Abstract
In this thesis, we investigate the thermodynamic and kinetic behaviors of high-entropy alloys via mechanical alloying. At first, a series of Cu-Ni-Al-Co-Cr-Fe-Ti-Mo alloys from binary to octonary one were prepared to examine the structure evolution during mechanical alloying process. Results reveal the alloying competition between elements during mechanical alloying. That is, not all the elements are alloyed at the same time but in a specific sequence. By examining the XRD patterns and EDS mapping images of the binary to octonary alloys the alloying sequence of this octonary alloy system is determined as Al → Cu → Co → Ni → Fe → Ti → Cr → Mo. This alloying sequence is also confirmed by another series of alloys with inverse element adding sequence. The alloying rate correlates best with the melting point among metallurgical factors. The mechanism for this correlation is explained through the effect of melting point on solid-state diffusion rate and mechanical disintegration which are both critical for the final alloying. As for the phase evolution, these alloys prepared by mechanical alloying also form BCC and FCC solid solution phases as that prepared by melting route. However, the binary and ternary alloys still remain crystalline structure even after milling for 60 h. The quaternary to octonary alloys finally transformed into amorphous structure after sufficient milling times. Only the octonary one has residual Mo after milling for 60 h due to its high melting point. The amorphization of these alloys is belong to type I as classified by Weeber and Bakker. The reasons include the increased number of elements, atomic size difference, and the continued lattice distortion imposed by milling. The amorphous quaternary to septenary alloy powders were further examined by thermal analysis and with annealing to observe the phase transformation during heating. It shows that recovery begins at 100°C, crystallization occurs in the range of 250 to 280°C, new phase forms or grain growth occurs at even higher temperatures. Simple phases are found to exist in the equilibrium state. This confirms again that high entropy effect enhances the formation of solid solution phases. However, glass transition temperature as that found in bulk amorphous alloys is not observed for the present amorphous alloys. Egami’s criterion for amorphization based on topological instability concept can also be applied to high-entropy alloys to explain the amorphization tendency. However, even larger size difference as that required by Inoue’s rule is still crucial to the existence of glass transition temperature. Furthermore, two equimolar alloys entirely composed of HCP elements were also prepared by mechanical alloying to investigate if they are also easy to form solid solution phases. However, no crystalline solid solutions and compounds form before full amorphization. The amorphization processes of these two alloys thus conform to type II. The inhibition of intermetallic compounds before amorphization is due to chemical compatibility among the constituent elements in company with high entropy effect and deformation effect which enhance the mutual solubility. Direct formation of the amorphous solid solution phase instead of the crystalline one attributes to their large range of atomic size.