Elevated-temperature deformation at forming rates of 10−2 to 102 s−1

Abstract

In the hot working at constant strain rate () of Al and ␣ Fe alloys at 0.5 to 0.9 T M (absolute melting temperature), steady-state deformation is achieved in similarity to creep, which is usually at constant stress. After an initial strain-hardening transient, the flow stress becomes constant in association with a substructure which remains equiaxed and constant in the spacing of sub-boundaries and of dislocations in both walls and subgrains. All these spacings become larger at higher temperature (T ) and lower values as well as with lower stress, being fully consistent with the relationships established in creep. Because hot working can proceed to a much higher true strain in torsion (ϳ100) and compression (ϳ2) as well as in extrusion (ϳ20) and rolling (ϳ5), it is possible to confirm that grains continue to elongate while the subgrains within them remain equiaxed and constant in size. When the thickness of grains reaches about 2 subgrain diameters (d s ), the grain boundaries with serrations (ϳd s ) begin to impinge and the grains pinch off, becoming somewhat indistinguishable from the subgrains; this has been called geometric dynamic recrystallization (DRX). In polycrystals as at 20 ЊC, deformation bands form and rotate during hot working according to the Taylor theory, developing textures very similar to those in cold working. In metals of lower dynamic recoverability such as Cu, Ni, and ␥ Fe, new grains nucleate and grow (discontinuous DRX), leading to a steady state related to frequently renewed equiaxed grains, containing an equiaxed substructure that develops to a constant character and defines the flow stress.

Hot Deformation of Austenitic Stainless Steel Type 316 up to Strain Rates of 100 s-1

“…14) Torsion, particularly, is often employed not only to simulate hot rolling but also to produce stress-strain curves for the derivation of constitutive equations suitable to be used in flow curve modeling. 12,[14][15][16][17] Mechanical testing, however, presents some limitations, the most serious of them, perhaps, regarding the maximum strain rate attainable during deformation. The usual strain rates reported by researchers do not surpass a value of around 10 s Ϫ1 and, only very rarely, tests are conducted at strain rates of the order of 50 s…”

“…In order to compute the increase in temperature due to adiabatic heating, it was assumed that the rise was uniform through the sample and that changes in the density and specific heat during a giving temperature interval dT was negligible. It was further assumed, as usually reported in the literature, 7,14,15) that all plastic work for a given strain interval, de, was converted into heat. All samples were deformed to an equivalent strain of 2 achieving a steady state stress in all cases.…”

Hot Deformation of 304 Type Austenitic Stainless Steel at High Strain Rates.

“…The usual strain rates reported by researchers do not surpass a value of around 10 s Ϫ1 and, only very rarely tests are conducted at strain rates of the order of 50 s Ϫ1 , as reported elsewhere. 8,13,14) Therefore, hot rolling loads at strain rates such as those found in the finishing stands of a hot strip and rod mills can be estimated only by the extrapolation of data obtained at considerable lower ranges of strain rates. This may be or may be not realistic since it seems that, up to these days, there is a lack of unequivocal experimental evidence for the hypothesis of calculating loads from extrapolated data.…”

Microstructure of Kinetic Spray Coatings: A Review

“…The activation energy required for SRV was lower than for SRX, since the dislocation glide and climb was fairly active in high stacking fault energy (SFE) materials such as Al. Therefore, most of the stored strain energy was relieved through SRV (Ref [91][92][93][94][95][96].…”