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a b s t r a c tThere is increasing interest in both relating the mechanical behavior of high-entropy alloys to their microstructural evolution and in their development for various applications. A special two-day international workshop on the above topic was held in Guiyang, China, in December 2014. The workshop gathered scientists and engineers to exchange information on recent progress in high-entropy alloys, to discuss the scientific issues and challenges to foster international collaborations, and to identify future directions. In this paper, a summary of this workshop is presented, including aspects of definition/terminology, phase formation, microstructure and phase stability, strengthening mechanisms, and hightemperature properties. Future research directions are also outlined. SynopsisThe continued development of high-performance materials is required for increased energy efficiency and sustainability. Traditionally, metallic alloys have been widely applied as structural materials. The conventional alloy design strategy usually is to start with one element as the principal constituent and adding other minor elements for the further optimization of properties and performances. After extensive efforts on these traditional alloys, the development of traditional alloys is approaching its limits. Recently, a new alloy design concept was proposed: the idea is to simultaneously alloy several principal elements (usually !5) to obtain high mixing entropy. As a result, many novel concentrated multicomponent alloys are in the process of being developed [1e6]. In a timely response to this rapid development, a two-day international workshop was held in Guiyang, PRC in December 2014 to exchange information on recent progress, foster international collaborations, and identify future research directions. The main theme of the workshop was the relationship between microstructures and mechanical properties. In this paper, a summary of this special workshop is presented, which includes; 1) Definition/terminology, 2) Main characteristics resulting from multiple primary constituents, 3) Atomic size effects on phase formation, 4) Microstructure and phase stability, 5) Strengthening mechanisms, 6) High-temperature mechanical properties, and 7) Future directions for research Definitions/terminologyAlthough "high-entropy alloys (HEAs)" have now become a new class of materials, the name has not been clearly defined since its inception. This often causes confusion in the scientific community.

Micropillars with diameters of 1 and 3.8 m were fabricated from Mg-based metallic glasses using focus ion beam, and then tested in compression at strain rates ranging from 6 ϫ 10 −5 to 6 ϫ 10 −1 s −1. The apparent yield strength of the micropillars is 1342-1580 MPa, or 60%-100% increment over the bulk specimens. This strength increase can be rationalized using the Weibull statistics for brittle materials, and the Weibull modulus of the Mg-based metallic glasses is estimated to be about 35. Preliminary results indicated that the number of shear bands increased with the sample size and strain rates.

Plastic deformation of Pd-40Ni-20P bulk metallic glass (BMG) was investigated by instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibited numerous serrations or pop-ins, but these serrations became less prominent as the indentation rate was increased. Using the tip velocity during pop-in as a gauge of serration activity, we found that serrated flow is only significant at indentation strain rates below about 1-10/s. This result suggests a transition in plastic flow behavior at high strain rates, in agreement with prior studies of BMGs under different modes of loading.

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