A Strategy for Designing Dual Phase High- or Medium-entropy Alloys Utilizing Spinodal Decomposition

Abstract

In the history of materials science, a wide variety of alloy designs have attempted to achieve metallic materials having high strength and light weight. For this purpose, the design of dual phase (DP) alloys and solid solution strengthened alloys has been conducted extensively in steel, titanium, and aluminum alloys, providing expanded material-design windows from single phase and low-alloy materials. Martensitic transformation, precipitation, and spinodal decomposition have been the most widely used strategies for obtaining DP in alloys. Recently, high-entropy alloys (HEAs) containing numerous principal elements of equiatomic or near-equiatomic ratio with a simple single phase, were proposed to achieve maximum solid solution strengthening effect. This concept is attractive for designing high-strength alloys because various alloying elements can be added to the alloys to produce a high solid solution strengthening effect. In addition to the solid solution strengthening, various strategies applied in conventional DP alloy design have been adopted in the HEA community (e.g., precipitation strengthened HEA, eutectic HEA, and martensitic DP HEA) to improve the mechanical properties of simple HEAs. Many of scientists and engineers have reported the numerous DP HEAs, however, most of the alloys cannot be even characterized as HEAs and their properties were not comparable with conventional alloys. This is resulted from the low solubility between alloying elements in multicomponent alloys. In consequence, only a very limited combination of HEAs has been studied and the alloy design strategy of HEAS has not been well established yet. The aims of this thesis consist of three specific goals. First goal of this thesis is a proposal of a new strategy for designing dual phase HEAs or medium-entropy alloys (MEAs) with excellent tensile properties. The design concept utilizing spinodal decomposition results in new alloys “spinodal decomposition MEAs (SDMEAs)”. Alx(CuFeMn)100-x (x = 0, 7.5, and 15 at%) alloys were developed by utilizing the immiscible nature of Cu-Fe alloys. The microstructures of the alloys show phase separation into Cu-rich and Fe-rich regions, and the addition of Al transforms the crystal structure from dual face-centered cubic to face-centered cubic and body-centered cubic. Spinodal-type decomposition of the microstructure enables further dissolution of Al into the matrix. The alloys exhibit high strength because of solid solution strengthening and interface strengthening caused by the dual medium-entropy phases. The presence of essential partially recrystallized microstructures also enhances the strength of the alloys. This new type of SDMEAs is expected to expand the design window in physical metallurgy. Second goal of this thesis is to understand microstructure, texture evolution, and mechanical properties of the developed SDMEA. The microstructure evolution of the alloy with different annealing conditions shows different recrystallization kinetics and grain growth behaviors of each phase. Typical cold-rolling and recrystallization textures of face-centered cubic and body-centered cubic alloys were observed in the alloy during the annealing heat treatment. The mechanical properties of the alloy annealed at higher temperature exhibit higher strength due to the distinct microstructure evolution, i.e. higher fraction of the strong Cu-rich BCC phase, in this alloy. This unusual strength enhancement of the alloy convinces us that the mechanical properties of the alloy can be controlled by thermomechanical processing to obtain desired microstructures and mechanical properties. Final goal of this thesis is to study further strengthening of the designed SDMEAs at cryogenic temperature. The designed SDMEAs were strengthened at 77 K by deformation twinning and deformation-induced phase transformation. Their mechanical properties and microstructure evolution were investigated. With increasing Al content in the alloys, the strengthening mechanism at 77 K varied from deformation twinning to deformation-induced phase transformation. When the Al content is 15 at%, the metastable Cu-rich β phase transformed into β1’ martensite during deformation at 77 K. The β1’ martensite plays an important role in the strengthening of the SDMEA.