Metals have been mankind's most essential materials for thousands of years; however, their use is affected by ecological and economical concerns. Alloys with higher strength and ductility could alleviate some of these concerns by reducing weight and improving energy efficiency. However, most metallurgical mechanisms for increasing strength lead to ductility loss, an effect referred to as the strength-ductility trade-off. Here we present a metastability-engineering strategy in which we design nanostructured, bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally proposed to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits: interface hardening due to a dual-phase microstructure (resulting from reduced thermal stability of the high-temperature phase); and transformation-induced hardening (resulting from the reduced mechanical stability of the room-temperature phase). This combines the best of two worlds: extensive hardening due to the decreased phase stability known from advanced steels and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), these two contributions lead respectively to enhanced trans-grain and inter-grain slip resistance, and hence, increased strength. Moreover, the increased strain hardening capacity that is enabled by dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase produces increased ductility. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide design in the near-infinite compositional space of high-entropy alloys.

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Hydrogen embrittlement (HE) is a well-acknowledged phenomenon in metals. It draws much scientists'' and engineers'' attention because it can lead to catastrophic failure of an industrial structure.Researchers use different methods to charge a test piece with hydrogen (H) and study the behavior of the material during or after charging. A common method is to use H 2 gas as the environment, for example Vehoff [1,2] used gaseous H to study the H effect on the fracture behavior of ferritic steels. More examples can be found in Ref. [3][4][5][6][7][8][9][10][11][12]. Furthermore, electrochemical cathodic charging is also commonly used, for example Wang et al. [13] used cathodic charging with an in-situ tensile test to study the effect of H on mobile dislocations, and the H concentrations were also determined. Other examples can be found in Ref. [14][15][16][17][18][19][20][21][22] as well.There is also a smart method for special materials, for example, Deng et al. [23,24] and Rogne et al. [25] used water vapor to charge Fe-Al intermetallic alloys since the H 2 O molecule can react with Al, producing alumina and H to invade the matrix.Due to the high diffusivity of H in body-centered cubic (BCC) lattice [13], it is always critical to have H pre-charging on the test piece because H can diffuse out even during a short transferring procedure.Therefore, in-situ H-charging is favored for the HE study of BCC structures.Currently, in-situ charging of ferritic steel is either done in high pressure hydrogen gas or under electrochemical control by cathodic H-charging. This makes the high-resolution observation of the sample during deformation challenging.Environmental scanning electron microscopy (ESEM) combined with miniaturized tensile stages provides a unique possibility to test the effect of H on mechanical deformation, however the limited maximum pressure inside the chamber and restrictions on the possible gases at that pressure in the chamber from the manufacturer hinders the HE tests inside the ESEM. At low pressures, the fugacity of H is not enough to have adequate physisorption and dissociation of the H 2 molecules on the surface to observe HE within a reasonable time scale. Converting H 2 gas to H plasma before letting it enter the ESEM chamber will increase the H fugacity in the chamber and provide possibilities to observe the HE processes inside the ESEM.Narita [26] used H glow charging to study the embrittlement effect in a Fe-Si system with a tensile test on single crystals and concluded that this method is capable of charging the samples with enough H to reveal an embrittlement effect without severe surface damage. Kimura and Birnbaum [27] observed that that H-plasma charging can cause a softening effect in the flow stress of pure iron samples. A recent work from Malitckii et al. [28] used H-rich plasma at elevated temperatures to charge two kinds of steels and successfully revealed the HE effect on them.

Effect of hydrogen on nanomechanical properties in Fe 22Mn-0.6C TWIP steel revealed by in-situ electrochemical nanoindentation, Acta Materialia,