Surface amorphous and crystalline microstructure by alloying zirconium using Nd:YAG pulsed laser
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I. INTRODUCTION
RICKEROY and Matthews proposed that surface engineering aims at designing a composite system, i.e., coating plus substrate, with a performance that cannot be achieved by either the coating or the substrate alone. Laser surface alloying is a most promising technique, because it can obtain the desired composition and microstructure, thus providing remarkable improvements in surface hardness, fatigue, wear, and corrosion resistance of materials without impairing their bulk properties.[2] Microstructure changed tremendously after laser alloying, i.e., refinement and homogenization of microstructure, extension of solid solubility and formation of nonequilibrium phases (noncrystalline phases and metastable crystalline phases). The focus of this study was laser surface alloying of an austenite stainless steel. The austenitic stainless steel is widely used owing to its high corrosion resistance, but due to its low hardness, its tribological properties were very poor. Moreover, because of its austenitic structure, it cannot be hardened by heat treatment and, therefore, there was no easy method for improving its wear resistance. Various investigations have been performed on synthesizing the particulate-reinforced metal matrix composite surface coating on stainless steels using laser surface alloying. As a result, hard and wear-resistant surfaces were produced. AISI 304 stainless steel was hardened by laser melt injection of 140-70 mesh TiC particles.[3] The microhardness of the austenitic matrix increased from 150 to about 200 to 250 Hv. Laser alloying of a 12 pct Cr stainless steel with Cr2O3 powder was also studied.[4] The surface-alloyed region exhibited a microhardness of about 1100 Hv due to the dispersion and precipitation of chromium carbides in addition to the martensitic structure. Laroudie et al.[5] investigated the surface hardening of the AISI 316 stainless steel by laser alloying with various precursors leading to the precipitation of hard carbides during the solidification. Rieker et al.[6] improved the abrasive wear resistance of a stainless steel by laser alloying with molybdenum and boron. Laser [1]
X.L. WU, Associate Professor, and Y.S. HONG, Professor, are with the State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China. Manuscript submitted February 29, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloying of 304 stainless steel, with molybdenum alone, led to an increase in hardness from 200 to 400 Hv for a surface alloy containing 10 wt pct of molybdenum.[7] Other authors studied the laser alloying of stainless steels with boron[8] and carbon.[9] In both cases, an increase in hardness vs the quantities of boron and carbon incorporated was evidence. The component surface treated for wear resistance is finding more use in harsh environments. For example, it was of much interest in design material that exhibited both superior wear resistance and corrosion resistance.[10] Unfortunately, the same micros
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