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Abstract
The fatigue crack growth behaviour of ultrafine grained (UFG) 6061 aluminium alloy processed by equal channel angular pressing (ECAP) is studied and discussed in relation to the crack path and grain microstructure. The UFG samples exhibited a lower fatigue threshold, a unique transitional regime where only modest increases in crack growth rates resulted with increases in stress intensity amplitude and higher crack growth rates in the steady state regime than their coarse grained counterpart. Electron back scattered diffraction (EBSD) was used for microstructural analysis in the crack tip process zone and outside the crack region to study fatigue induced changes and also provide a detailed analysis of crack path. A combination of intergranular and transgranular crack growth was reported at the fatigue threshold. Grain boundary sliding is a likely mechanism for intergranular crack growth in the near threshold regime and is proposed to transfer plastic strain leading to a reversible plastic zone size that is ~10 times the grain size. The relative toughness of grain boundaries to grain interiors is estimated to be 0.68. It is proposed that ECAP weakens grain boundaries, resulting in more near threshold intergranular crack growth, grain boundary sliding and a lowering of the fatigue threshold with ECAP passes. The transitional regime featured a granulated surface where increased roughness-induced crack closure (RICC) effects may occur. A purely intergranular crack growth mode may result from a concentrated incompatibility of plastic deformation in adjacent grains. The UFG samples show a higher crack growth rate in the steady state regime which may be attributed to lower roughness induced crack closure effects and some cleavage crack growth. Microcracks, crack branching, grain coarsening and recovery were observed in the crack tip process zone but did not significantly retard crack growth rate in the UFG samples.