Journal article

Functionally graded metals and metal-ceramic composites .1. Processing

The processing, mechanical analysis, and performance of functionally graded 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, termed' constructive processes', produces gradients by stacking selectively two or more starting materials, allowing full and potentially automated control of compositional gradients. The second class is termed 'transport based processes' and utilises natural transport phenomena to create compositional and microstructural gradients during production of a component. Timescales for relevant transport phenomena are evaluated and compared to usual processing times to assess their role and importance in both functionally graded materials process classes. Constructive processes comprise chiefly powder consolidation processes, such as solid state powder metallurgy or reactive sintering, and coating processes, 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 processing, and also in well established materials processes, most notably in steel surface hardening. Each process type is described in turn, with examples drawn from current research on functionally graded advanced materials, and also from engineering practice in steel. The review focuses on underlying principles and critical issues for each process, using current theory or simple timescale analysis to rationalise some observations made in graded materials processing. Respective merits of each process class are also discussed, to conclude that transport based processes have limitations compared with constructive processes, but may in many instances provide an economical and flexible route to graded materials production if they are well understood from a modelling standpoint.


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