Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly re® ning the microstructure, which is found to have a strength in excess of 2 . 5 GPa together with an ability to¯ow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographi c details are reported of the very ® ne bainitic microstructure associated with the incredibly low transformatio n temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than~10 17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation .

Mixed microstructures consisting of ®ne plates of upper bainitic ferrite separated by thin ®lms of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental veri®cation of the design process.MST/4644

Mixed microstructures consisting of ®ne plates of upper bainitic ferrite separated by thin ®lms of stable retained austenite have seen many applications in recent years. There may also be some martensite present, although carbides are avoided by the judicious use of silicon as an alloying element. The essential principles governing the optimisation of such microstructures are well established, particularly that large regions of unstable high carbon retained austenite must be avoided. With careful design, impressive combinations of strength and toughness have been reported for high silicon bainitic steels. The aim of the present work was to ascertain how far these concepts could be extended to achieve unprecedented combinations of strength and toughness in bulk samples subjected to continuous cooling transformation, consistent with certain hardenability and processing requirements. Thus, this paper (part 1 of a two part study) deals with the design, using phase transformation theory, of a series of bainitic alloys, given a set of industrial constraints. Part 2 of the study concerns the experimental veri®cation of the design process.MST/4644

No abstract

Various near net shape items have been produced by superplastic forming and thixoforming magnesium alloys. Fine grained EA55RS magnesium alloy has been superplastically formed to produce sabot petals and various other items without obvious increase in grain size. Although the laminated appearance of the starting material was less pronounced after processing, the microstructure of superplastically formed material varied throughout any particular piece. ZC71 magnesium alloy has been thixoformed to produce items with a finer grained microstructure than that of the start material. The thixoformed items had an average grain size of about 25 gm and exhibited only a little local variation in their microstructures.