TABLE OF CONTENTS Two 'far from equilibrium' processing techniques, such as RS and MA have been available for the last three decades to produce materials with properties much superior to those obtained by conventional ingot metallurgy (IM) techniques.
The RS technique refers to cooling of metallic melts (most commonly using melt spinning or atomisation techniques) at rates of about 10 4 10 6 K. It is well established that RS can result in one or more of the following constitutional and microstructural modifications [1]:
refinement of grain size and second phase particles or segregations;
extension of solid solubility;
formation of non-equilibrium crystalline or quasi-crystalline intermediate phases;
production of amorphous phases.
On the other hand, MA involves repeated welding, fracturing, and rewelding of powder particles in a dry high-energy mill. The process can also lead to the above-mentioned metastable effects, but entirely in the solid state. With either of the techniques, the product obtained (powder only in MA and powder, foil, wire or ribbon in RS) has to be consolidated into larger, more stable forms. To retain all the advantages gained by RS or MA, consolidation should be done at relatively low temperatures. Thus, the challenge is to achieve the necessary consolidation within a temperature window, which is often quite narrow. The consolidation methods described in Chapter 7 are applicable to either of these two types of materials.
A comparison of the RS and MA techniques is given in Table 11.1.
