Modeling Enhanced Diffusion Mass Transfer in Metals during Mechanical Alloying

Abstract

The subject of this Chapter is an urgent cross-disciplinary problem relating to both Mass Transfer and Materials Science, namely enhanced, or abnormal diffusion mass transfer in solid metals and alloys under the action of periodic plastic deformation at near-room temperatures. This phenomenon takes place during the synthesis of advanced powder materials by mechanical alloying (MA) in binary and multi-component systems, which is known as a versatile means for producing far-from equilibrium phases/structures possessing unique physical and chemical properties such as supersaturated solid solutions and amorphous phases (Benjamin, 1992; Koch, 1992; Koch, 1998; Ma & Atzmon, 1995; Bakker et al., 1995; El-Eskandarany, 2001; Suryanarayana, 2001; Suryanarayana, 2004; Zhang, 2004; Koch et al., 2010). Along with MA, this phenomenon is relevant to other modern processes used for producing bulk nanocrystalline materials by intensive plastic deformation (IPD) such as multi-pass equal-channel angular pressing/extrusion (ECAP/ECAE) (Segal et al., 2010; Fukuda et al., 2002), repetitive cold rolling (often termed as accumulative roll bonding-ARB, or folding and rolling-FR Sauvage et al., 2007; Yang et al., 2009), twist extrusion (Beygelzimer et al., 2006) and high-pressure torsion (HPT) using the Bridgman anvils (X.Quelennec et al., 2010). It is responsible for the formation of metastable phases such as solid solutions with extended solubility limits during IPD, demixing of initial solid solutions or those forming in the course of processing, and is considered as an important stage leading to solid-state amorphization in the course of MA. In these and similar situations, the apparent diffusion coefficients at a room temperature, which are estimated from experimental concentration profiles, can reach a value typical of a solid metal near the melting point, D~10−8-10−7 cm2/s, and even higher. Mechanical alloying (MA) was discovered by J.S.Benjamin in early 1970es as a means for producing nickel-base superalloys with dispersed fine oxide particles (Benjamin & Volin, 1974). Later it was found that MA brings about the formation of non-equilibrium structures in many metal-base systems, such as supersaturated solid solutions, amorphous and quasicrystalline phases, nanograins etc., and it had acquired a wide use for the synthesis of novel metallic and ceramic materials. MA of powder mixtures is performed in attritors, vibratory and planetary mills and other comminuting devices where particle deformation occurs during incidental ball-powder-ball and ball-powder-wall collisions. An important