Jingguo WANG
,
Guoliang CHEN
,
and Zuqing SUN (State Key Lab. for Advanced Metal Materials
,
University of Science and Technology Beijing
,
Beijing
,
100083
,
China)Hengqiang YE (Lab. of Atomic Imaging of Solids
,
Institute of Metal Research
,
Academia Sinica
,
Sh
材料科学技术（英文）
The microstructures and phases of ternary TiAl+Nb alloys containing 50-60 at.-%Al, 0-21 at.-%Nb have been studied using transmission electron microscopy (TEM) and X-ray powder diffraction. The phases present in the alloys and their distribution were found to be a sensitive function of composition. The phase relations between γ-TiAl and γ1 (a new ordered ternary intermetallic compound based on γ-TiAl) were determined. Essentially single γ phase was determined for alloys with relativety low Nb content (≤10 at.-%Nb). the γ1 phase was determined to exist in the composition range containing higher Nb contents (15-21 at.-%Nb). Between γ and γ1 phases, with intermediate Nb contents there is a transitional phase γ1 (a superstructure of γ-TiAl). As for the influence of Al concentration on the phase relations. the γ1 phase was inclined to form in the alloys with relatively high Al contents. The ordering transformation of γ, to γ1 is a continuous ordering process and the transition may be second order.
关键词:
Guo HE
,
Zan BIAN
,
Guoliang CHEN
,
Jian LU
,
Dianjing CHEN
,
Guochao TU
,
Guojun CHEN
,
Xiaojun HU
材料科学技术（英文）
Cylindrical and sheet samples of bulk metallic glassy with a nominal composition of Zr52.5N14.6Al10CU17.9Ti5 (at. pct) were prepared by melt injection casting. The crystalline precipitates formed during the casting were studied by metallographic observations and selected-area electron diffractions. The effect of crystalline precipitates on the mechanical properties were investigated by tensile and compressive tests at room temperature. Oxygen contents and the sample sizes (or cooling rates) strongly affect the formation of the crystalline precipitates. Overheating the alloy melt up to 200 K above its melting temperature can effectively prevent the formation of the crystalline precipitates to get fully glass samples with diameters up to 2 mm for cylinders and thickness up to 1 mm for sheets even the oxygen content is as higher as 0.08 wt pct. With increasing the sample sizes, the crystalline precipitates increase in volume fraction and size. The formation of the precipitates experienced two stages, i.e., initially nucleation and isotropic growth, and then anisotropic growth, finally forming faceted morphologies. Fully glassy Zr52.5Ni14.6Al10Cu17.9Ti5 alloy exhibits excellent tensile and compressive properties at room temperature. The presence of crystalline precipitates significantly decreases the tensile and compressive properties. With increasing the crystalline precipitates, the area of vein patterns on the fracture surface decreases, but the fracture steps increase, and the fracture mode changes from ductile to brittle resulting from the larger stress concentration caused by the larger sizes and faceted shapes of the crystalline precipitates.
关键词:
Guo HE
,
Zan BIAN
,
Guoliang CHEN
材料科学技术（英文）
Bulk metallic glass Zr52.5Ni14.6Al10Cu17.9Ti5 was prepared by melt injection casting method. Its glass transition and crystallization temperatures were determined by differential scanning calorimetry (DSC) to be 631 K and 710 K respectively. By analysis of X-ray diffractometry (XRD) and transmission electron microscopy (TEM), the predominant crystallized phase of Zr2Ni0.67O0.33 distributed an glass state matrix was detected after annealing at 673 K for 600 s. The transformation to Zr2Ni0.67O0.33 and a small amount of ZrAl and Zr2Cu took place after annealing for 600 s at temperature from 703 K to 723 K. With increasing annealing temperature from 753 K to 823 K, the amounts of ZrAl and Zr2Cu increased, but the size of the crystals did not significantly change. The transformation to Zr2Ni0.67O0.33 is interface-controlled, but is diffusion-controlled to Zr2Cu and ZrAl. With increasing annealing temperature up to 200 K above T-x, the nanometer grains became very fine because of the increase of nucleation rate for Zr2Cu and ZrAl.
关键词:
Timothee Nsongoa
,
Guoliang CHEN
,
Xiaodong NI
材料科学技术（英文）
The embedded atom method was used to theoretically analyze the effect of the lattice parameter variation on the order-disorder transformation in binary alloys. The simple model was considered in which the configuration energy E as function of the lattice parameter a and the long-range order parameter sigma was given by Taylor expansion at sigma =0. The following results were found for AB alloy, at T=0 K, the stable state exists in the completely ordered phase which was also found in the case of A(3)B or AB(3) compounds The order-disorder was found to be a second-order transition. Only one kind of order-disorder was found for AB alloy. Three groups of order-disorder transformation can be observed for the A(3)B or AB(3) compound. For the group I when the temperature is below the critical temperature, the order-disorder is a first order transformation. For the group II, the order-disorder transformation is a first order transition. For the group III, the order-disorder transformation is found to be a second order transition. The lattice parameter variations have a significant effect on E-2 coefficient, which is related to the ordering energy. These results are, in a good agreement with those obtained by using EAM (embedded atom method) and CVM (cluster variation method), calculations.
关键词:
