Introduction

COLD SINTERING or high pressure consolidation at ambient temperature, or at temperatures not exceeding 450 °C, of rapidly solidified and mechanically alloyed powders, as well as of very fine elemental powder blends, is an alternative method to achieve full density without compromising very fine and/or metastable microstructures (Ref 1, 2, 3, 4).

Development of fine microstructures is a primary goal in the design of high performance materials (Ref 5, 6). Mechanical properties of materials with complex microstructures are controlled by the superposition of several strengthening mechanisms. Refinement of grain size, second phase precipitates or dispersoids, and the development of a homogeneous microstructure result in a combination of high strength and ductility.

Fine-scale microstructures can be obtained using advanced powder metallurgy processing techniques. Rapid solidification of powders yields supersaturated solid solutions, fine-scale precipitates, fine grain sizes, and homogeneity of composition and microstructure, as well as results in the retention of metastable phases including amorphous metal glasses (Ref 6, 7, 8, 9). Alternatively, mechanical alloying of metal powders and ceramic particles results in extremely fine microstructures with a uniform distribution of dispersoids in the metal matrix (Ref 10, 11, 12, 13). Similar features are observed in metal-oxide composite powders obtained by chemical methods (Ref 14, 15, 16) or by blending very fine micron/submicron elemental metal and ceramic powders (Ref 17, 18).

To produce high performance materials from rapidly solidified or other fine-scale powders, consolidation to full density is mandatory. Hot processing methods, such as extrusion, hot isostatic pressing (HIP), or powder forging, are normally employed for the consolidation of rapidly solidified or mechanically alloyed powders (Ref 7, 13, 16). The temperatures of hot processing are often higher than the subsequent service temperature. Thus, the refined microstructures obtained by rapid solidification or by mechanical alloying can coarsen, and the metastable constituents can dissolve in the matrix during hot consolidation. In the blends of very fine elemental powders, grain growth can take place.

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