particles; Coherence mismatch; LSW theory; Particle size distributions;

Received： 20 September 2017

1 Introduction

Cu-Ni-Si alloys are widely used in conductor components and lead frame materials due to their excellent strength and conductivity [ 1, 2, 3, 4, 5] .It is generally that Cu-Ni-Si alloys are usually strengthened using a fine dispersion of Ni₂Si particles [ 6, 7, 8] ,while Cu-Ni-Co-Si alloys have higher strength,electrical conductivity and stress relaxation compared with Cu-Ni-Si alloys.

Izawa et al. [ 9] found that larger quantities of Co in CuNi-Co-Si alloys resulted in an alloy with higher dislocation densities.Increased strength resulting from greater Co content was attributed to a decrease in the interprecipitate spacing and an increase in the dislocation density.Xiao et al. [ 10] found that precipitation stimulated by Co leads to higher mechanical property and spinodal decomposition suppressed by Co results in higher electrical conductivity in Cu-Ni-Co-Si alloys.

Wang et al. [ 11] found that the strengthening of Cu-NiSi-Co alloy is mainly attributed to precipitates of both Ni₂Si and Co₂Si phases,with the same structure and very similar lattice parameters.The strength is strongly dependent on various characteristics of (Ni,Co)2Si particles such as content,particle size,particle distribution and coarsening rate.Thus,it is important to investigate the influence of aging conditions on the coarsening behavior of (Ni,Co)2Si particles.

The main theoretical approach to simulating volume diffusion-controlled coarsening has been developed by Lifshitz,Slyosov and Wagner (LSW) theory [ 12, 13] .The LSW theory predicts the coarsening kinetics of particles,and it can be represented as follows:

where t is aging time; is the average particle size after aging time t;r₀ is the average particle size att=0;K is coarsening rate constant.For spherical precipitates,the rateconstant can be derived as ^[14] .As the diffusion coefficient (D) is given by D=D₀e^-Q/RT,the coarsening rate constant (K) can be formulated as follows:

where D₀ is a frequency factor;C_e is the concentration of solute in equilibrium with particles in infinite size;σis the particles-matrix interfacial energy per unit area;V_m is the molar volume of the particles;R is the gas constant;T is the absolute temperature;and Q is the activation energy for particles coarsening.

The purpose of the present work is to evaluate the coarsening behavior of (Ni,Co)2Si particles in Cu-Ni-CoSi alloys through transmission electron microscopy (TEM)and high-resolution transmission electron microscopy(HRTEM).The research focused on mismatch of the particle,morphology evolution,particle size distributions and coarsening kinetics of (Ni,Co)2Si during aging at 450,500,550 and 600℃over different durations.

2 Experimental

Cu-1.2Ni-1.6Co-0.6Si (wt%) alloy ingots were prepared using a medium-frequency induction furnace.The Cu,Ni and Co blocks were first melted in the furnace.Then,intermetallic Cu-Si alloy of the required quantities was added to the molten bath.The stages of melting and casting operations were carried out in a N₂ atmosphere to prevent the alloy from oxidizing.After surface defects were removed,the ingots were hot-rolled in a furnace at 920℃for 1.5 h,which reduced the thickness of the ingots from35 to 7 mm.The resultant strip was solution treated at1000℃for 1.5 h followed by water quenching.The alloy sheets were aged at 450,500,550 and 600℃for different times followed by air-cooled to room temperature (25℃).The TEM and HRTEM samples were prepared using a conventional electro-polishing method using an electrolyte of 25 vol%HNO₃ and 75 vol%CH₃OH at-30℃.JEM2100 LaB6 operating at 200 kV was employed to carry out the most of the electron microscopy.

3 Results

3.1 Mismatch of particle

Figure 1 shows HRTEM images of Cu-1.2Ni-1.6Co-0.6Si alloy aged at 500℃for 1 h after solid solution.The (200)crystal plane of copper matrix is parallel with the (-321)crystal plane of the precipitated phase:(200)_Cu//(-321)_p.The degree of mismatch (δ) can be found in Refs [ 15, 16, 17] .

where d₁ and d₂ represent the interplanar distance of the two phases on the two sides of the phase interface along one direction,respectively (d₁>d₂).The degree of mismatch is

which is below 0.05,indicating complete coherence between precipitated phase and matrix.

Figure 2 shows HRTEM image of Cu-1.2Ni-1.6Co-0.6Si alloy aged at 500℃for 4 h after solid solution.The(01-1) crystal plane of the copper matrix is parallel with the (-210) crystal face of the precipitated phase:(01-l)cu//(-210)p.The degree of mismatch is calculated by:

which is>0.05 but<0.25,indicating a semi-coherent interface between precipitated phase and matrix.

The precipitates grow aging temperature or duration increases.Therefore,the coherence between precipitated phase and matrix evolves into semi-coherence or even noncoherence.Such a transition greatly influences alloy performance.HRTEM analysis demonstrates that coherence between precipitated (Ni,Co)₂Si phase and matrix is lost within a radius of～12 nm.Flexibility caused by lattice mismatch is the driving force for interfacial dislocation.Once coherence is lost,an increase in interfacial energy caused by interfacial dislocation is equivalent to a decrease in elastic energy caused by lattice distortion and relaxation.The critical radius for the transition from coherence to semi-coherence is given as follows [ 18] :

where r_t is average radius of precipitated particles,G is shear modulus,σ_dis is interfacial energy due to the formation of a misfit dislocation at the interface,v is Poisson's ratio,b is Burgers vector,δ'is the decrease in lattice mismatch caused by introduced interfacial dislocation andβis the coefficient associated with theδ'and v.For the copper matrix and (Ni,Co)2Si phase,G=50 GPa,b=0.255 nm,δ'=0.013 [ 19] and the Poisson's ratio v=1/3.Thus,the critical radius for lattice mismatch is10.3 nm.That is to say,the precipitated phase would start to grow after coherence is lost at a radius of～10-12 nm.

Fig.1 HRTEM images of Cu-1.2Ni-1.6Co-0.6Si alloy aged at 500℃for 1 h:a HRTEM image of Cu matrix and precipitates,b diffraction spots of Fourier transform of a,c calibrated results and d lattice fringes of inverse Fourier transfer a

Fig.2 HRTEM images of Cu-1.2Ni-1.6Co-0.6Si alloy aged at 500℃for 4 h:a HRTEM image of Cu matrix and precipitates,b diffraction spots of Fourier transform of a,c calibrated results and d lattice fringes of inverse Fourier transfer a

Figure 3 illustrates the aging process of the alloy at different temperatures.The dimensions of the precipitated phase were measured after different aging treatments through Digital Micrograph analysis software.The precipitated phase is coherent with the matrix at low temperatures.As shown in Figs.1,2 and Eq.(4),coherence is lost when the precipitated phase has a diameter of～12 nm,when semi-coherence is detected.The transition from coherence to semi-coherence is complete as aging temperature or aging time increases and particle diameter has reached～25 nm.The precipitated phase and the matrix have the following relationships based on average size of the precipitated phase [ 20, 21] .Coherent stage:the mean particle diameter is less than 12 nm and nearly all precipitates are coherent;intermediate stage:the mean particle diameter is greater than 12 nm and smaller than25 nm,with coherent and semi-coherent precipitates coexisting;semi-coherent stage:the mean particle diameter is greater than 25 nm and nearly all precipitates are semicoherent.

Fig.3 Particles size of alloy at different aging conditions

3.2 Morphology evolution of (Ni,Co)2Si particles

Figures 4,5,6 and 7 show TEM images of Cu-1.2Ni-1.6Co-0.6Si alloy after solid solution strengthening and aged at different temperatures.The bright-field images show that the precipitated phases are uniformly distributed and the majority of the precipitates are disk shaped.As aging temperature or aging time increases,the precipitates grow continuous and become more irregular,indicating loss of mismatch.

3.3 Particle size distributions of (Ni,Co)2Si particles

The experimental particle size distribution histograms are plotted in Fig.8 together with the theoretical distribution functions predicted from LSW theory.When aged for less than 4 h,the distribution of particles size shifts mildly to the left with the maximum being (0.9-1.0) .For aging time longer than 4 h,particles sizes are in an intermediate range,with overall particle size increasing.Coherence is lost at an aging temperature of 500℃after about 16 h.Coherence coexists with semi-coherence at an aging temperature of 450-550℃.When aging time is prolonged to greater than 4 h at 500℃,the rate of growth is initially slow but becomes faster.Many particles are already semicoherent when aged at 500℃for 4 h.Moreover,the coarsening rate increases dramatically when aging time is prolonged.

3.4 Coarsening kinetics of (Ni,Co)2Si particles

The evolution of average (Ni,Co)2Si particles size with aging time at 450,500,550 and 600℃is plotted in Fig.9.It can be seen that the average particles size (r³) is basically in a linear relationship with aging time (t).This relationship conformed to Eq.(1),demonstrating that the coarsening of (Ni,Co)2Si particles obeys the LSW theory.The data in Fig.9 indicate that the (Ni,Co)2Si radius increases in accordance with bulk diffusion-controlled kinetics [ 22, 23, 24] .The slope of each line in Fig.9 yields the temperature-dependent coarsening rate constant (K),which is3.32,8.57,13.29 and 30.72 nm³·min^-1,respectively,at the temperature of 450,500,550 and 600℃.

Fig.4 Bright images of precipitates aged at 450℃:a 1 h,b 2 h,c 4 h and d 16 h

Fig.5 Bright images of precipitates aged at 500℃:a 1 h,b 2 h,c 4 h and d 16 h

Fig.6 Bright images of precipitates aged at 550℃:a 1 h,b 2 h,c 4 h and d 16 h

Fig.7 Bright images of precipitates aged at 600℃:a 1 h,b 2 h,c 4 h and d 16 h

Fig.8 Comparisons of LSW theoretical distribution function with experimental distribution of (Ni,Co)₂Si particles aged at 500℃for different durations:a 1 h,b 2 h,c 4 h and d 16 h

The activation energy for (Ni,Co)2Si particles coarsening is determined from the slope of the plot of ln(KT/C_e) versus 1/T in Fig.10.Assuming that the solute atoms that form (Ni,Co)2Si particle are made of Ni,Co and Si atoms,the term C_e is the sum of Ni,Co and Si contents of the matrix.The activation energy is determined to be(216.21±5.18) kJ·mol^-1.The activation energy for (Ni,Co)2Si coarsening correlates well with the value of Cu-Ni binary (228 kJ·mol^-1) and Cu-Si binary (187 kJ·mol^-1) [ 25] .The results indicate that the coarsening of (Ni,Co)2Si particles is mainly controlled by the volume diffusion of Ni,Co and Si in copper matrix.

Fig.9 Relationship of average radius and cube root of time in Cu-1.2Ni-1.6Co-0.6Si alloy

Fig.10 Determination of activation energy

3.5 Aging hardening of alloy

The effect of aging temperature and aging time on the microhardness of Cu-1.2Ni-1.6Co-0.6Si alloy is plotted in Fig.11.It is obvious the microhardness decreases rapidly with aging temperature increasing.The drastic drop of microhardness at 600℃is the result of rapid (Ni,Co)2Si particles coarsening at high temperature as shown in Figs.6 and 9.The coarsening of (Ni,Co)2Si particles ought to be the leading cause for the decrease in microhardness.The results also reveal that higher microhardness at 450and 500℃can be attributed to the fine distributions of (Ni,Co)2Si particles in Cu matrix.

Figure 11 also shows that the microhardness has a maximum value with aging time increasing.The maximum microhardness values for 450,500,550 and 600℃are HV225.6,HV 230.8,HV 197.8 and HV 182.7,and the corresponding (Ni,Co)2Si particles sizes are 12,13,16 and24 nm,respectively.According to the hardening theory [ 26, 27, 28] ,when a particle is smaller than the critical size,it can be cut or deformed by dislocations or weakly coupled dislocation pairs.In this case,the strength increases with the particles size.Otherwise,if a particle size is larger than the critical size,the cutting of the particles becomes difficult and the dislocations find ways to pass around the particles.When by-pass occurs,the strength decreases with the increase in the particle size.Therefore,there is a drastic drop in microhardness with the particle size larger than the critical size when aging at 550 and 600℃.

Fig.11 Variation of microhardness with aging time at 450,500,550and 600℃

4 Discussion

The heterogeneous precipitate distribution and coarsening process is related to the solute diffusion,and migration of solute atoms in the matrix requires the movement of vacancies.Therefore,the interaction between Ni and Co atoms and vacancies becomes the main factor controlling precipitation and coarsening.Reduced vacancy concentration reduces the mobility of vacancies,which reduces the probability of transition of vacancies according to the laws of thermodynamics.The probability of migration of vacancies (Γ) can be described as [ 29, 30] :

whereβis the entropy factor;Z is coordination number;y is the atomic thermal frequency;ΔU_m is the activation energy of vacancy movement;K is a constant;and T is the absolute temperature.

It is expected,therefore,that at low temperature most vacancies will be bound to atoms and their mobility is hampered by the need to associate with a solute atom.This increases the number of ordered vacancies in the system,leading to a decrease inβamount,which decreases the probability of migration of the vacancies (Γ).The (Ni,Co)2Si particles are,therefore,thought to act as vacancy traps retaining large numbers of excess vacancies and inhibiting the particles at the initial stage of aging.

Mobile vacancies can be engaged with solute atom when aging time is extended.Ni and Co have an unfilled inner layer,which exhibits a metastable structure in Cu alloys.This characteristic structure connects Ni,Co with mobile vacancies.The vacancies around Si atoms carry Ni and Co atoms,so that when the atoms gather to a certain extent,they cause nucleation of the particles.A high proximity of particles along the length of the vacancy leads to rapid coarsening during extended duration of aging.

5 Conclusion

In the present work,the coarsening behavior of (Ni,Co)2Si particles in Cu-Ni-Co-Si alloy was investigated.The critical radius for coherence mismatch is found to be1 0.3 nm,according to the theory of transition from coherence to semi-coherence.In fact,it is observed that at an aging temperature of 450-600℃,the radius for coexistence of coherence and semi-coherence of the (Ni,Co)2Si phase is 12-25 nm.Particles larger than 25 nm are generally semi-coherent.The coarsening behavior and particle size distribution of the particles conform more to the LSW function during aging treatment.The average particles size(r³) is basically a linear relationship with aging time (t).The activation energy for (Ni,Co)2Si coarsening is accurately determined to be (216.21±5.18) kJ·mol^-1 when considering the effect of temperature on the solution concentrations in matrix.The coarsening of (Ni,Co)2Si particles in Cu-Ni-Co-Si alloy is controlled by the diffusion of Ni,Co and Si in Cu matrix.The growth of particles over long periods of the time suggests that vacancies can be trapped within the structure for long periods despite their mobility.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos.51561008 and51461017) and Jiangxi Yorth Major Natural Science Foundation(Nos.20171ACB21044 and 20161BBE50030).We thank professor Rui-Qing Liu and Dr.Hang Wang for enlightening discussions.

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