The strengthening mechanisms of hot-rolled steels micro-alloyed with Ti(ST-TQ500)and Nb-Ti(NTTQ500)were investigated by examining the microstructures of steels using optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM).The results revealed almost no differences in the solute solution strengthening and fine-grained strengthening of the two steels,whereas the contributions of precipitation strengthening and dislocation strengthening were different for ST-TQ500 and NT-TQ500.The measured precipitation strengthening effect of ST-TQ500 was 88 MPa higher than that of NT-TQ500;this difference was primarily attributed to the stronger precipitation effect of the Ti-containing nanoscale particles.The dislocation strengthening effect of ST-TQ500 was approximately 80 MPa lower than that of NT-TQ500.This is thought to be related to differences in deformation behavior during the finishing rolling stage;the inhibition of dynamic recrystallization from Nb in NT-TQ500(Nb-Ti)may lead to higher density of dislocations in the microstructure.
Chuan-feng MENGYi-de WANGYing-hui WEIBin-qing SHITian-xie CUIYu-tian WANG
As a new type of high manganese steel,the twinning induced plasticity(TWIP)steels have attracted a growing interest in the automotive industry due to their good performance.Thin plates of TWIP steel were welded by laser beam welding(LBW)and gas tungsten arc welding(GTAW).The microstructure result shows that GTAW joint has obvious heat-affected zone(HAZ),while the HAZ of LBW joint is almost invisible.The X-ray diffraction result shows that the phase compositions of both joints are austenitic and no phase transition occurs.Energy dispersive spectrometry result shows that there is violent evaporation of Mn element in LBW joint,while the proportion of Mn element in GTAW joint is almost unchanged.Tensile tests and micro-hardness measurements were performed to take into account the mechanical properties of joints manufactured by the two different processes.The micro-hardness profiles of both joints present a typical saddle distribution,and the hardness of GTAW seam is lower than that of LBW seam.The failure positions of LBW joints are all located in base metal while the GTAW joints are all at the weld toe due to the softening of HAZ.By means of scanning electron microscopy,a typical ductile fracture is observed in LBW joint,while a brittle fracture with quasi-cleavage fracture characteristic is observed in GTAW joint.
The fatigue performance and fracture mechanism of laser welded twinning induced plasticity(TWIP)steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.The optical microscopy was used to analyze the evolution of microstructure.The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers.The micromechanical behaviors of the various zones were characterized using nanoindentation.The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal.The microstructures,tensile properties and fatigue strength of base metal as well as welded metal were analyzed.The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy,in order to identify fatigue crack initiation sites and propagation mechanisms.Moreover,the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.
The effect of the rare earth element Er on the microstructures and properties of Mg-Al intermetallic were studied in this experiment. Metallographic and X-ray diffraction(XRD) results showed that the microstructures of Mg-Al-Er alloys varied with Er content. The Mg-44Al-0.5Er and Mg-43.8Al-1.0Er alloys were both composed of Mg17Al12 matrix and Al3 Er phase, whereas Mg-43Al-3.0Er and Mg-42Al-5.0Er were composed of Mg17Al12 matrix, Al3 Er phase, and Mg-Mg17Al12 eutectic. The Mg-42Al-5.0Er alloy showed the highest microhardness, and the values remained nearly stable as Er content increased from 1.0 wt.% to 5.0 wt.%. The dispersed second phase Al3 Er caused the grain refinement of the Mg-Al-Er alloy, which was the main reason for the improvement in microhardness. The corrosion resistance of the Er-containing alloys initially increased and then decreased with increasing Er content. All the Er-containing alloys had the ability to suppress hydrogen evolution, which was the main reason for the higher corrosion resistance of the modified alloys than that of the Mg-44.3Al alloy. Considering the higher hardness and dispersity of the Al3 Er phase, Mg-43.8Al-1.0Er exhibited higher wear resistance than the as-cast Mg-44.3Al alloy.
In order to increase the depth or concentration of Ti ion implantation of pure iron, the surface mechanical attrition treatment(SMAT), which can fabricate a nanometer-grained surface layer without porosity and contamination in a pure iron plate, was used before ion implantation. Ti ion was implanted into the SMA treated sample and coarse-grained counterpart by using a metal vapor vacuum arc source implanter. The changing of depth and concentration of Ti was studied in a function of implantation time.By optical microscopy, transmission electron microscopy and X-ray diffraction, the grain size of the nano structured surface was studied. Micro-hardness, friction and wear behavior of nano surface layers were studied. By energy dispersive X-ray spectroscopy and Auger electron spectroscopy, the chemical composition and concentration of Ti ion in the surface implantation layer were studied. Experimental results showed that the concentration of Ti increased dramatically compared with untreated coarsegrained samples, which is attributed to the existence of higher density of defects(supersaturated vacancies, dislocations, non-equilibrium grain boundaries etc.) and compression stress field in the SMA treated nanocrystallined surface layer. The interaction between the defects and the implanted solute atoms leads to the increment of solid solubility. But the implantation depth showed inconspicuous change. It is shown that the ion range is just relevant to the energy and mass of the ion, dose of injection,the mass and density of target material.
Xu LiYanli AnYinghui WeiHuayun DuLifeng HouChunli GuoHongbo QuYide Wang
