MODEL OF THE EFFECT OF GRAIN SIZE ON PLASTI-CITY IN ULTRA-FINE GRAIN SIZE STEELS
Jin LIU1,Guohui ZHU1,2()
1 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083 2 School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243000
Cite this article:
Jin LIU,Guohui ZHU. MODEL OF THE EFFECT OF GRAIN SIZE ON PLASTI-CITY IN ULTRA-FINE GRAIN SIZE STEELS. Acta Metall Sin, 2015, 51(7): 777-783.
Based on our earlier preliminary work, a model was developed for prediction of the critical grain size where the plasticity would be decreased as the grain refined. In the model the effect of grain size on the fracture strength was combined. The prediction of the model exhibited that in the range of grain size of 10 mm to 0.2 mm as an example, the total elongation of the steels would be firstly increased. But when the grain size was refined to 2.5 mm and below, the total elongation of the steels was not increased but decreased sharply, which was good agreement with the experimental results published recently. Present work illustrated that the dominant mechanism of the elongation decreased in the ultra-fine grain size materials is due to increase in resistance force of grain boundaries on the dislocation sources resulting in the difficulty of activation of dislocation movements. Its expression would be the decrease of the plastic strain in macro-level.
Fund: Supported by National Natural Science Foundation of China (No.51071026) and Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education
Fig.1 Relationship between fracture strength σf and grain size d in polycrystalline materials
Fig.2 Schematic of dislocation pile-up group in a single grain (td—given applied stress)
Fig.3 Relationship between the displacement of a single grain Df0 and d under the corresponding fracture strength
Fig.4 Schematic of grain boundary blocks the process of Frank-Read source (FR source) emits dislocations (Both da and db represent grain size and da>db)
Fig.5 Relationship between the probability of activated FR sources in polycrystal Ff and d under the applied stress of 616 MPa, 1946 MPa and the corresponding fracture strength
Fig.6 Relationship between the total fracture elongation ef and d
Fig.7 Experimental data of uniform elongation eu or total elongation ef versus d in ultra-fine grain size steels[5,6,9,37~42] (Solid symbols represent eu and hollow symbols represent ef)
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