Papers by Author: Yong Hua Rong

Abstract: Since Hsu in our group proposed a novel quenching-partitioning-tempering (Q-P-T) process in 2007, Q-P-T steels with ultrahigh strength and ultrahigh strength-ductility are greatly developed. In this paper a novel Q–P–T process based on design of microstructure is first introduced, including the comparison of Q-P-T process with a novel quenching-partitioning (Q&P) and traditional quenching and tempering (Q&T) process. The mechanical properties of Q-P-T steels at room temperature and elevated temperatures are then exhibited, including nanolath martensitic steel with the tensile strength of over 2000MPa and ultrahigh strength-ductility steel with the product of strength and elongation of over 30000MPa%. The mechanisms of ductility enhancement by retained austenite are finally summarized, including a new effect proposed by us, named DARA (dislocation absorption by retained austenite) effect after TRIP (transformation induced plasticity) effect and BMP (blocking microcrack propagation) effect proposed by other investigators 50 years ago.

Abstract: A nanocrystalline surface layer is produced in Co plate by surface mechanical attrition treatment (SMAT). The characterization of microstructure and composition indicates that elements of Fe, Cr diffuse from hardened steel balls into the surface layer during SMAT. The diffusion phenomenon results in the composition deviation in the surface layer, leading to higher value of saturation magnetization (M_s) for nanocrystalline Co surface layer in comparison with its coarse-grained counterpart.

Abstract: A series of (Fe50Ni50)xCu1-x granular films were prepared using magnetron controlled sputtering method. The magnetic-transport and microstructure of FeNi-Cu films deposited at room temperature and then annealed at various temperatures were investigated through TEM, XRD and conventional four probes method under room temperature, respectively. The giant magnetoresistence (GMR) as a function of FeNi volume fraction for as-deposited FeNi-Cu films reached a maximum of about 1.8 % at the volume fraction of 32 %. With increasing the annealing temperature, the GMR of films with the volume fraction less than 26 % reaches a peak at certain annealing temperature. While for films with the volume fraction larger than 26 %, the GMR have almost no changes at first and then decrease with increasing the annealing temperature. In addition, the relationship between magneto-transport and the microstructure, morphology of granular films is discussed.

Abstract: Previous works on the kinetics of martensitic transformation in shape memory materials through Landau theory and the application of the phase field theory to study phase transformations in alloys are briefly reviewed. Based on field model to improper martensitic transformation proposed by Wang and Khachaturyan in 1997, a simpler model is suggested. Using this model, the motion speed and shape of parent/martensite and martensite-martensite interface are quantitatively described, which are important factors to be considered in design of smart device using shape memory materials as a main element.

Abstract: The quantitative relationships are suggested that the applied stress decreases the nucleation barrier and activation energy of nucleation of martensitic transformation, and strain increases the nucleus sites. Taking Fe-20Ni-0.5C and Fe-25Ni-0.66C alloys as examples, their different martensitic morphologies in thermal-mechanical processing can be explained and the origin of such a difference may be revealed based on the above theoretical analysis. Accordingly, the control of martensitic morphologies in thermal-mechanical processing of ferrous alloys will become possible.