Microstructural Development and Evolution of Liquid Phase Sintered Cr-Cu Composites

Features of the microstructure formation of Cr-Cu composites under impregnation followed by liquid phase sintering of reduced and e lectrolytic chromium powders at 1200 oC in a vacuum of (2-4) ⋅10Pa have been studied. The refractory component part icle size distribution in the microstructure of samples with reduced chromium sintered for 60 min is shown to obey a normal logarithmic law; with distri bution parameters being sensitive to the volume fraction of the refractory particles. The ca lculated values of the dihedral angle are close to the value of one of the modes in the exper imental dihedral angle distribution for the microstructure of electrolytic chromium based sampl es (115 o ). The interfacial and interparticle surface energies ratio σsl/σss>0.5 is shown to correspond to theory for the Cr sCul system in equilibrium, which indicates the presenc e of skeleton structure elements in the course of composition formation under liquid phase sintering (including the case of excess liquid phase). Experimentally determined interparti cle and interfacial surface areas, solid particle contiguity and continuity are discussed in terms of concurrent diffusion-controlled particle coarsening (in Lifshitz, Slyozov and Wagne r theory) and particle coalescence (in German’s model). The kinetics of shrinkage for the composites with 50...55 % solid-phase volume-fractions at heating and isothermal sinterin g in a vacuum at a temperature of 1200 oC in terms of linearly viscous rheological theory a e discussed.


Introduction
Cr-Cu composites are widely used as contact materials for medium-voltage and highcurrent interrupters.A variety of their properties (abrasive, erosion-resistant, specific resistant and others) are produced by тхе liquid phase sintering process.The properties of these depend largely on the microstructure developed during sintering.Liquid-phase sintered microstructures are characterized in terms of the solid phase volume fraction, V p , scale (average solid particles radius), R , the surface-to-volume ratio, S v. Solid phase morphology is conditioned on interfacial and interparticle surface energies ratio σ sl /σ ss and can be quantified by тхе dihedral angle ψ, (between two particles on the boundare with liquid phase), contiguity, C c (interparticle surface area as a fraction of the total surface area), and continuity, C p (the number of solid-solid particle contacts per particle).This paper is concerned with an experimental study relating the scale and morphology of liquid phase sintered Cr-Cu composites and features of densification of these under isothermal and nonisothermal sintering conditions in a vacuum.

Experimental procedure
Electrolytic and reduced by hydride calcium powders of chromium (99.93 and 99.3 % purity respectively) as well as copper melted in a vacuum and electrolytic (99.999 and 99.3 % purity respectively) are used.The specimens were produced by impregnation of freely poured chromium with copper at 1200 o C in a vacuum of (2-4)⋅10 -3 Pa (of excess liquid phase or equal chromium and copper mass) and subsequent isothermal liquid sintering for 3-90 min.Other specimens were produced by blending powders оф reduced chromium and electrolytic copper with subsequent compacting (pressure of 200 MPa) into 10 mm diameter and 10 mm high cylindrical powder compacts with a green theoretical density of about 70 %.The compacts were sintered in a vacuum of 10 -5 Pa with a heating rate up from 2 to 33 K/min at 1200 o C and isothermal sintering at this temperature for 1 hour.Quantitative metallography was performed on suitably prepared metallographic samples.The average solid particles size -R , the interfacial surface to-volume ratio, S s-l (between solid and liquid phases) and interfacial surface to-volume ratio, S s-s (between two or three solid particles), were determined using the line intercept method [1], and the volume fraction -V p was determined as a check on composition.The contiguity, C C , was defined by the ratio [1]: The continuity, C p , was determined using the Gurland method [2], by counting the number of interparticle contact particles, M s-s , and total number of particles, N total , on a twodimensional plane of polish (per unit area), by the ratio: It was also necessary to define at what stage of particle fusion «two» particles should be considered as «one».In this regard, it was postulated that if the neck diameter was at least two-thirds of the diameter of the smaller of the contacting particles, the particles were considered as having fused into one.Computer automated analysis of the images was carried out.Measurement of at least 2000-3000 particles-matrix and 300-400 interparticle line intercepts were necessary to obtain accurate values for S s-s S s-l , C c and C p .

Experimental results
Solid-phase volume-fractions in the specimens after impregnation of freely poured chromium with copper were estimated as V p =37…40 % vol.(in the case of excess liquid phase) and as V p =50…55 % (in the case of equal chromium and copper mass).Fig. 1 shows that in the composites formed from reduced chromium the number of interparticle contacts and the average particle size increase with increasing sintering time and solid phase volume fraction.Coalescence occurs in the composites studied even at an excess of liquid phase.Particle size distribution parameters appear sensitive to the solid-phase volume-fraction (fig.2).With an increase in the volume fraction of the solid phase, the shift of the distribution curve in a region of great significance occurs as well as an increase in the degree of the The results of studies show that particle size growth kinetics obey the law of R 3 ∼t (fig.3), by the mechanism of diffusion-controlled coarsening theory developed by Liifshitz, Slyosov and Wagner (LSW-theory, [3]) described by the relation: where: o R is the average initial solid particle radius; k is the coarsening rate constant; t is the sintering time.
The experimentally determined coarsening rate constants are: LSW-theory modified by Ardell [4] predicts that while the basic t 1/3 kinetics are maintained the coarsening rate increases with increasing solid volume fraction.The theoretical rate constant can by determined by the relationship: where: σ s-l is the surface energy solid-liquid; Ω is the atomic volume of solid phase; (where α is a thermodynamic characteristic of system, . The calculated time of initial stage sintering t init ≈ 30 min, is in very good quantitative agreement with experimental data (tab.I).(5) where: σ s-s and σ s-v -are surface energy on solid-solid and solid-liquid boundaries respectively.

Tab. I. Experimental ratio
The equilibrium dihedral angle can be estimated by the equation: The equation ( 6) is obtained via substitution into expression (5) using the Young-Neumann relation-ship: (where: σ s-g , σ l-g -surface energies on solid-gas and liquid-gas boundaries respectively).The dihedral angle in the Cr-Cu system in equilibrium, calculated by formula ( 6) is equal ψ=128-140 о (used following data: σ l-g = 1300 mJ/m 2 for copper and σ l-g = 1631 mJ/m 2 for chromium [5]; σ s-s ≈ 0.4σ s-g is the averaged value for polycrystalline chromium [6]), σ s-g = 2400 mJ / m 2 as determined by the zero creep method [6] or σ s-g = 1876 mJ/m 2 calculated from σ s-g = 1.15σ l-g [7] ; and Θ=39 o under the experimental conditions [8]).The calculated ψ is close to the value of one of the modes in the experimental planar dihedral angle distribution in the microstructure of electrolytic chromium based samples (115 o ) (fig. 4, a It is established that in the microstructure of composites formed reduced chromium contiguity, C c , attains values 0.30 and 0.35 and the number of solid-solid particle contacts per particle, C p , established over 0.4 and 2 for V p =37…40 and V p =50…55 % vol.respectively, at 90 min sintering time (fig.5, a, b).According to the Gurland's criteria (C p >1.5, [2]) for composites with V p =50…55 % vol. a rigid skeleton is formed.The area of the interparticle surface-to-volume ratio S s-s remains the essentially constant (within error-margin of experiments) up to 60 and 90 min sintering for V p =37…40 and V p =50…55 % vol.respectively (fig.5, a, b).The absence of changes in S s-s in the initial stage of sintering (fig.6 where: is the loss in total surface-to-volume ratio divided by the initial surface area; γ is the exponent depending on the transport mechanism in neck growth between particles; C is the kinetic constant. Since The estimate of γ of from experimental data of surface area reduction kinetics showed that (fig.6, b):  The heating of specimens is accompanied by "increase of the specimens", which continues up to temperatures of 900-900 o C at all heating rates (fig.7, a), the finite volume changes of specimens at nonisothermic sintering depend on the heating rate and maximum temperature (tab.II).The effect of "increase" more strongly appears with an increase of heating rate, probably, because of the development of the process of local non-uniform densification in different zones of specimens.
The evaluation of kinetics during isothermal sintering at 1200 o C was carried out.The densification kinetics under isothermal hold can be estimated in the framework of a phenomenological sintering theory, accounting for rates of change in porosity controlled by the particle accommodation mechanism (the Ashby model), by ratio [9]: where: K -rate coarsening constant; q -constant, , A≈100, B≈10.
The theoretical estimation of densification kinetics of Cr-Cu composite with V p =55 % vol.carried out using experimentally determined data , is different from experimentally observed volume changes of specimens (fig.7, b).The difference between the experimental data and theoretical estimation of densification probably, may be explained by anomalous porous growth because of local non-uniform densification and the rigid skeleton solid phase formation under sintering.The features of shrinkage kinetics of the Cr-Cu composites are in need of further studied.

Conclusions
A rigid skeleton structure is formed in the liquid phase sintered Cr-Cu composites.It is confirmed by: calculated thermodynamic ratio σ sl /σ ; agreement of theoretical estimation and experimental data of the dihedral angles; presence of skeleton elements in the microstructure of composites and number of contacts per unit particle more than 1,5.Experimentally determined solid particle growth kinetics follows a cubic law R 3 ~t, that indicates domination of diffusion-controlled particle coarsening mechanism (by Liifshitz and Slyosov and Wagner theory).
Coarsening rate constants for composition with 37-40 and 50-55 % solid-phase volume-fractions are determined.Evaluation of the diffusion coefficient of chromium in copper at 1200 o C is carried out.Stationary particle size distribution is established for 30 min sintering.

Fig. 3
Fig.3 Plot of average solid particle size R in relation to the sintering time in the microstructure of composites.Curves (1) and (2) -V p =37-40 % and V p =50-55 % vol.respectively.

Fig. 4 Fig. 5
Fig. 4 Microstructure of specimen formed by electrolytic chromium (a) and histogram for planar dihedral angle ψ distribution in the microstructure (b).

3 .
Taking into account that at the first stage of liquid phase sintering of Cr-Cu composites changes Ss-s are still sufficiently small, and also that a volume of refractory particles remains constant, than for Cr-Cu composites initial and instantaneous interphase surface to-volume ratio).

Fig. 7
Fig.7 Effect of heating rate of specimens on the shrinkage kinetics for composites with V=50-55 % vol.(a) and porosity change of the composite during sintering at 1200 o C (b).In fig.(a): curves (1) and (2) -2 and 8 o C/min heating rate respectively; in fig.(b): curves (1) and (2)calculated data from equation (6) and experimentally data respectively.