Influence of Grain Coarsening on the Creep Parameters During the Superplastic Deformation of a Severely Friction Stir Pr
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nostructured materials usually present good superplastic response at high strain rates and relatively low temperatures due to their exceptional microstructures in the form of highly misoriented ultra-fine grains.[1–6] In this regard, friction stir processing (FSP) is a severe plastic deformation (SPD) technique based on the concepts of friction stir welding (FSW)[7] capable of producing ultra-fine grain sizes (2 and Qap „ QGB, QL.[27–31] On the other hand, grain coarsening during the superplastic deformation alters the resulting values of the apparent creep parameters.[33] In this study, superplastic ultra-fine grained FSPed Al7075 alloy data showing non-model nap and Qap values are rationalized taking into account the L dependence of the GBS constitutive equation. We used 3-mm-thick sheets of a commercial Al 7075 (5.68 wt pct Zn, 2.51 wt pct Mg, 1.59 wt pct Cu, 0.19 wt pct Cr, 0.19 wt pct Fe, 0.052 wt pct Si, 0.025 wt pct Ti, 0.007 wt pct Mn, bal. Al) aluminum alloy in the T6 temper, with average L ~ 60 to 100 lm in RD and ~10 lm in TD, that were subjected to FSP as described elsewhere.[11] Three different L were obtained using different processing conditions, i.e., L(1) = 1065 nm (1400 rpm, 500 mm/min, conventional backing anvil), L(2) = 530 nm (1000 rpm, 500 mm/min, liquid nitrogen refrigerated backing anvil) and L(3) = 385 nm (1000 rpm, 1000 mm/min, liquid nitrogen refrigerated backing anvil). For discussion purposes, the L(2) condition has been selected as the reference material. The high temperature, 473.15 K to 723.15 K (200 C to 450 C), mechanical behavior was characterized by means of constant crosshead speed tensile tests (CCST) equivalent to an initial e_ = 102 s1 which have been compensated to an equivalent constant e_ at 102 s1 throughout the test (constant strain rate tests, CSRT), and by strain rate-change tensile tests (SRCT) ranging e_ = 101 105 s1. Since e_ decreases with increasing e in CCST, the observed r values correspond to different e_ when deforming the material, which is noticeable at the high e values usually considered in superplastic materials. In our calculations, it is crucial to use r e data keeping e_ constant. Therefore, we first calculate the true
e_ ð_et Þ for each deformation as e_ t ¼ v=ðl0 þ dÞ, where v = 0.065 mm/s is the crosshead speed, l0 = 6.5 mm is the initial gauge length and d is the instant displacement. Once e_ is known, the CCST stress (rCCST) can be compensated by comparing Eq. [1] at two different e_ , assuming no change in the deformation mechanism during the test. Therefore, the compensated stress (rcomp) can be obtained using rcomp ¼ rCCST ðe_ i =_et Þ1=nap , where e_ i ¼ 102 s1 is the initial e_ and nap is the apparent stress exponent, obtained from the CSRT (see Figure 2 description). The tests were performed using a universal Instron 1362 testing machine equipped with a four-lamp ellipsoidal furnace. Dog-bone tensile samples with 6 9 2 9 1.8 mm3 gage dimensions were electro-discharge machined along the traverse processing direction in such a way that t
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