Superplasticity at high strain rates in fine-grain ultrahigh-carbon steel

An ultrahigh-carbon steel alloy containing 1.25 weight percent carbon and 10 weight percent aluminum as its primary alloying additions (UHCS-10Al) is studied. This ferrous alloy was subjected to various processing procedures in order to create materials with very fine grain sizes. The goals of achieving superplasticity and high strain-rate superplasticity were achieved in the UHCS-10Al alloy. The elevated temperature mechanical behavior of the processed UHCS-10Al materials was evaluated and explained using constitutive equations from creep models.; Solidified powders of UHCS-10Al material were consolidated into bulk material by hot isopressure extrusion. This material exhibited two dominant creep mechanisms: (1) solute-drag creep with a stress exponent of n = 3 and an activation energy for creep equal to that for chemical diffusion; (2) grain-boundary sliding creep with a stress exponent of n = 2 and an activation energy for creep equal to that of lattice diffusion. Under conditions of grain-boundary sliding creep a superplastic elongation of 750% in tension was observed. The flow stress in compression under grain-boundary sliding was observed to be 64% greater than that in tension. Only a slight difference between tension and compression of 19% was observed for solute-drag creep, indicating a significant difference in behavior between tension and compression for grain-boundary sliding.; Additional bulk UHCS-10Al material was manufactured from solidified powders by mechanical attrition and subsequent consolidation. Mechanical attrition of powders in a ball-mill device created an extremely fine microstructure in the UHCS-10Al material after consolidation by hot isopressure extrusion. The attrited UHCS-10Al material was tested in both tension and compression at elevated temperature. The observed mechanical behavior is explained by a model combining two grain-boundary sliding mechanisms, one involving grain-boundary diffusion and one involving dislocation-pipe diffusion. This model explains the stress exponent of n {dollar}approx{dollar} 3 observed for the attrited material. The attrited UHCS-10Al material exhibited high strain-rate superplasticity with an elongation of 350% at a strain-rate of {dollar}dotepsilon{dollar} = 10{dollar}sp{lcub}-1{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}. The ability of materials with a stress exponent significantly greater than n = 2 to achieve high tensile elongation is explained with a numerical model of neck growth. It is shown that even a value of n = 3 allows for significant elongation before failure by necking.