FUNCTIONALLY GRADED METALS AND METAL-CERAMIC COMPOSITES .1. PROCESSING

The processing, mechanical analysis, and performance of functionally g raded materials containing a metallic phase is reviewed in two parts. This first part focuses on processing, and distinguishes two principal process classes in graded materials production. The first class, term ed' constructive processes', produces gradients by stacking selectivel y two or more starting materials, allowing full and potentially automa ted control of compositional gradients. The second class is termed 'tr ansport based processes' and utilises natural transport phenomena to c reate compositional and microstructural gradients during production of a component. Timescales for relevant transport phenomena are evaluate d and compared to usual processing times to assess their role and impo rtance in both functionally graded materials process classes. Construc tive processes comprise chiefly powder consolidation processes, such a s solid state powder metallurgy or reactive sintering, and coating pro cesses, such as plasma spraying and vapour deposition. Transport based processes include processes which rely on transport of heat, mass, or fluid. Examples of these can be found in advanced materials processin g, and also in well established materials processes, most notably in s teel surface hardening. Each process type is described in turn, with e xamples drawn from current research on functionally graded advanced ma terials, and also from engineering practice in steel. The review focus es on underlying principles and critical issues for each process, usin g current theory or simple timescale analysis to rationalise some obse rvations made in graded materials processing. Respective merits of eac h process class are also discussed, to conclude that transport based p rocesses have limitations compared with constructive processes, but ma y in many instances provide an economical and flexible route to graded materials production if they are well understood from a modelling sta ndpoint.