The Use of Phase Transformations in the Design of Alloy Steel
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THE USE OF PHASE TRANSFORMATIONS IN THE DESIGN OF ALLOY STEEL
J. W. Morris, Department of Materials and University of
Jr. Materials Science and Mineral Engineering and Molecular Research Division, Lawrence Berkeley Laboratory, California, Berkeley, CA 94720
ABSTRACT the use of phase This paper is intended to describe and illustrate transformations in the design of new alloy steels. The general method of Two examples are discussed: the development of alloy design is described. new ferritic structural steels for use at cryogenic temperature, and the development of 'dual phase' steels for automotive use.
INTRODUCTION The phase transformations that are considered in this paper are moderately well understood. The subject is how they are employed in the systemaThe tic design of new materials that have predetermined sets of properties. notion that one can use the theory of phase transformations to engineer the It should, therefore, properties of materials is a relatively modern one. be worthwhile to outline the method and its context before turning to specific examples. The vast majority of the engineering materials that are used in modern They are either the accidental technology are not 'designed' in any sense. product of research directed toward other ends (serendipity) or the residue of a more or less comprehensive screening of all materials that might conceivably work in a particular application (Edisonian testing). This will But engineering systems are becoming increasingly continue to be the case. complex, and new concepts in engineering devices are increasingly dependent on the availability of materials that have exceptional or highly specific Since satisfactory materials often do not combinations of properties. exist, and since it cannot be assumed that they will be accidentally discovered, it is increasingly important that materials scientists learn how to invent them in a systematic and timely way. Modern materials science offers the outline of a method for creating a new material that can be implemented successfully if a sufficient body of fundamental information is available. The procedure is an iterative one the relation step one utilizes In the first that involves three steps. between the structure of a material and its properties to select a composition and a microstructure that is hypothetically capable of achieving the the connection In the second step one utilizes target set of properties. between the structure of a material and the procexssing it has received to design a processing sequence that is hypothetically able to produce the In the third step one analyzes the material and desired microstructure. tests its properties, uses the results to refine the processing sequence and the target microstructure, and iterates the whole procedure until a satisBecause the structure-property and structurefactory result is obtained. processing relations are imprecisely known, the method can be difficult to apply. Nonetheless it is often effective, and will become more so as the theory of material behavior deve
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