Crack paths and hydrogen-A f inssisted crack growth response in AlSi 4340 steel
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I.
INTRODUCTION
IT is well known that high-strength steels exposed to hydrogen-bearing environment exhibit high susceptibility to hydrogen embrittlement under sustained load. Studies of crack growth in hydrogen and hydrogen sulfide have indicated that the kinetics of Stage I[ crack growth show substantially different response in two temperature regions ~-5 (see Section III and Figures 1 and 2). At "low" temperatures, crack growth reflects control by transport of gases to the crack tip (or external transport) or reaction of the gas with newly created surfaces at the crack tip, or hydrogen diffusion (or internal transport) to the embrittlement region ahead of the crack tip. The crack growth rates conform to the temperature and pressure dependence of the rate controlling process. 3-6 The transfer of control from one process to another and the resultant change in crack growth rates, as one changes environmental conditions in the "low" temperature region, are reasonably well understood. 6 At "high" temperatures, the crack growth rates are substantially lower than those predicted by the low-temperature rate controlling process if it had remained in control. The reasons for the decrease in growth rate with increasing temperatures in this region, however, are less clear. The decrease may result from a decrease in the rate of supply of hydrogen engendered by changes occurring outside of the steel, L2 or from a phase transformation at clean fracture surfaces, 7 or from changes in fracture paths caused by the distribution of hydrogen into different regions of M. GAO is Visiting Scholar, on leave from the Department of Materials Science, Shanghai Jaio Tong University, Shanghai, People's Republic of China. M. LU was formerly Research Associate, Department of Mechanical Engineering and Mechanics. R.P. WEI is Professor of Mechanics, Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015. Manuscript submitted July 21, 1983. METALLURGICALTRANSACTIONSA
microstructure. Clarification of this issue is essential to the further understanding of rate controlling processes for Stage II crack growth response and of hydrogen embrittlement mechanisms. Hydrogen embrittlement apparently involves a region of material ahead of the crack tip. In other words, it occurs by a "volume embrittlement" mechanism as opposed to a "surface" mechanism. 8-" This volume embrittlement, or reduction in local fracture stress, results from interactions of the microstructure with the deleterious species (e.g., hydrogen) that enter the material to reduce the interatomic bond strength, or to alter the local chemistry or microstructure. As a result of these interactions, cracks tend to follow specific paths through the microstructure. The particular paths and rates of growth are expected to depend on the local concentration of the deleterious species and on the rate of supply of these species to the various fracture sites. The paths, therefore, are expected to depend on temperature and pressure of the environment. Changes in frac
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