Effects of grain size on mechanical properties of nanostructured copper alloy by severe plastic deformation (SPD) process

1 Introduction

Nanostructured materials are polycrystalline materials with grain size in the nanometer range, usually less than 100 nm ([1] Furukawa et al. , 2002; [2] Kumar et al. , 2003; [3] Horita et al. , 2001; [4] Segal, 1995; [5] Koch, 1992). It is a form of advanced material processing using severe plastic deformation (SPD) to enhance certain material properties.

Interest in the processing of nanostructured materials through the application of SPD has grown significantly over the last decade ([6] Valiev, 2003; [7] Iwahashi et al. , 1998). SPD process is the "top-down" approach that starts with the bulk materials with a relatively coarse grain size and break down the microstructure into a nanostructure by imposing very high-shear deformations into the material under superimposed hydrostatic pressure leading to exceptional grain refinement of nanocrystalline materials ([8] Valiev et al. , 2002).

The influences of grain sizes on the flow stress in polycrystalline materials has been quantitatively described by Hall-Petch in the 1950s with experiment and theoretical research continuing up to the present ([9] Gleiter, 1989).

In term of yield stress, Hall-Petch is expressed as σ₀ =σ₁ +kd^-1/2 , where σ₀ , yield stress; σ₁ , frictional stress opposing dislocation motion; k , constant and d , grain diameter, similar results are obtained for hardness, with H₀ =H₁ +kd^-1/2 , the hardness of a materials, where H₁ and K are appropriate constants associated with the hardness measurement. HV is proportional to the yield stress through the expression HV∼3σ_y ([9] Gleiter, 1989; [2] Kumar et al. , 2003). More recently, it has been suggested that the Hall-Petch relationship breaks down for certain materials when the grain size is reduced to a certain level and it has been suggested that an inverse Hall-Petch relationship exist for grain sizes reduced beyond a certain points (Figure 1 [Figure omitted. See Article Image.]).

The two most commonly used SPD methods to nanostructured metals and alloys are the high-pressure torsion and the equal channel angular press (ECAP) technique, but the most attractive technique is ECAP ([11] Zhu et al. , 2004). The ECAP method can be used to controlling the microstructure of metals effectively and improve...