Effect of Ion Irradiation on Dislocation Processes in Stainless Steel
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Effect of Ion Irradiation on Dislocation Processes in Stainless Steel Josh Kacher, Grace S. Liu, May Martin, I.M. Robertson University of Illinois at Urbana-Champaign Urbana, IL 61801, USA ABSTRACT The effects of ion irradiation damage on dislocation generation and propagation in austenitic stainless steels were studied by means of in situ transmission electron microscopy and electron tomography. Tensile samples were irradiated in situ to a dose on the order of 1017 ions/m2 with 1MeV Kr+ and strained at 300 K as well as 673 K. Dislocation motion through the irradiation-obstacle field was jerky and discontinuous, dislocation pile-ups formed in grain interiors and at boundaries, long straight dislocations were generated decorating the channelmatrix walls, and dislocation cross-slip within the channel created debris along the channel leading to channel widening. Electron tomography was applied for the first time to reveal new detail about the dislocation reactions in the channel wall. INTRODUCTION Irradiation damage causes a degradation of the mechanical properties of metals, including a loss of ductility, an increase in both tensile and yield strength, and the formation of an upper and lower yield point [1]. Central to this material degradation is the interaction of dislocations with the irradiated matrix and, through the passage of dislocations, the formation of defect-free channels [2]. In situ heavy-ion irradiation and straining experiments in the transmission electron microscopy (TEM) of stainless steels have shown that previously mobile dislocations are locked in place by the irradiation defects, which in this case were Frank loops. The dislocation sources, however, continued to generate dislocations after the irradiation at the same rate as before irradiation, which suggests that the mechanical response is determined by dislocation propagation not nucleation. The passage of individual dislocations through the irradiated matrix was observed to be jerky and segmented, and to occur in a discontinuous manner. The collective strength of the Frank loop population represented a strong barrier to dislocation motion as evidenced by the formation of dislocation pile-ups close to the dislocations source or in the grain interior at no obvious barrier. Such distributions would result in an increased stress state at the grain boundary, which may serve as an important contributing factor to irradiation-induced stress corrosion cracking [3]. The current work focuses on the dislocation processes that produce defect-free channels and the attendant dislocation structure within and bordering the channels. These processes were studied by means of TEM as well as electron tomography. Heating, straining, and irradiation were all conducted in situ using the IVEM-facility at Argonne National Laboratory. EXPERIMENT Stainless steel samples from three different alloys, a 21%Cr 32%Ni (21Cr32Ni), a 15%Cr 12%Ni (15Cr12Ni), and a commercial 304 stainless steel (SS304) alloy, were prepared for TEM investigation by cutting them to the prescribed
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