The role of nitrogen on the deformation response of hadfield steel single crystals
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8/8/03
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The Role of Nitrogen on the Deformation Response of Hadfield Steel Single Crystals D. CANADINC, I. KARAMAN, H. SEHITOGLU, Y.I. CHUMLYAKOV, and H.J. MAIER We studied the role of nitrogen content on the stress-strain response of Hadfield steel (HS) single crystals under compressive loading. Two different nitrogen concentrations were examined for each orientation (0.05 wt pct and 1.06 wt pct) with drastic increase in critical resolved shear stresses (CRSSs) and strain-hardening coefficients compared to HS without nitrogen. The stress-strain response was strongly dependent on both the crystallographic orientation and the nitrogen concentration. Transmission electron microscopy (TEM) results revealed that, for the HS with 1.06 wt pct nitrogen, the hardening is influenced by the coexisting deformation twins and precipitates, which both act as strong obstacles against dislocation motion. A visco-plastic self-consistent (VPSC) model was modified to account for precipitation and twinning length scales in HS with 1.06 wt pct nitrogen for selected crystallographic orientations. Incoherent precipitates in the hardening formulation were treated as factors affecting the mean free path of dislocations. The model also accounts for plastic relaxation of precipitates with increasing strain and accurately predicts the stress-strain response.
I. INTRODUCTION
RECENT developments in nitrogen steel processing technologies have led to an increased use of nitrogen alloying as a strengthening mechanism for steels.[1,2] By alloying with nitrogen, which is a strong austenite stabilizer, solid solution hardening is obtained without a significant loss of fracture toughness. Possessing a higher solubility than carbon in austenitic iron forming and stronger substitutional-interstitial (s-i) atom couples, nitrogen is preferably used for the strengthening of austenitic stainless steels.[3,4,5] Most of the previous work on the strengthening of steels by nitrogen alloying was conducted on austenitic stainless steels. It has been reported that flow stress in austenitic stainless steels increases with increasing nitrogen concentration, which also influences the thermally activated component of the flow stress.[4,5] Similar observations were made in single crystals.[2,6–9] In our previous studies, we investigated Hadfield steel (HS) (an austenitic manganese steel) single crystals under compression and tension. The unusual strain-hardening behavior of HS was attributed to interstitial solid solution hardening, early activation of deformation twinning, slip-twin interaction, and dynamic strain aging of Mn-C couples.[10,11] The present study is undertaken to investigate the effect of nitrogen on the critical resolved shear stresses (CRSS) of slip and twinning, different stages of deformation, strainhardening coefficients, and the deformation mechanisms of HS. These observations are necessary because nitrogen is more soluble than carbon, and Mn-N s-i atom couples are stronger than Mn-C couples.[12] Therefore, a difference in D.
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