Ultrasonic characterization of surface modified layers
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I.
INTRODUCTION
RAPID solidification processing (RSP) refers to solidification processing where the cooling rate exceeds 102 K 9 s -~. The emergence of this innovative approach to materials processing has resulted in the ability to produce novel microstructures which bestow improved properties upon materials processed by these techniques (Figure 1). These high technology materials possess microstructures with enhanced chemical homogeneity, extended solid solubility, microcrystallinity, refined precipitate distributions, and metastable phases, some of which are amorphous. This has resulted in significant improvements in corrosion, erosion, wear resistance, and improved strength. One of the methods for the production of rapidly solidified microstructures is that of surface modification by directed high energy beams (lasers and electron beams). These sources of high intensity heat are capable of rapidly melting (e.g., in less than a few milliseconds) a thin surface layer before heat has appreciably diffused to the substrate. This results in a very steep temperature gradient normal to the surface, and thus a mechanism for rapid quenching of the melt (by the substrate) upon removal of the heat source. By rapidly scanning the heat source over the sample surface, cooling rates in the 103 to 108 K 9 s -L range are achieved and solidification velocities of up to 1 to 10 m 9 s -L induced. The application of this process to near net shape components eliminates the need for appreciable further processing and thus can optimize process productivity. Two broad classes of RSP surface microstructures are currently being investigated. One is concerned with the formation of amorphous surface layers on crystalline metal substrates. The second is the formation of modified crystalline surface layers with or without chemical modification. Chemical modification with this technique can be achieved by incorporating alloying powders into the melt prior to its solidification. The formation of amorphous surface layers or the layer glazing process, as it is sometimes referred to, has been demonstrated to be technically feasible, but its widespread application is limited by two problems: the difficulty of producing deep (>20 p~m) amorphous layers and the elimiBRET J. ELKIND and MOSHE ROSEN are with the Materials Science Department, The Johns Hopkins University, Baltimore, MD 21218. HAYDN N. G. WADLEY is with National Bureau of Standards, Gaithersburg, MD 20899. Manuscript submitted August 15, 1985. METALLURGICALTRANSACTIONS A
nation of cracking. The former problem arises either from the reduction of temperature gradient for deeply melted layers, which causes a reduction of solidification velocity to a value below the threshold for glass formation,2 or from the occurrence of amorphous to crystalline transitions in the heat affected zones of previously vitrified material during sequential melting passes in adjacent material. The cracking problem, on the other hand, arises because of the large tensile residual stresses that are produced by solid
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