Papers by Keyword: Fine Grain

Abstract: The hot deformation behavior and microstructure of a powder metallurgy (P/M) Ni-base superalloy with different original microstructure were studied by isothermal compression tests. The isothermal compression tests were conducted on Gleeble-3500D simulator with the temperature range of 1000°C~1100°C and the strain rate of 0.001s^-1~0.1s^-1. The results showed that the flow stress of the specimens with fine grains (10μm) and ultrafine grains (3μm) gained by hot extruding (HEX) were much less than the that with the average grain diameter of 30μm by hot isostatic pressing (HIP). At the strain rate of 0.001s^-1，the as-HIPed specimens with the average grain diameter of 30μm showed steady-state deformation at 1100°C only， whereas the as-HEXed specimens with the average grain diameter of 10μm and 3μm showed steady-state deformation both at 1050°C and 1100°C. The flow stress showed decreasing trend as the average grain diameter decreasing. The activation energy of hot deformation decreased form 622.79 kJ·mol^-1 to 302.36 kJ·mol^-1 as the average grain diameter decreased from 30μm to 3μm. When the as-HEXed specimen with the average grain diameter of 3μm was deformed at the condition of (1050°C, 0.001s^-1), the flow stress was lower than that at the condition of (1100°C,0.001s^-1), and the former also gained much finer and uniform grain, the later gained mixed grains.

Abstract: In this work, the conventional rolling (CR) and warm rolling (WR) have been carried out with an ultra purified ferritic stainless steel. After different rolling processes and subsequent recrystallization annealing, different recrystallized microstructure characteristics had been obtained. It was observed that as compared to the conventional process, the warm rolling and subsequent recrystallization annealing could obviously refine the recrystallized microstructure of the hot band. The ductile-to-brittle transition for the ultra purified ferritic stainless steel can be closely related to the occurrence of deformation twinning. The refined microstructure can decrease the temperature at which twinning occurs and, consequently, the ductile-brittle transition temperature for the ultra purified ferritic stainless steel was lowered to be below -40°C, and its notch toughness was significantly improved when the fine grain route can be applied.

Abstract: The good formability and corrosion resistance of 6N01 Al alloy allow it to be utilized in high-speed train systems, and weight reduction of railway vehicles is possible by improving the strength of this alloy. This study examined the effect of the fine-grained structure on the mechanical properties of the alloy formed by a combination of heat treatment and severe plastic deformation such as forging and rolling. The role of the fine-grained structure in determining the plastic formability was also investigated. The 0.2% proof stress and tensile strength of the heat-treated and multi-axial alternative forging (MAF) processed materials were both greater than 300 MPa. Subsequent cold rolling of these alloys increased both the 0.2% proof stress and tensile strength to over 450 MPa with a grain size of less than 1 μm. The fine-grained structure was confirmed to be effective in improving the strength of the 6N01 Al alloy.

Abstract: Cavitation erosion behavior of 17-4PH steel used for large turbine blades and laser surface melting layer were investigated in simulated seawater 3.5%NaCl solution through an ultrasonic vibration system. The microstructure and the eroded surface and cross-section morphology were observed by SEM and OM, then the results were compared and analyzed. The results showed that the microstructures of 17-4PH stainless steels consist of martensitic matrix, ferrite and residual austenite. Compared with 17-4PH substrate, it found the microstructure of laser surface melting treatment became dense and homogenous, the microhardness increased 80HV. During cavitation test, the cavitation incubation period of laser surface melting sample raised 1 time than 17-4PH steel, and enhanced its cavitation resistance through fine grain strengthening.

Abstract: In order to study the characteristics of fine grained polycrystalline metals, it is important to recognize the function of grain boundaries (GB), crystal defects such as dislocation and/or nanoscale voids, since the fraction of GB increases as grain sizes decreases, the deformation process of these metals could be different from those in larger size grains. In this study, we first evaluate the hypothesis that GB behaves as dislocation source and sink during the deformation of fine grained metal, then compare the behavior between GB and a tiny defect from the view point of dislocation source and sink phenomena. Since continuous dislocation supplies could be considered as the key issue to improve the toughness of fine grained metals, this concept could be helpful to design next generation polycrystalline metals.

Abstract: It had been already reported that the resistance of compression at warm condition can be decreased by the preliminary torsion working at AZ31B magnesium alloy. In the present study, it was found that the dynamic recrystallization occured during warm working by torsion. Dynamic recrystallization was slightly seen in the fractured edge of the bar at a rotation speed of 1rpm at temperature 573K and 623K. The amount of torsion to fracture was increased with increasing of deformation temperature. Remarkable dynamic recrystallization could be seen in the center of bar at the rotation speed of 1rpm at temperature of 673K.

Abstract: This paper investigates experimentally the edge chipping of a rock to assess the cutting performance of a conical and a pyramidal tip. It was found that the conical tip generates many radiated cracks and results in a larger amount of fine rock grains due to crashing. The size of chips produced by the pyramidal tip is bigger. It was concluded that the critical chipping energy has approximately a linear relation with h9/4 of which h is the depth of cut of the tip.

Abstract: Fine grained copper was studied using the stress relaxation technique and creep testing in nano-indentation, to determine the activation volume involved in the micro-mechanism of the deformation. This material exhibits a near-perfect elasto-plastic deformation, featured by a steep work-hardening, after the elastic domain, followed by flow at a constant stress. Measurements of the activation volumes in the various domains reveal the role of the dislocations and the variation in the dislocation density in the deformation mechanism. This emphasizes the importance, in the determination of the activation volume, of the deformation domain investigated as well as the testing technique used and whether in both cases, the measurement is carried out in a transient domain or condition where variation in dislocation density occurs.

Abstract: Experiments on surface features were carried out in honing zirconia engineering ceramics using fine grains. In ultrasonic honing, the surface fracture ratio increased quickly when the depth of cut is more than 3 μm, while in common honing it performed an ascending trend from depth of cut 1.5 μm. The value of surface roughness in ultrasonic honing increased along with the depth of cut; while it descends contrast to the depth of cut when the honing velocity is less than 0.58 m/s in traditional honing. As for honing velocity over 0.74 m/s, it deceased firstly to the minimal value for depth of cut 1μm, then began to ascend along with depth of cut. The value of surface roughness descended to minimum value for the range of honing velocity 0.41-0.58 m/s in traditional honing, while the optimized honing velocity ranged 0.58-0.75 m/s in ultrasonic machining. The surface roughness in ultrasonic honing was superior to that in traditional honing.

Abstract: TiAl alloys are powerful candidates for light weight high temperature structural materials because of their excellent high temperature strength, low density and good oxidation resistance. Unfortunately, TiAl alloys are difficult to machine and hot working due to ordered structure, which impede large-scale application. Synthesis and superplastic deep drawing of a TiAl alloy were studied. Mechanically synthesized fine crystalline Ti/Al composite powders as precursor of TiAl alloy. The TiAl alloy with fine grain via reactive sintering was obtained. The deformation behavior under biaxial stress by means of deep drawing was achieved. The optimum temperature is 1100°C≤T≤1150°C. The microstructures corresponding to deep drawing in the part were studied.