Papers by Author: Li Jian Rong

Abstract: The effect of V contents (0, 0.03, 0.08 and 0.14%) on microstructure and mechanical properties was investigated in a Fe-Cr-Ni-Mo high-strength steel. Optical microscope (OM), electron back scattered diffraction (EBSD), transmission electron microscopy (TEM) and electron probe micro-analyzer (EPMA) were used to observe the microstructure varying in this paper. The results show that the prior austenite grain was refined obviously as increasing V content. Simultaneously, the amount of acicular ferrite (AF) increased and the size of that decreased. In addition, the size of carbide gradually decreased with increasing V content, and there appeared M₂C, M₆C and MC types of carbide after addition of V in the steel, while it is mainly M₂₃C₆ in the 0V steel (M is any combination of Cr, Mo, V or Mn). The strength and elongation increased gradually as increasing V content, meanwhile the impact toughness decreased gradually. The excellent combination of mechanical properties can be obtained in the steel with about 0.03% V content.

Abstract: Tempering temperature dependence of the amount of the reversed austenite in the range of 570 oC to 680 oC was investigated by means of X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) in a low carbon Fe-13%Cr-4%Ni-Mo (wt.%) martensitic stainless steel. It was found that the reversed austenite began to form at the tempered temperature slightly above the As temperature. As the tempered temperature increased, the amount of the reversed austenite changed little in the temperature range of 580-595 oC. Then, the amount of the reversed austenite increased sharply with the increased tempered temperature. When the tempered temperature increased to about 620 oC, the amount of the reversed austenite exhibited a peak. Afterward, it decreased quickly at the elevated tempered temperature. The microstructural evolvement of the reversed austenite at different tempering temperature was also observed by TEM.

Abstract: One internal friction peak associated with the B2↔B19’ transformation appears on the cooling curve of porous NiTi shape memory alloy and the dense NiTi alloy shows the maximum peak. The tan δ value increased with the increasing of strain amplitude and the decreasing of frequency. Tan δ value of porous alloy mainly comes from the energy absorbing of the matrix at the small strain amplitude, however, if the strain amplitude is large, the tan δ value comes from the energy consumption that overcomes the friction between folds and the plastic contribution.

Abstract: Porous Ti51Ni(49-x)Mox (x=0, 0.7, 1.0, 1.2) shape memory alloys were successfully fabricated by the self-propagating high-temperature synthesis (SHS) method. The effect of Mo content on microstructures, transformation characteristics and compressive properties of porous TiNiMo alloys was investigated systemically. It has been found that Mo doping into porous TiNi alloys will induce R phase transformation. A small amount of Mo addition (0.7at.%) improves compressive properties of porous TiNiMo alloy due to Mo solution strengthening and the obvious ductile fracture is observed on the fracture photography. However, the compressive strength and compressive strain of porous TiNiMo alloys with excessive Mo content decrease sharply and the failure manner turned into brittle fracture mode, which results from a large amount of Ti2Ni and Ti4Ni2O phases precipitated at grain boundary. Porous Ti51Ni48.3Mo0.7 alloy with suitable transformation temperature and high compressive strength is very promising for use as biomaterial and damping material.

Abstract: The corrosion behavior of cold worked Al-Mg-Sc-Zr-Ni alloys prepared by vacuum induction melting in acidic chloride solution was studied. The morphological characteristics of the corroded specimens were examined by OM(optical microscopy), SEM
and EDX techniques. The results indicated that the intergranular and exfoliation corrosion susceptibility dramatically depended on the Ni content. The Al-Mg-Sc-Zr alloy with 0 and 0.5 wt.% Ni were lightly susceptible to intergranular corrosion as the precipitation of Mg2Al3 phases presented at grain boundaries and the Al3Ni particles were finely dispersive. The intergranular corrosion was enhanced by the Al3Ni particles enrichment and became pitting corrosion with increasing Ni to 1.0 wt.%. Finally, the exfoliation corrosion happened to the alloy with 2.0wt.% Ni. This trend correlated well with the electrochemical property and distribution of Al3Ni phases. The corrosion potential of Al3Ni intermetallic phase is nobler than the β phase and the matrix, which result in an appearance of galvanic coupling. In addition, the increase of Al3Ni particles enlarged the attack area and the inhomogeneous segments of Al3Ni and Al3Mg2 phases accelerated the localized corrosion.

Abstract: In this paper, a new technique for fabricating grating on the surface of porous TiNi SMA is proposed. The grating is directly written onto the surface of the specimen using the FIB milling. No photoresist is required during the lithography process. From the experimental results, it can be obviously seen that the grating fabricated by FIB milling has a better quality than that by electron beam lithography. Using the self-made FIB grating, the in-plane deformation of the porous TiNi SMA in microscale is studied by SEM moiré method. Moiré and the microscopical structures are synchronously observed, including microcracks, martensites and grain boundaries.

Abstract: In order to improve the performance of pipe couplings, a technique has been developed to electroplate copper on the inner surface of Ni-Ti-Nb pipe-joints after pre-deformation. Gas leak tightness and tensile tests were conducted to evaluate the performance of the couplings both with and without copper coating. It was found that electroplating a thin copper layer on the inner surface of the pipe-joint effectively solved the problem of leakage and greatly enhanced the pull-out force of the couplings. Furthermore, the pipe-joints without ribs can be demounted easily after being cooled to the liquid nitrogen temperature and benefit the maintenance of the pipeline.

Abstract: A ternary alloying element Niobium addition to the Ni-Ti shape memory alloy could optimize transformation temperatures, mechanical properties. In this paper, with the help of ambient tensile test and scan electron microscope observation, the influences of Nb-content on the mechanical properties of (Ni47Ti44)100-xNbx alloys were investigated in detail. The results indicated that both the fracture strength (sb) and the elongation (d) decreased gradually with the increasing of the Nb content. The fracture surfaces showed the fracture character of the alloys was clearly gliding fracture, which is typically transgranular fracture mode.