Effects of high-pressure heat treatment on the solid-state phase transformation and microstructures of Cu_(61.13)Zn_(33.94)Al_(4.93) alloys

The phase transformation activation energy of the Cu61.13Zn33.94A14.93 alloys, which were treated at 4 GPa and 700 ℃ for 15 minutes, was calculated by means of differential scanning calorimetry curves obtained at various heating and cooling rates. Then, the effects of high-pressure heat treatments on the solid-state phase transformation and the microstructures of Cu61.13Zn33.94A14.93 alloys were investigated. The results show that high-pressure heat treatments can refine the grains and can change the preferred orientation from （111） to （200） of α phase. Compared with the as-cast alloy, the sample with high-pressure heat treatment has finer grains, lower β＇→β and/β→β＇ transformation temperature and activation energy. Furthermore, we found that high cooling rate favours the formation of fine needle-like α phase in the range of 5-20℃/min.

Strengthening iron enrichment and dephosphorization of high-phosphorus oolitic hematite using high-temperature pretreatment

The efficient development and utilization of high-phosphorus oolitic hematite is of great strategic significance for the sustainable supply of iron-ore resources in China.In this paper,the mechanism of high-temperature pretreatment for enhancing the effect of iron enrichment and dephosphorization in the magnetization roasting–leaching process was studied by X-ray diffraction(XRD),vibration sample magnetometer(VSM),scanning electron microscopy and energy dispersive spectrometry(SEM–EDS).Compared with the process without high-temperature pretreatment,the iron grade of the magnetic separation concentrate after high-temperature pretreatment had increased by 0.98%,iron recovery rate had increased by 1.33%,and the phosphorus content in the leached residue had decreased by 0.12%.High-temperature pretreatment resulted in the dehydration and decomposition of hydroxyapatite,the dehydration of limonite and the thermal decomposition of siderite,which can produce pores and cracks and weaken the compactness of the ore,improve the magnetization characteristics of roasted ore,and strengthen the iron enrichment and dephosphorization during the magnetization roasting and leaching process.

High-temperature Tensile Behavior of Laser Welded Ti-22Al-25Nb Joints at Different Temperatures

The high-temperature tensile behavior of laser welded Ti-22Al-25Nb (at%) joints was investigated at 500,650,800,and 1 000 ℃.The temperatures for tensile tests were selected according to the phase transformation sequence of Ti2AlNb-based alloys.At temperatures lower than the B2+O phase field (500 ℃) and higher than the B2+O phase field (1 000 ℃),the joints fracture in the base metal in ductile fracture mode.By contrast,the joints exhibit obvious high-temperature brittleness in the B2+O phase field (650 °C and 800 ℃).Heat treatments were conducted with respect to the thermal history of tensile specimens.Intergranular microcracks along the grain boundary of B2 phase are found in the fusion zone after the heat treatments at 650 ℃ and 800 ℃.The high-temperature brittleness at 650 ℃ and 800 ℃ is attributed to the B2→O transformation along the grain boundary.The stress concentration caused by the volume change of B2→O transformation also contributes to the high-temperature brittleness of laser welded Ti-22Al-25Nb joints.

Structural Stability of Natural Magnesiochromite at High-Temperature-Pressure Conditions

The podiform chromitites in the Luobusha ophiolite have been thought to experience a very deep formation,but the maximum depth is still an open issue.Here,we have investigated the structural stability of natural magnesiochromite using the synchrotron-based powder X-ray diffraction and diamond anvil cells up to 48.6 GPa and 2450 K.The results have shown that spinel-type magnesiochromite first decomposes into corundum-type‘Cr_(2)O_(3)’+B1-type‘MgO’at 11–14 GPa and 1250–1450 K,then modified ludwigite(mLd)-type‘Mg_(2)Cr_(2)O_(5)’+corundum-type‘Cr_(2)O_(3)’at 14.3–20.5 GPa and 1300–2000 K,and finally CaTi_(2)O_(4)-type phase at 24.5 GPa.During the quenching procession from high-temperature-pressure conditions,the mLd-type phase appeared again and was kept at ambient conditions.We also obtained the isothermal equation states of spinel-type and CaTi_(2)O_(4)-type phases,revealing the composition effect on their elasticities.Based on the updated results,we propose chromitites could not experience pressure exceeding∼14.3 GPa(approximate maximum depth∼400 km)in the subduction-recycling genesis model.

Phase Transformation in Al/Zn Multilayers during Mechanical Alloying

In this work,mechanical alloying of the alternating stacked pure Al and Zn thin foils was accomplished via high-pressure torsion(HPT).In the alloyed Al-Zn system,an exotic phase transformation from hexagonal close-packed(HCP)to face-centered cubic(FCC)was identified.The atomic-scale evolution process and underlying mechanism of phase transformation down to atomic scale are provided by molecular dynamics simulation and high-resolution transmission electron microscopy.The HCP→FCC phase transformation was attributed to the sliding of Shockley partial dislocations generated at the Al-Zn grain boundaries,which resulted in an[2110][011]and(0001)/(111)orientation relationship between the two phases.This work provides a new approach for the in-depth study of the solid phase transformation of Al-Zn alloys and also shed lights on understanding the mechanical properties of the HPT processed Al-Zn alloys.

MD Simulation of Structural and Mechanical Transformation of Single-Walled Carbon Nanotubes Under Pressure

We investigate the structural and mechanical properties of single-walled carbon nanotubes(SWNTs)under hydrostatic pressure,using constant-pressure molecular dynamics(MD)simulations.We observed that all the SWNTs,independent of their size and chirality,behave like a classical elastic ring exhibiting a buckling transition transforming their cross-sectional shape from a circle to an ellipse.The simulated critical transition pressure agrees well with the prediction from continuum mechanics theory,even for the smallest SWNT with a radius of 0.4nm.Accompanying the buckling shape transition,there is a mechanical hardness transition,upon which the radial moduli of the SWNTs decrease by two orders of magnitude.Further increase of pressure will eventually lead to a second transition from an elliptical to a peanut shape.The ratio of the second shape transition pressure over the first one is found to be very close to a constant of∼1.2,independent of the tube size and chirality.

Study on Phase-Formation Mechanism and Intergrowth for Bi(Pb)-Sr-Ca-Cu-O System

In this paper, the phase-formation mechanism of Bi-based superconductors was systematically investigated by using high-temperature X-ray diffraction, differential thermal analysis (DTA) and crystallization of amorphous state. The transformations among Bi-based 2201, 2212 and 2223 phases were observed, and the intergrown phenomenon of the 2201, 2212 and 2223 phases was explained. It was proposed that there exist a composition equilibrium between 2212 phase and Ca2CuO3 and a competition of thermody-namic stability among Ca2CuO3, 2212 and 2223 phases. This is why it is difficult to prepare the 2223 phase. After Pb was doped in Bi-Sr-Ca-Cu-O system, Ca2CuO3 phase in the Bi-Sr-Ca-Cu-O system was replaced by Ca2PbO4, so that the composition equilibrium and competition of thermodynamic stability mentioned above were avoided.