EBSD法による超細粒結晶銅の疲労き裂伝ぱ挙動の解明

概要

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Fatigue crack propagation behavior of ultrafine-grained copper precessed by equal channel angular pressing, ECAP, was investigated by electron backscatter diffraction, EBSD, technique as well as atomic force microscopy, AFM. The results show that the crack propagation rate, da/dN, was smaller in ECAP-processed specimen than the coarse-grained counterpart at higher stress intensity factor range, ΔK. The decrease in the da/dN in spite of the small grain size is attributed to the grain coarsening at the crack tip during fatigue which results in the increase in the roughness induced crack closure. In the range below ΔK<5MPa√<m>, Region I, large-scale grain coarsening in the limited number of grains is dominant due to the small ΔK and the large number of fatigue cycles to cross the plastic zone, N_p. In the range above ΔK>5MPa√<m>, Region II, small-scale grain coarsening in many grains is dominant due to large ΔK and small N_p. The grain coarsening was introduced as a result of the migration of high angle grain boundary. The large initial transgranular strain and unstable grain boundaries are considered to be the causes of the strain-induced grain boundary migration. The X-ray stress at the crack tip was found to be smaller than δ_<0.2> due to the strain relaxation and grain coarsening.
2009-06-25