A Deformation Mechanism Map for the 1.23Cr-1.2Mo-0.26V Rotor Steel and Its Verification Using Neural Networks
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SEVERAL deformation mechanism maps have been constructed for different engineering alloys in recent years. Typically, constitutive rate equations are used to describe the contribution of different mechanisms, including dislocation gliding, dislocation creep or power law creep (dislocation glide plus climb), and diffusion creep mechanisms, i.e., Nabarro–Herring creep and Coble creep (Table I).[1–3] Most current maps are primarily constructed and/or validated using experimental data or microstructural evidences while others are based on the assumption that observations made in simple metals and alloys are also applicable to complex engineering materials. However, a general consensus is slowly emerging that grain boundary sliding (GBS) accommodated by different processes plays a major role in complex engineering alloys demanding a reassessment of conventional assumptions. Also, new approaches need to be developed that can allow a cross-check of the validity and an expansion of the emerging maps. For instance, deformation mechanism maps for turbine blade superalloys have often been constructed under the assumption that diffusion creep was prevalent at lower stresses.[4–6] However, creep and rupture life data over a wide range of temperature have showed that GBS dominates creep deformation in long-term creep tests.[3,7–11] Therefore, using test data where power law NAFISA BANO, Student, and MICHEL NGANBE, Associate Professor, are with the Department of Mechanical Engineering, University of Ottawa, 161 Louis-Pasteur, Ottawa, ON K1N 6N5, Canada. Contact e-mail: nafi[email protected] ASHOK K. KOUL, President, is with the Life Prediction Technologies Inc., 1010 Polytech Street, Ottawa, ON K1J 9J1, Canada. Manuscript submitted May 20, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
breakdown (PLB) was clearly discernible, Castillo et al. modified the conventional map of IN738LC by considering GBS as a separate deformation mechanism as shown in Figure 1.[9,12–16] Furthermore, Banerjee et al. modified the deformation mechanism map of a Pb-Sn eutectic solder alloy considering GBS as the dominant creep mechanism. Based on microstructural evidences, they further sub-divided the GBS dominance region into two parts depending on the dominance of different GBS accommodation processes, i.e., GBS accommodated by wedge type cracking below 0.5T/Tm and GBS accommodated by creep cavitation above this temperature (Figure 2).[17,18] The quantification of the GBS contribution in material deformation has also remained the focus of extensively research in recent years.[19–24] Langdon[25,26] developed a rate equation for GBS taking into account the movement of dislocations within, or adjacent to, boundary planes, through a combination of dislocation glide and climb steps. The rate of sliding was considered to be controlled by the rate of accommodation through intragranular slip. The model showed good agreement with experimental results under creep and superplastic deformation conditions.[27] Wu and Koul[28] modified the Langdon’s GBS model to dev
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