Structural characterization of nanocrystalline and amorphous tungsten(molybdenum)-iron(manganese) alloys prepared by mechanical alloying.

摘要

Pure Fe and W powders, and the powder mixtures of W{dollar}sb{lcub}rm x{rcub}{dollar}Fe{dollar}sb{lcub}rm 1-x{rcub}{dollar} (0.20 {dollar}le{dollar} x {dollar}le{dollar} 0.75), W{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}50{rcub},{dollar} Mo{dollar}sb{lcub}50{rcub}{dollar}Fe{dollar}sb{lcub}50{rcub}{dollar} and Mo{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}50{rcub}{dollar} were milled in a SPEX high energy mixer/mill under an argon atmosphere for different periods of time. Various parameters of the milling process, such as milling time, milling atmosphere, ball-to-powder weight ratio, and the composition were studied for the W-Fe system. X-ray diffraction was employed to characterize the microstructure of the milled powders, and electron microscopy was used to determine the particle morphology.; It was found that for pure Fe, only a nanocrystalline phase was formed after prolonged milling time (up to 100h), while for W{dollar}sb{lcub}rm x{rcub}{dollar}Fe{dollar}sb{lcub}rm 1-x{rcub}{dollar} (x {dollar}ge{dollar} 0.5), within 24h, an amorphous phase of W-Fe was formed in addition to the nanocrystalline W phase. Similar results were also observed for W{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}50{rcub}{dollar}, Mo{dollar}sb{lcub}50{rcub}{dollar}Fe{dollar}sb{lcub}50{rcub}{dollar} and Mo{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}50{rcub}{dollar} alloys.; The structure of the nanocrystalline phase of the milled powders was evaluated from the broadening of the powder pattern peaks using the Warren-Averbach method and integral-breadth method to determine the particle size and microstrain. After prolonged milling time ({dollar}ge{dollar}20h), the particle size {dollar}langle{dollar}D{dollar}rangle{dollar} and microstrain {dollar}langle(epsilonsb{lcub}rm L=50{rcub})sp2ranglesp{lcub}1/2{rcub}{dollar} of W in the alloys reached limiting values of 50A({dollar}pm{dollar}5A) and 0.6%({dollar}pm{dollar}0.1%), respectively.; The total interference function I(K) and the reduced atomic distribution function G{dollar}sb{lcub}rm I{rcub}{dollar}(r) of the amorphous W{dollar}sb{lcub}50{rcub}{dollar}Fe{dollar}sb{lcub}50{rcub}{dollar} alloy were obtained from the X-ray diffraction pattern. Both functions were dominated by the W-W and W-Fe atom pairs. The first maximum of G{dollar}sb{lcub}rm I{rcub}{dollar}(r) was found to be at r{dollar}sb1{dollar} = 2.74A, close to the interatomic distance in crystalline W.; To study the effect of isomorphous substitution of Mo for W and Mn for Fe, W{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}50{rcub},{dollar} Mo{dollar}sb{lcub}50{rcub}{dollar}Fe{dollar}sb{lcub}50{rcub}{dollar} and Mo{dollar}sb{lcub}50{rcub}{dollar}Mn{dollar}sb{lcub}50{rcub}{dollar} alloys were also investigated. Comparisons among these four alloys showed that the structures were similar, indicating that Mo and Mn can be used as isomorphs for W and Fe, respectively.; W{dollar}sb{lcub}50{rcub}{dollar}Fe{dollar}sb{lcub}50{rcub}{dollar} powders, mechanically alloyed for different periods of time, corresponding to predominantly nanocrystalline or amorphous structure, were compacted at room temperature and pressures up to 1720 MPa (250 ksi). A maximum density of {dollar}sim{dollar}70% of the theoretical value was obtained for a compact which was 11 mm in diameter and 1.3 mm thick. Electron microscopy was used to investigate the effect of ball milling on particle morphology and interparticle bonding. After 100h of milling, the size of the powder particles was reduced from 12{dollar}mu{dollar}m for W and 5{dollar}mu{dollar}m for Fe to an average size of {dollar}sim{dollar}1{dollar}mu{dollar}m.