ABSTRACT
The concentration and segregation of dissolved impurities and the distribution, size, and strength of second-phase particles have a strong in§uence on the §ow stress, regardless of the strain [10-15]. The grain size and shape and previous deformation affect the high-temperature yield strength considerably, but they usually have less effect as the strain is increased toward the steady-state value [16-18]. For satisfactory analysis, the microstructure actually present at the start of a test should be determined. Because of the size difference, it is not usually possible to simulate in test specimens the macrostructures found in production workpieces, in particular those produced by casting. There is validity to using worked specimens as many problems in processing arise near the ‰nish where the temperature is low; the workpiece has already gone through several stages of processing so that the cast structure has already been broken down to a more uniform condition. Moreover, the objective of high T, high ε testing may be the determination of the fundamental mechanisms, and inherent behavior without the disturbance of as-cast variations in composition or
structure [19,20]. The problems of cast structure will be examined in greater detail in consideration of the various alloys (Sections 1.6 and 14.6.1); the degrees of homogenization required will be discussed under speci‰c processes. The bene‰ts and disadvantages of reduced section dimensions in continuous castings, such as strip casting, will receive attention.
