A study on deformation behaviour of Cu- Al2O3 metal matrix composite with the variation of size and volume fraction of reinforcement particle

Abstract

Metal matrix composites are revolutionary materials of future which have endowed the material industry with excellent properties which can be tailored in context to desired applications and requirements. The Cu-Al2O3 metal matrix composites apart from resisting softening at high temperatures have applications such as spot welding electrodes, lead frames commutators for starting helicopters, catalysts with a high degree of conversion, coatings with low porosity and high adhesion. In the present investigation Cu–Al2O3 micro- and nanocomposites with different volume fractions of alumina (5, 10, 15 and 20 vol. %) were fabricated by powder metallurgy route. The specimens were sintered at different temperatures (850◦C, 900◦C and 1000◦C temperature) to study the effect of sintering temperature on physical and mechanical properties of the composites. These micro- and nano-composites have been characterized using X-Ray diffraction, scanning electron microscopy and atomic force microscopy followed by density, microhardness and wear measurements. Deformation mechanism dominated by dislocation motion has been emphasized comprehensively and critically by changing the particle size and volume fraction of the reinforcement. The mismatch in several physical and mechanical attributes of the matrix and reinforcement entities give rise to misfit strain, thermal misfit, lattice parameter misfit, elastic inhomogeneity misfit. Our investigation focuses comprehensively on thermal misfit in metal matrix composites. The differential elastic modulus of reinforcement Al2O3 particle and matrix copper leads to development of misfit strain. It restricts the mobility of dislocation in the matrix leading to strengthening contribution of misfit hardening which is expressed as Where τ is the misfit strain hardening, G is the shear modulus, 𝜺 is the misfit strain, r is the radius of the particle, f is the volume fraction of the reinforcement. The flexural and compression tests were carried out to investigate the mechanical behavior of MMCs. The thermal mismatch of both entities (matrix and reinforcement) plays a vital role in the thermo-mechanical reliability of the composite. Thermal misfit dislocations at the interface results in generation of strain field at the particle-matrix interface. The amount of geometrically-necessary dislocation density generated in the matrix- reinforced particle interface can express as Where ρg dislocation density, ∆α is the difference in thermal expansion coefficient, ∆T is the change in temperature; f is the volume fraction of the reinforcement √2 rp is the dislocation loop radius. The specimens of Cu – 5 % Al2O3 having particle size (10 µm, <50 nm) were treated at -80 ◦C for a time period followed by immediate exposure at +80 ◦C for the same period of time. The thermal shock procedure was also followed in the reverse order. The flexural tests were conducted at room temperature maintaining a span length of 26mm and strain rate of 0.5 mm/mm min. An attempt was made to emphasize on the response of strengthening effect of particle reinforced composite with various thermal shock treatments (up and down cycle). The in-situ flexural tests were carried out at +250°C, +100°C. The strain hardening effect, ductility, flexural moduli were reflected from the three point bend test results.
The following observations have been made from the present investigation:
Micro-composites
I. Volume fraction of the reinforcement: With increases in reinforcement content particle clustering increases.
II. Sintering temperature: CuAlO2 phase appears from Cu2O at 1000◦C sintering temperature. Particle clustering decreases with increases in sintering temperature. Annealed twins are more likely to observed in the micro composites sintered at 900◦C less likely at 1000◦C. With increases in sintering temperature densification values of the micro-composites increases.
III. Mechanical behavior: Density decreases with increases in reinforcement content. Increases in the reinforcement content increases the micro hardness values. Hardness values decreases with increase in sintering temperature from 900◦C to 1000◦C. wear resistance decreases with increase in reinforcement volume fraction. With increases in reinforcement content the ultimate compressive strength decreases. Cu–15 % Al2O3 composite with 10 micron have highest ultimate compressive strength. Maximum flexural strength decreases with increases in alumina content. . Particle pullout, interface de-cohesion, and matrix cracking are the dominant failure phenomenon.
IV. Thermal shock: Maximum flexural strength increases for both up and down thermal shock treatment. Particle cracking is the dominant failure mechanism in the down thermal shock whereas up thermal shock treated specimens fail by interfacial de-cohesion.
V. In-situ high temperature TRS- At high exposed temperature maximum flexural strength increases. As the test temperature increases from 100◦Cto 250◦C the flexural strength decreases by 33%.
Nano-composites
I. Volume fraction of the reinforcement: Increase in the volume fraction of the reinforcement sintered density decreases.
II. Sintering temperature: At high sintering temperature hardness value decreases with increases in sintering temperature.
III. Mechanical behavior: Hardness values increases with increases in the reinforcement content. Ultimate compressive strength of the nano composites were decreases with increase in the reinforcement content. Strain hardening exponent also decreases with increase in reinforcement content. Cu–7 % Al2O3 nanocomposite provides the maximum bending strength. Mixed (cleavage and dimple) mode failure of the nanocomposite has been observed.
IV. Thermal shock: Maximum flexural strength increases for both up and down thermal shock treatment. Matrix deformation is the predominant failure mechanism for down thermal shock. In up thermal shock treated sample interfacial cavitation is the principle damage mechanism.
V. In-situ high temperature TRS: At high exposed temperature maximum flexural strength increases. As the test temperature increases from 100◦Cto 250◦C the flexural strength decreases by 20%
The present research confirms that the deformation behavior of particle reinforced metal matrix composites are affected by particle size, volume fraction of the reinforcement, particle distribution. Particle cracking, particle/matrix interface de-cohesion, particle pullout and localized melting are some of the failure mechanisms operational in MMCs which lead to the premature and unpredictable failure of composite materials.