Papers by Author: Yong Ning Liu

Abstract: The electrochemical property of molybdenum disulphide (MoS₂) as anode materials for lithium ion batteries was studied using two-electrode Li-ion cell. The first reversible capacity of MoS₂ treated by using ball milling and doped graphite was 617mAhg^-1 and 506mAhg^-1 respectively. But the reversible capacity of pristine MoS₂ was 661mAhg^-1. The results indicated that the processes of ball milling and doped graphite of MoS₂ can not widely enhance the reversible capacity.

Abstract: With massive trials, spheroidized by austeniting at 810°C and cooling by 1°C/min, a 1.6C (pct) Ultra-high Carbon Steel shows a microstructure of uniformly distributed fine carbides in the ultra-fine ferrite matrix. The grain size of ferrite matrix and spheroidized carbides are about 5um and 0.1～2um, respectively. Further investigation by TEM shows that much dislocation together with twins is obtained for the UHCS, and generally finer grains have higher dislocation density. The spheroidized steel exhibits high tensile strength of 910 MPa and high yielding strength of 653 MPa at room temperature, together with excellent elongation of 18.3%, which shows the UHCS can entirely satisfy certain grades of engineering materials and means the steel may substitute present engineering steel considering lower cost. Furthermore, the steel owns good high-temperature superplasticity, the elongation of 216% obtained at 800°C under a strain rate of 2.5×10-4. Initial analysis suggests that the superplastic deformation mechanics of the steel is grain boundary sliding and grain rotating (GBSR), coordinated by migration of dislocation.

Abstract: Ultrahigh carbon steel containing 1.6 wt pct C was processed to create microduplex structure consisting of fine-spheroidized carbides and fine ferrite grains. Elongation-to-failure tests were conducted at strain rates from 10-4s-1 to 15×10-4s-1, and at temperatures from 600 °C to 850 °C. The steel exhibited superplasticity at and above 700 °C when testing at a strain rate of 10-4s-1, and at 800 °C when testing at strain rates of 7×10-4s-1 and slower. The grains retained the equiaxed shape and initial size during deformation; dynamic grain growth was not observed after superplastic deformation, whereas carbide coarsening was observed. It is concluded that the fine ferrite grains or austensite grains are stabilized by the grain boundary carbides, and grain-boundary sliding controlled by grain boundary diffusion is the principal superplastic deformation mechanism at temperatures in the range of 700-850 °C.

Abstract: The post-dynamic softening of Nb micro-alloyed steel and plain carbon steel was investigated through stress relaxation method. Meanwhile, By comparison with C-Mn steel, the effect of deformation temperature and strain on post dynamic recrystallization of Nb steel was studied . The result shows that element Nb in solution state can dramatically improve the apparent activation energy of static recrystallization. At lower temperature, the recrystallization can be retarded and precipitation of Nb(C, N) occured. If strain is more than the critical strain of dynamic recrystallization, the metadynamic recrystallization takes place. If fully metadynamic recrystallization takes place, the kinetics is no longer dependent on strain of the kind of steels.

Abstract: A ultrahigh carbon steels (UHCS) containing 1.6 wt pct carbon was studied. Through spheroidizing process by divorced-eutectoid transformation (DET), the forged microstructure was spheroidized and the microstructure was fine carbide particles distributed in ferrite matrix. Second-time heat treatment included two kinds of technologies: normalizing and quenching + tempering. Finally, the UHCS obtained ideal mechanical properties. The yield strength and tensile strength of the UHCS were higher than that of 40CrNiMo, moreover plasticity of the UHCS was equal to that of 40CrNiMo. So the UHCS was an excellent structural material.

Abstract: 1.4 %C ultra high carbon steel (UHCS) was prepared in order to study the structure of martensite transformation and mechanical properties. Ultra-fine spherical carbide and ultra-fine austenite grain size were obtained. A great deal of lath martensite was observed after quenching. The phenomenon does not agree with the traditional knowledge that the lath martensite would disappear when carbon content is in excess of 0.8% in austenite. The strength, fatigue properties and fracture toughness have been measured. A good combination of strength, toughness and fatigue properties come from fine and uniform distributed carbide particles and ultra-fine austenite grain size. Fracture strength increases by 48%, yield strength increases by 15% and plasticity keep the same comparing with that of hardened and tempered 40CrNiMo. The carbon content of ultrahigh carbon steels (UHCS) is in the range of 1.0-2.1% [1, 2]. Traditional heat treatments for normal steels will cause the microstructure of UHCS to be coarse and do not produce optimal properties. With controlled rolling and special heat treatment, UHCS can be in ferrite, pearlite, bainnite or martensite structures, which all have different mechanical properties. The yield stress of a 1.8%C, 1.6%Al ferrite UHCS can reach 1500MPa, which is much higher than that of high strength and plain alloy steels [3]. The tensile strength of a 1.25%C-1.5%Cr pearlite UHCS can reach 1810Mpa and its elongation can be 18%. When it is treated into martensite, its compression strength reached to 4690Mpa and compression strain reached to 26% [1, 4], which is comparable to WC-12Co. Such good mechanical properties can be ascribed to the ultra fine grain sizes because of the undissolved carbide particles which resist growth of austenite grain during heating. Another reason could be the lath martensite structures. O.D.Sherby [4] had reported that there was a lot of lath martensite in quenched UHCS. The UHCS was considered not only as tool steels but also as good structure materials. Fracture and fatigue properties are important for structure materials. However, they have rarely been studied. The present paper is going to study the martensite structure and mechanical properties of a prepared 1.4% C UHCS.