Papers by Keyword: Mechanical Alloying (MA)

Abstract: The Ti₅₀Fe₂₂Ni₂₂Sn₆ amorphous powder was prepared by mechanical alloying with a high-energy planetary ball mill. Crystallization kinetics of the milled amorphous powders was investigated by DSC. Thermal analysis showed that when the heating rate increasing from 10K/min to 40K/min, the super-cooled liquid region of milled amorphous alloy increased from 93K to 110K. On the basis of the obtained DSC curves, the characteristic temperatures of the amorphous powders could be measured. The equations of Kissinger and Ozawa were used to calculate the crystallization activation energies of the milled amorphous alloy for comparing. The activation energy for the glass transition E_g had the maximum values of 650KJ/mol and 629KJ/mol calculated from Kissinger and Ozawa, respectively. In addition, the second crystallization process had a higher activation energy value comparing with the others crystallization events.

Abstract: This paper introduces the theoretical model of mechanical alloying. In view of the varied angle in the actual impact process, the Gauss distribution of angle is drawn into the model based on the angle factor. The distribution of temperature rise is obtained and fitted. It accords well with the Gauss distribution in some extent. The results show the thermal effect of powder depends on milling technology, the impact angle and others. Compared with the previous models, the presented model is well in consistent with the fact.

Abstract: A new ternary Ti-based amorphous alloy, Ti₅₀Fe₂₅Ni₂₅, is prepared by the mechanical alloying. The milling is performed in a high-energy planetary ball mill under argon atmosphere. Fully Ti₅₀Fe₂₅Ni₂₅ amorphous alloy powder is obtained after milled 160h. The milling speed is 300rpm and the weighs ratio of ball to powder is 10:1. The structural features are studied by X-ray diffraction and field emission scanning electron microscope, and the thermal stability is investigated by a differential scanning calorimeter. The super-cooled liquid region of the amorphous alloy increases from 98K to 119K as the heating rate increasing from 10K/min to 40K/min. The effective activation energy of crystallization is estimated with modified Kissinger’s plot. The initial crystallization activation energy E_x1 and the first crystallization peak E_p1 are 155.9KJ/mol and 188.5KJ/mol, respectively.

Abstract: The composite materials with a nominal composition of Cr₂Nb-24wt.%Ti were fabricated by mechanical alloying followed by hot pressing. The microstructures and properties were investigated on the composites contained with Laves phase prepared through 20 hours mechanical alloying of chromium, niobium and titanium elemental powders and hot pressing at 1250°C for half an hour. The results indicate that the near full-dense Laves phase Cr₂(Nb,Ti) based alloy with homogeneous composition and microstructure is obtained by mechanical alloying and hot pressing techniques. The dispersed soft second phase Nb solid solutions with the fine grain size less than 1μm are distributed uniformly on the matrix. The sample has a relative density of 99%, fracture toughness of 5.32MPa•m^1/2 and compress strength of 2080MPa. Due to the effect of fine-grain and alloying addition, the toughening of the Cr₂(Nb,Ti) based alloy has been fully realized.

Abstract: The nanocrystalline Cu-5wt.%Cr alloy powders were prepared by mechanical alloying. The structural changes were characterized by X-ray diffraction (XRD) technique. A thermodynamic analysis was carried out to predict the change in the solubility limit of this system. It was found that the energy resulting from the MA process is sufficient to increase the solid solubility of immiscible Cr-Cu system. The solid solubility may be extended up to 5 wt.% Cr in Cu after 20 h milling. The formation of the supersaturated solid solution leads to the decrease of Cu lattice parameter. However, it decomposes with the further increase of the milling time, which leads to the increase of Cu lattice parameter.

Abstract: The FeAl/Al₂O₃ composites were fabricated by hot-pressing process in this research. The Fe-Al intermetallics compounds powders were fabricated by mechanical alloying and heat treatment. The FeAl powders and Al₂O₃ powders were mixed and the FeAl/Al₂O₃ composite powders were prepared. The FeAl/Al₂O₃ composites bulks were fabricated by hot-pressing process at 1300°C for 2h under the pressure of 35MPa. The phase composition and microstructure of the FeAl intermetallics compounds powders produced by mechanical alloying and heat treatment were investigated. The phase composition and microstructure of the FeAl/Al₂O₃ composites produced by hot-pressing process were investigated. The XRD patterns results showed that the Fe-Al intermetallics compounds powders were fabricated by mechanical alloying for 60h. The FeAl intermetallics compounds powders were fabricated by heat treatment at 800°C, 900°C and 1000°C. The microstructure showed that the mean particles size of the FeAl intermetallics compounds powders produced by mechanical alloying and heat treatment was rather fine and about 4-5μm. The XRD patterns results showed that there existed FeAl phase and Al₂O₃ phase in sintered composites. The FeAl/Al₂O₃ composites bulks exhibited the homogenous and compact microstructure. The mean particles size of FeAl was about 4-5μm and the mean particles size of Al₂O₃ was about 4-5μm. The microstructure of the FeAl/Al₂O₃ composites became more homogenous and compact with the increase of FeAl content.

Abstract: Amorphization and crystallization behaviors of Ti55Al45 powders during mechanical alloys (MA) and subsequent Spark Plasma Sintering are studied. It is found that the nanocrystallization process of the Ti-Al alloy proceeds and the sintering temperature can control the microstructure of alloy. The sintering of the compacts is carried out at the temperatures of 1100—1200°C with a compaction pressure of 30MPa and a heating rate of 30°C min-1. Specimens with high densities and approaching the equilibrium state can be obtained in short time of 180s by spark sintering than conventional sintering. Such shorter high temperature is important to prevent grain growth. The microstructures of the alloy contains equiaxed gamma TiAl with sub-micron grain size and small amount alpha Ti3Al phase.

Abstract: The possibility of synthesizing nanocrystalline (W, Ti)C is investigated by spark plasma sintering of mechanically alloyed elemental mixed powders of W, Ti and C. The results show that wet-milling generates WxTi1-x instead of directly forming (W, Ti)C, and makes the graphite C transform into amorphous phase surrounding the formed WxTi1-x. Meanwhile, the grain size of the 70 h milled elemental mixed powders reaches about 50 nm under the wet-milling condition. Subsequently, nanocrystalline (W, Ti)C is successfully synthesized by spark plasma sintering of the 70 h milled elemental mixed powders. The formation of (W, Ti)C phase by SPS from the WxTi1-x and amorphous C generated by MA is simply explained based on our proposed model.

Abstract: The amorphization and crystallization of mechanical alloyed Cu₅₀Zr₄₀Ag10 alloy have been investigated using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results demonstrate that a full amorphous phase of Cu₅₀Zr₄₀Ag₁₀ can be obtained through mechanical alloying. The amorphous phase begins to show the initial mechanical crystallization when the milling time is 108 h and subsequently the main milling product is still amorphous structure with increasing milling time up to 208 h. Therefore, the amorphous alloy exhibits the excellent mechanical stability during mechanical alloying. The nearest-neighbor distance of atoms firstly increases then reduces with the increasing milling time, indicating that there is a closely correlation between the initial crystallization behavior and short range ordering.

Abstract: The Fe₃Al/Al₂O₃ composites were fabricated by hot-pressing process at 1300°C for 2h under the pressure of 35MPa, by using the Fe₃Al intermetallics compounds powders fabricated by mechanical alloying and heat treatment. The phase composition and microstructure of the Fe₃Al intermetallics compounds powders and Fe₃Al/Al₂O₃ composites were investigated. The XRD patterns results showed that the Fe-Al intermetallics compounds powders were prepared by mechanical alloying for 60h and heat treatment process at 800°C and 1000°C. The XRD patterns results showed that there existed Fe₃Al phase and Al₂O₃ phase in sintered composites. The Fe₃Al/Al₂O₃ composites exhibited homogenous and compact microstructure, the Fe₃Al particles were homogenously distributed in Al₂O₃ matrix. The mean particles size of Fe₃Al was about 3-4μm and the mean particles size of Al₂O₃ matrix was about 4-5μm.