Microstructural characterization
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INTRODUCTION
S E L E C T I V E modification of surface microstructures and mechanical properties by various rapid cooling methods, such as splat quenching, melt spinning, and laser cladding, has received significant attention in recent years) ~-51 In view of high cooling rates with laser surface treatments, e.g., 103 to 107 K / / s , nonequilibrium crystalline and amorphous fine microstructures possessing increased solid solubility limits, high microhardness, and good toughness can be developed. Such microstructures are difficult to obtain with conventional surface treatment processes. Crystalline and amorphous microstructures exhibiting improved corrosion and wear resistance have been produced by laser surface alloying ferrous and nonferrous alloys with Cr, Ni, Mo, and Co. 15,61 The formation of amorphous and nonequilibrium y and g phases with extended solubilities of C, Cr, Mo, and W and improved mechanical properties have been reported for rapidly quenched Fe-X-C (X = Cr, Mo, W) ternary systems, t7,81 Laser melt quenching and transformation hardening of steels revealed the formation of hard and tough microstructures comprising fine-grained, highly dislocated lath K. NAGARATHNAM, Graduate Research Assistant, and K. KOMVOPOULOS, Associate Professor, are with the Department of Mechanical Engineering, University of Calilbrnia, Berkeley, CA 94720. Manuscript submitted August 7, 1992. METALLURGICAL TRANSACTIONS A
and internally twinned martensite with some retained interlath austenite.[9.1~ Fine microstructures consisting of Co-rich primary dendrites and interdendritic eutectic consisting of M7C 3 and M23C 6 carbides in an fcc y phase have been obtained by means of metal arc welding, tungsten inert gas welding, and laser cladding of Cobased hardfacing alloys.[~] Laser cladding of Co-Cr-W-C and Ni-Cr-B-Si-C alloys on AISI 304 stainless steel produced fine-grained dendritic microstructures possessing high hardness due to the formation of carbides and carboborides at grain boundaries)~21 The improved wear resistance of laser-clad Fe-Cr-Mn-C alloy was associated with the fine distribution of complex carbide precipitates, such as M 6 C and M7C3, in a ferritic matrix. 113,141Microstructural characterization of laser-alloyed Fe-C-Cr demonstrated the formation of a fully martensitic phase with dislocated lathtype substructures and twinned austenite needles or M23C 6 carbides in a dislocated ferritic matrix, depending on the concentration of Cr. 115'161 Refined microstructures possessing metastable crystalline and amorphous phases have been reported for laser surface alloying with a similar Fe-Cr-C ternary system, l~TI The influence of processing conditions on the microstructure fineness, distribution of alloying elements, geometrical characteristics (e.g., thickness and width of clads), size of the heat-affected zone, microhardness, and toughness of laser-clad Fe-CrW-C alloy has been investigated, t~sl VOLUME 24A, JULY 1993--1621
Although microstructural modification of various Febased alloys by rapid solidification t
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