Analisi del comportamento plastico di un acciaio austenitico deformato in torsione ad alta temperatura

Flow behaviour of a torsionally deformed AISI 3161 was investigated in the range of temperatures 850-1100°C with equivalent strain rates 0.0001-0.006 s-1, For temperatures higher than 900°C the 30 ?m grained material presented an unusual linear relationship at low strains between equivalent stress, ?Eq, and equivalent strain, ?Eq, whilst at 850°C a parabolic ?Eq- ?Eq relationship was observed. The flow behaviour of a 100 ?m grained stainless steel torsionally deformed at 1000°C with equivalent strain rate of 0.006 s-1 was also reported to follow the expected parabolic ?Eq-&epsiEq relationship and produced a significantly higher flow stress than the 30 ?m grained material, about 40 % higher at 0.5 equivalent strain. The strain hardening behaviour of the alloy was investigated through plotting strain hardening rate ?Eq = d?Eq/d?Eq in vs. ?Eq. The data curves for the material having the parabolic ?Eq- ?Eq relationships, i.e. 30 ?m grains at 850°C and 100 ?m grains at 1000°C, resulted to have the expected trend, since a wide region of the curves at intermediate stresses could befitted with the linear equation ?Eq = ?o ·(1 - ?Eq/?v), resulting in a value of ?v equal to ET/N with N = 54.8 ± 2.0, where ET is the Young modulus at the test temperature. At higher temperatures and 30 ?m grains, where linear ?Eq- ?Eq curves were found, no linear fitting was possible in the ?Eq vs. ?Eq, plot, i.e. Stage III could not be detected. In the ?Eq vs. ?Eq plot the deviation at high stresses from the linear behaviour, conventionally called Stage IV of strain hardening, occurred at the distinctive value of the ratio of ?Eq/?Eq = 1.0. Through linear fitting method, Stage IV was found to finish with an abrupt deviation to a null strain hardening rate at the distinctive value of the ratio TD = ?Eq/?Eq ~ 0.2. Both values were consistent with values reported in literature for similar austenitic stainless steel deformed in similar conditions. Beyond TD dynamic recrystallisation takes place, i.e. Stage V of hardening. The flat regions of the data curves ?Eq, vs. ?Eq of the 30 ?m grained AISI 316L, i.e. the material presenting anomalous flow behaviour, were found also to begin at the distinctive value of the ratio of ?Eq/?Eq = 1.0 and to finish to the similar distinctive value of the ratio TD ~ 0.2. Microstructure investigations reported that beyond TD the grain structure of the small grained alloy was consumed and replaced by new finer structure, which is typical of dynamic recrystallisation. Since the values of the beginning and the end of the flat regions were consistent with the distinctive values of Stage IV for the normal flow behaviour material, it was concluded that the flat regions in the anomalous flow behaviour material were Stages IV of strain hardening. Also TEM results were further confirmations of such plastic flow analysis, since the observed dislocation structure was typical of Stage IV of strain hardening: the material presented dislocation cells clear of dislocations with cell walls sharp and bi-dimensional and the crystallographic misorientation among different cells was quantified through the analysis of the selected area diffraction patterns to be about 2°. In the small and big grained materials an unusual grain shape evolution from pre-deformation equiaxed to rhomboidal was observed during torsion, which was rationalised in (Angella G. et al, Acta Mater 53 (2005) 1263) in terms of strain induced grain boundary migration (SIGBM) combined with the torsion stress field. Such unusual rhomboidal grain microstructure was reported at temperatures higher than 900°C regardless the grain size, and was concluded that SIGBM in conjunction with small grain size caused the anomalous behaviour of the 30 ?m grained AISI 316L at high temperatur