Papers by Keyword: Grain Refining

Abstract: In Cr-rich CoCrFeMnNi alloys, the precipitation of the σ phase at grain boundaries during recrystallization is so fast that ultrafine-grained structure is formed due to the pinning effect of the precipitates. The average grain size of the fcc parent phase is found to be consistent with modified Zener-Smith model. If conventional alloys come to equilibrium, volume fraction of precipitates should approach a saturation value. However, it is interesting to note that the volume fraction of the σ phase in Cr-rich CoCrFeMnNi alloys is inversely proportional to the average grain size of the fcc parent phase. For instance, in Co₂₀Cr₂₅Fe₂₀Ni₁₅Mn₂₀ alloys, the volume fraction changes from 6.5% to 1.2% with increasing average gran size from 14 μm to 210 μm even after annealing at 1273 K for 100 h. It is well known that heterogeneous nucleation of precipitates at grain boundary is energetically favorable and fast diffusion through grain boundary can assist the precipitation. However, they cannot account for the grain size dependence of the volume fraction after reaching equilibrium. Based on stereology, the reciprocal of grain size is proportional to grain boundary area. Thus, chemical fluctuation at grain boundaries (e.g. segregation) is considered to be related to the unusual precipitation at the grain boundaries.

Abstract: In Al_xCoCrFeNi high entropy alloys (x = 0.3–0.5), the NiAl phase with the B2 structure is precipitated rapidly along the fcc grain boundaries. During recrystallization after conventional cold rolling, the NiAl precipitates effectively suppress the grain growth, which results in the ultrafine-grained microstructure. It should be noted that no severe plastic deformation is necessary to obtain the microstructure. The volume fraction of the NiAl precipitates increases with increasing x. As a result, the average grain size of the fcc matrix (d_m) after the recrystallization decreases with increasing x, and therefore, a minimum d_m of 0.5 μm can be obtained at x = 0.5. The grain refinement by the NiAl precipitates is consistent with the Zener-Smith model. At x = 0.5, the alloy with d_m = 0.5 μm exhibits a yield stress of 1163 MPa and an elongation of 24% at room temperature.

Abstract: In this paper, alumina ceramics were prepared with alumina powder and lanthanum nitrate. The influence of La₂O₃ on the microstructure and properties of alumina ceramics were studied. The result showed that the addition of La₂O₃ contributed to the mechanical property of the samples. The alumina ceramics with adding of 0.05wt% La₂O₃ sintered at 1490°C had the best mechanical properties. And the flexural strength, fracture toughness of the ceramics could reach 571.902MPa and 5.82MPa•m^1/2，which were improved by 37.55% and 10.65% respectively compared with the alumina ceramics without La₂O₃. Besides, the average grain size of alumina ceramic is about 2.8μm, and the effect of inhibition on grain growth of alumina ceramics was obvious.

Abstract: Al–Si A413 treated and untreated alloys were cast and poured at approximately 720 oC, 700 oC, and 680 oC in a cooling slope to obtain the semisolid material feeding the ceramic nozzle (150 cm³) at the lower roll (single-roll melt-dragged processing)—this drags the metallic slurry via the chill/columnar layers at a rate of 0.2 m/s, forming a molten-metal strip with a thickness of 2 mm and width 45 mm, approximately. The untreated alloy poured at 720 oC formed coarse structures of α-Al dendrites, as well as a coarse eutectic of Al–Si and microshrinking on the surface of the casting strip facing the atmosphere. The Al–Si 413 alloy poured at 680 oC and treated with Al5Ti1B (0.1%) led to microstructural refinement, resulting in α-Al globular structures, the absence of microporosities on the surface facing the atmosphere, and a finer and more homogeneous distribution of the eutectic grains with smaller Si particles. The AlTiB master alloys are not used as a grain refiner in Al–Si alloys because of Si poisoning. This subject is discussed in this paper. The addition of the inoculant and 0.2% of the Al–Si eutectic morphology modifying agent (Al–10%Sr) refined both the α-Al and eutectic phases more efficiently in the cast strip poured at 700 oC and 680 oC. This suggests that the inoculant did not interfere with the action of the modifying agent. As a result, molten metal strips of higher mechanical strengths and ductilities were obtained.

Abstract: Semisolid process can be used as grain refining method to replace conventional chemical grain refining approach. Typical semisolid slurry making techniques were introduced in the paper as potential semisolid grain refining methods. The temperature and morphology control for grain refining through semisolid process were enhanced to achieve fine particle size and large particle number rather than spheroidal particle shape or low viscosity of the slurry comparing with conventional rheo-forming process. The grain refinement effect was demonstrated by a 40 mm cylinder casting of AZ31 alloy.

Abstract: The interaction between Zr addition and intensive melt shearing on grain refinement of commercial purity Mg has been investigated experimentally. It was found that, without intensive melt shearing, the grain structure of Mg is changed from columnar grains to equiaxed grains with the increase in Zr concentration. However, with intensive melt shearing, the grain structure of Mg undergoes a complex change as a function of Zr concentration; for instance, the grain structure showed equiaxed grains at 0.1wt. % Zr concentration, while it was changed to columnar grains at 1wt. % Zr concentration. It was noted that, particularly, under the intensive melt shearing condition, the grain size of Mg with minor addition of Zr (0.1%) was further decreased to 134 ± 4 μm compared with that of Mg without Zr addition (217 ± 15 μm).

Abstract: Results of experiments, focused on development of modified AZ61 alloy structure after hot forming, are summarized in this paper. Microstructure changes of the alloy after rolling and forging at temperatures of 380°C and after ECAP processed at 250°C were observed. After forging, the original grain size was reduced almost 14 times down to a value around 9 μm. After ECAP the grain size reduction was even better, almost 60 times. Important role of the ß-phase (Mg₁₇Al₁₂) in this alloy, during plastic forming, was confirmed. Independently of processing technology, the best final properties of the AZ61 alloy were supported with very fine particles distributed in the Mg matrix. The experiment succeeded in proving the influence of precipitates on restoration processes too.

Abstract: The effect of intensive melt shearing and Zr concentration on grain refinement of Mg‑0.5Ca-xZr alloy has been investigated experimentally. It was found that without intensive melt shearing, the grain size of Mg-0.5Ca alloy decreased from 290 µm to 76 µm when the Zr concentration is varied between 0 and 1 wt.%. However, intensive melt shearing makes the grain size of the same alloys first decrease and then increase with increasing Zr addition. The minimum grain size (85 µm) was achieved at 0.4 wt.% Zr.

Abstract: With orthogonal tests the effects of grain refiners and modifiers, Al-10Sr, rare earth (RE) and Al-Ti-5B, on the mechanical properties of Al-Si alloy for automobile cylinder blocks were studied. The best result occurred when the proportion of refiners and modifiers was Al-10Sr = 0.1wt%, RE = 0.3wt%, Al-5Ti-B = 0.8wt%. With SEM and EDS the as-cast microstructures of materials after grain refining and modification were observed and analyzed. Tiny branch-shaped AlSi phase, thin-striped AlCu phase, new small bone-shaped AlSiMn phase, new leaf-shaped or feathered B(AlCuCe) phase as well as the phases of other metal compounds made the as-cast microstructures more complicated and more compact after grain refining and modification. Some brittle phases reduced in size obviously and distributed in the grain boundaries of plastic α-phase in network, which acted as intercrystalline strengthening to certain degree. During the tensile deformation these brittle phases could reduce the stress concentration, impeded dislocation movement, and then strengthened the alloy matrix. In addition after grain refining and modification the porosity was significantly reduced.

Abstract: The microstructures have been investigated in steel plates with different rolling processes to find the reasons of yield ratio overseted standard in few high strength building steel plates produced in Echeng Iron and Steel Corporation. The effects of rolling path, rolling path depress ratio on microstructure and properties. Some reasons of yield ratio overseted standard have been analyzed. The results show that decreasing rolling paths and increasing depress ratio refined grains but unchanged area ratio and interlamellar spacing of pearlite. Yield strength excessively increased but tensile strength unchanged increased yield ratio. So, excellent properties can be obtained by applying the optimization process.