Influence of the ECAP Processing Parameters on the Cyclic Deformation Behavior on Ultrafine‐Grained Cubic Face Centered Metals

Abstract

Ultrafine-grained (UFG) materials produced via equal channel angular pressing (ECAP) show high strength under monotonic loading as well as strongly enhanced fatigue lives in the Wö hler-S-N-plot compared to their conventionally grain sized (CG) counterparts. [1][2][3] For the AlMg model system, where the Mg content was systematically varied between 0.5 and 2 wt% by May et al., [4] where the fatigue life increases by several orders of magnitudes. Improved fatigue lives were found for many other UFG materials [3,5] like aluminum alloys, [6] copper, [7,8] titanium alloys, [9] and steels. [10][11][12] Despite the positive effects of the UFG microstructure on the monotonic behavior and fatigue life, pronounced differences became evident when comparing the data available in literature. Hence the influences of the processing parameters of the ECAP process have to be investigated in more detail.When the strain amplitude is the relevant quantity of design, fatigue life enhancement by an UFG microstructure has to be discussed carefully. As already predicted by Mughrabi and Hö ppel in 2001, a crossover between the fatigue life curves of the CG and UFG condition occurs. [13] This crossover is related to the fact that the fatigue life is dominated by the strength of the material at small plastic strain amplitudes and by the ductility of the material when the plastic strain amplitude exceeds or overcomes the elastic strain amplitude. In this context the point of crossover, also denoted as point of transition, is dependent on the materials properties and hence on the ECAP.Besides the fatigue life also the cyclic stability is strongly dependent on the processing parameters like already shown by Niendorf et al. [10][11][12] on IF steels or by Vinogradov [14] on copper.In order to understand the influence of the ECAP processing parameters on the cyclic deformation behavior and fatigue life in more detail this work focuses on the the AlMg model system with up to 2 wt% of Mg. For the comparison with commercial alloys AA6061 and AA6082 are used.

Ultrafine versus coarse grained Al 5083 alloys: From low-cycle to very-high-cycle fatigue

“…In LCF, the reduction in life of the UFG material compared to its CG counterpart decreases as the plastic strain amplitude increases, up to the point that the two curves seem to cross at the highest amplitude -which should not be considered as a definitive result, owing to the limited number of data-points and the scatter often found, even in this regime [29]. This result is opposite to the trend reported in several papers (for example [4]). However, these papers mostly concern microstructurally unstable, cyclically softening UFG materials in which softening and grain coarsening, associated with strain localization tend to increase with the strain amplitude, leading to a more pronounced reduction in life than at lower amplitudes.…”

“…Besides, observations made on ECAPed materials may themselves differ, depending on the ECAP route (i.e. rotation of the billet between consecutive extrusions) [4], or even on the number of ECAP passes [5]. Furthermore, the gain or loss in fatigue resistance due to grain refinement into the UFG regime is likely to be dependent on the intended life and on the loading mode.…”

“…A higher tensile strength as compared to coarse grained (CG) materials is expected to improve the resistance of UFG materials to high or very high cycle fatigue (denoted below as HCF and VHCF), which is generally observed [1,2,[5][6][7], while a lower ductility is expected to reduce the performance in low-cycle fatigue (LCF), which is generally observed in strain-controlled tests [1,[8][9][10], but not in stress-controlled tests [11,12]. While the entire S-N curve of a UFG material generally stands above that of its CG counterpart, their strainlife curves often cross [1,4].…”