Papers by Author: Zheng Hong Guo

Abstract: The internal friction and elastic modulus variations caused by the structural rearrangement fcc↔fct in Mn-Fe (Cu) antiferromagnetic alloys were studied in this paper. Antiferromagnetic transition exhibits weak first-order features due to the formation of microtwins by modulus softening mechanism. Antiferromagnetic transition also assists subsequent transformation to form twinned martensite. The small hysteresis between direct and reveres martensitic transformations indicates the thermoelastic feature. Both the martensitic and its reverse transformations also depend on the modulus softening mechanism.

Abstract: A novel heat treatment process, that is, quenching-partitioning-tempering (Q-P-T) process, has been developed as a new way to obtain ultrahigh strength martensitic structural steel containing retained austenite and alloying carbide. In order to display merit of the Q-P-T process, a medium carbon Nb-microalloyed steel is treated by Q-P-T 1-step process and Q-P-T 2-step process, as well as treated by the transformation induced plasticity heat treatment process and quenching and tempering process, respectively. The results show that Q-P-T samples possess better mechanical properties than those treated by other heat treatment processes. The origin of the good mechanical properties is analyzed based on the phase and microstructure characterization using X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

Abstract: According to the design principle of microstructures for high strength steel and a new quenching-partitioning-tempering (Q-P-T) process recently proposed by Hsu, a microalloying Fe-Mn-Si base steel by the Q-P-T process has been designed. The results indicate that the Q-P-T steel exhibits ultra-high tensile strength combining with good ductility and toughness, and it is a new family of advanced high-strength steels. The microstructures of samples by different Q-P-T processes were characterized by means of optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy, and the relation between microstructures and mechanical properties was analyzed

Abstract: An ultrahigh strength dual-phase (DP) steel with low ratio of yield strength/ultimate tensile strength (YS/UTS) was designed based on the simulation using JmatPro software so as to improve formability as well as to extend its application in automobile industry. Results show the DP steel suffered from water quenching (WQ) technology exhibits high ratio, 0.872, of YS/UTS, which loses the advantage of formability of DP steels and restricts its application in automobile industry. Therefore, the controlled slow-cooling rate (CSCR) technology is employed to this DP steel, and the low ratio, 0.458, of YS/UTS is obtained. Although the tensile strengths of the DP steel suffered from two kinds of technologies are over 1000 MPa, The YS of the DP steel with CSCR technology is 480 MPa and is much lower than 983MPa of the DP steel with WQ technology, which are attributed to relative large grains and small volume fraction of martensite in the former based on the characterization of microstructure by optical microscope, scanning electron microscope, transmission electron microscope and electron backscattering diffraction.

Abstract: The possibility of Al2Cu( q) precipitation in nanosized Al-4wt%Cu alloy is predicted based on the theory of homogeneous nucleation. The result indicates that the initial concentration of Cu in parent phase has little influence on the nucleation event when the grain size is larger than a critical size. On the other hand, when the grain size is smaller than the critical size, the formation of a stable Al2Cu nucleus will be prohibited completely due to the insufficient initial concentration of Cu.

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.