Papers by Keyword: Microstructure

Abstract: The microstructure, tensile and stress rupture properties of K492Malloy have been investigated in the present study. The results revealed that γ matrix, γ′ phase, carbide and eutectic in the interdendritic region within grain interior and along grain boundary were observed after solidification. After heat treatment, γ' precipitates with two obviously distinct size existed in the dendrite core and interdendritic region, respectively. Meanwhile, the chain-like (W, Mo)₆C and Cr₂₃C₆ carbides precipitated along grain boundary. The investigation of mechanical properties suggested that the tensile strength was initially increased but then decreased with increasing the temperature from 25oC to 760oC. The stress rupture life was 68.2h and 35.8h at 760oC / 655MPa and 870oC / 365MP, respectively. The columnar grain and carbide along grain boundary resulted in intergranular brittle fracture in both test conditions. As a result, the elongation under the conditions of 760oC/655MPa and 870oC/365MP was 1.5% and 1.4%, respectively.

Abstract: Two experimental single crystal superalloys, the Ru-free alloy and the Ru-containing alloy with [001] orientation, were prepared in a directionally solidified furnace, while other alloying element contents were kept unchanged. The effects of Ru on the microstructure and tensile property at 25°C, 760°C, 980°C and 1100°C of the single crystal superalloy were investigated. The results show that the size of γ′ particles became uniform, and the cubic shape of the particles turned a little regular after Ru addition. The yield and ultimate tensile strengths of the single crystal superalloy increased with variant degree at different temperatures. The fracture elongation and area deduction decreased at the testing temperatures of 25°C and 760°C, but those increased at 980°C and 1100°C. The tensile fracture mechanism of the single crystal superalloy at different temperature did not change. Finally the relationship between the improvement of tensile strength and Ru addition was discussed.

Abstract: The creep behavior of a Re-free single crystal Ni-base superalloy at high temperature has been investigated by means of creep property measurement and microstructure observation. The experimental results show that the creep life of the alloy under the testing condition of 1040 oC/137 MPa was measured to be 556h. In the primary stage of creep, the deformation mechanism of the alloy is dislocations slipping within the γ matrix, and the γ′ phase in the alloy is transformed into the rafted structure along the direction vertical to the stress axis. In the latter stage of creep, the alternate activation of the main/secondary slipping dislocations may twist the rafted γ′ phase to promote the initiation and propagation of the cracks along the interfaces of γ′/γ phases up to the creep fracture. And the various configurations of the rafted γ′ phase appeared in the different regions of the sample, thereinto, the coarsening and twisting of the rafted γ′ phase in the regions near the fracture may be attributed to the bigger plastic deformation.

Abstract: The deformation and damage behaviors of a 4.5% Re nickel-based single crystal superalloy during creep at medium temperature were conducted by means of creep properties measurement and microstructure observation in the present investigation. The results show that the creep life of the superalloy was measured to be 226h at 820MPa/820^oC, displaying a better creep resistance. After the alloy crept up to fracture, the γ′ phase in the region near fracture was contorted and coarsened, which is attributed to the severe plastic deformation. In the latter stage of creep, the main/secondary slipping systems were alternately activated to twist the γ′ and γ phases, which promoted the initiation of the cracks along the interfaces of γ′ /γ phases. And the micro-cracks may be propagated both along the direction vertical to the stress axis and along the direction of slipping traces, which is thought to be the main deformation and fracture mechanisms of the alloy in the latter stage of the creep.

Abstract: The effect of V content on the mechanical properties and microstructure of DZ417G alloy was studied in the present investigation. DZ417G alloy with different element V contents was smelted , then tested the mechanical properties and observed microstructure. The results show that with the increase of element V content, the morphology of rod type M₂₃C₆ carbides turned into block-type MC carbides, the size of carbides became bigger, and the amount of carbides decrased, but the dispersion degree of carbide in the matrix was increased. The alloy tensile strength and plastic properties were increased at high temperature with the increment of V content. There were finer dimple structures on the tensile rupture fracture surface of the alloy with high element V than the alloy with low element V under the high temperature conditions. The results of creep rupture life tests show that with increasing V content the creep rupture life increased at the test condition of 760 °C, 725MPa. The creep rupture life is 90h of the alloy with 0.62% V element content, and the creep rupture life increased to more than 245h when the element V content increased to 0.96%.

Abstract: Multiphase steels were obtained by using Gleeble-1500 simulator and TMCP, and were characterized by optical microscopy, SEM, TEM, EBSD (electron back-scattered diffraction) and other tests to investigate its microstructure and mechanical properties. During the simulation, the deformation temperature is 850°C, and the steels are air cooled to 750-600°C and then quenched to room temperature. The results indicate that the microstructure of the specimen is composed of ferrite and bainite. With the lowering of quenching temperature, the proportion of ferrite increases and the proportion of bainite decreases, and the bainite laths is shorter. The fine (Nb, Ti) C particles and dislocations appear in ferrite and lath bainite, and the amount of high angle grain boundary decrease after the initial increasing. The microaolloyed hot-rolled multiphase steel plate was developed by two-stage rolling, subsequently quenching to room temperature or air cooling to 600°C, then quenching to room temperature. Two typical microstructures: acicular ferrite and ferrite-bainite multiphase were obtained. The ferrite-bainite multiphase steel showed better mechanical properties, and the yield strength, tensile strength, yield ratio, uniform elongation and percentage elongation were 488Mpa, 845Mpa, 0.58, 10.3% and 21% respectively. The refinement of bainite structures, fine (Nb, Ti) C particles and the dislocations in bainite increase the strength.

Abstract: A new-type of Cr12MoVNbRE die steel was designed and investigated. The phase transition temperature was measured by dilatometer. Effects of austenizing temperature and time on the microstructure and mechanical properties of the Cr12MoVNbRE die steel were studied. The results show that the hardness decreases with the increasing austenitizing time. As for the austenitizing temperature, it increases firstly, and then decreases when the austenitizing temperature increases. The most suitable austenitizing temperature and time are 1010 °C and 20 min respectively. The high hardness of the Cr12MoVNbRE die steel is mainly attributed to the martensite with high alloy content and fine uniformly dispersed carbides.

Abstract: The carburizing behavior of low-carbon Cr-Ni-Mo, the nitriding behavior of C-Cr-W bearing steel and the carburizing process prior to nitriding of low-carbon Cr-Ni-Mo bearing steel were mainly investigated in this paper through the contrastive analysis of microstructures and properties by employing scanning electron microscope (SEM), transmission electron microscope (TEM) and X-Ray Diffraction XRD, etc. The result indicated that compared to the simple carburizing and nitriding process, the material that experienced the duplex surface finishing process could obtain a good infiltrated layer’s microstructure and excellent mechanical properties. The thickness of infiltrated layer was 1.8mm. The hardness of surface could reach 1100HV, and the infiltrated layer had a reasonable hardness gradient distribution. Meanwhile the test materials through duplex surface finishing process exhibited good friction properties in the aspect of point contact in dry friction.

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: Mg-2Zn-0.3Gd sheets processed by large strain hot rolling with one pass of 80% reduction at 200°C and 250°C were selected to investigate the rolling temperature effect on the microstructure, texture and mechanical properties of Mg-2Zn-0.3Gd sheets after rolling and subsequent annealing. It was found that the rolling temperatures in the present study seemed to have no obvious effect on the microstructure of Mg-2Zn-0.3Gd sheets during large strain hot rolling process. High density of shear bands and numerous intersected twins but free of DRX grains were observed in the microstructure of both sheets. The Mg-2Zn-0.3Gd sheets showed non-basal textures with peaks tilting to TD after annealing. While the peak intensity of (0002) pole figure increased as the rolling temperature decreasing. Tensile testing results revealed that the Mg-2Zn-0.3Gd sheets rolled at both temperature displayed high room temperature ductility about 40% after annealing, which is due to the existence of non-basal texture.