Effect Of Microstructures On The Deformation Behaviors Of Al_0.1CoCrFeNi Metastable High-entropy Alloy

Combination of high yield strength,high plasticity and high fatigue limit is a long-standing goal in structural materials.Face-centered cubic(fcc)medium-entropy and high-entropy alloys(MEAs and HEAs)usually exhibit high tensile strength and high plasticity.Their fracture toughness is also at a high level.However,due to the absence of the obstacles to dislocation slip,their yield strength is generally lower than 400 MPa at room temperature,which is cannot satisfy the requirements of the structural materials in modern industrial applications.Traditional hardening methods,such as solid-solution strengthening,dislocation strengthening,grain-boundary strengthening,and precipitation strengthening,vastly increase the yield strength of fcc MEAs and HEAs but often accompanied with a sacrifice of plasticity.Inspired by the traditional transformation induced plasticity(TRIP)structural materials,combined with the recent first-principles studies on the mechanical-metastability of fcc MEAs and HEAs at room and low temperatures,we investigated the mechanical-metastability and plastic deformation behaviors of fcc Al0.1CoCrFeNi HEA under room-temperature tension.We found a fcc-to-hexagonal closed-packed(fcc?hcp)and hcp-to-fcc(hcp?fcc)martensitic transformation among fcc MEAs and HEAs,which proves that both the fcc and hcp structures are metastable at room temperature.The main results of the thesis are summarized as follows:Upon tensing,martensitic transformation occurs in the as-cast Al0.1CoCrFeNi HEA.At the onset of deformation,a fcc?hcp transformation occurs.The nanoscale hcp martensite is formed at the boundaries of the thick deformation twins.With increasing the strain to 16.2%,the volume fraction of the hcp martensite approaches the maximum of 16.7%.Further straining causes a hcp?fcc transformation,and decreases the volume fraction of the hcp martensite(only 1.48%aftter fracture)and the thickness of the hcp martensite.This martensitic transformation is important to strain-harden the as-cast Al0.1CoCrFeNi HEA.Grain-refinement severely inhibits the martensitic transforming and twinning in Al0.1CoCrFeNi HEA.Compared with the as-cast HEA,grain-refinement in the 1073 K-annealed HEA increases the average twin spacing,and reduces the twin thickness.The reduction of the twin thickness degrades the local stress concentration at the twin boundaries,suppressing the fcc?hcp transformation.In the second regime of strain-hardening process,the strain hardening rate of the 1073 K annealed HEA decreases with straining,while that of the as-cast HEA increases,contributing more uniform elongation in the as-cast HEA than in the 1073 K-annealed HEAThe correlation between cold-rolling,low-and high-temperature annealing,microstructure and deformation behaviors is established.The yield strength versus average grain sizes of the Al0.1CoCrFeNi HEA follows the Hall-Petch relationship,and exhibits a high lattice resistance and a Hall-Petch coefficient.The cold-rolled and 673 K-annealed HEAs have high strength.The deformation of these two HEAs are dominated by dynamic recovery process.Thus,the strain-hardening rate rapidly decreases to near zero with straining,and the plasticity is very low(<2%).With refining the grain size,the fcc?hcp martensitic transformation and twinning in the as-cast,1273 and 1073 K-annealed HEAs are gradually inhibited,leading to that,in the second regime of strain-hardening process,the strain-hardening rate of the as-cast HEA increases,and those of the 1073 and 1273 K-annealed HEAs decrease(the rate of reduction in the 1273 K-annealed HEA is much smaller than that of the 1073 K-annealed HEA).We developed a methodology to design and fabricate bulk mechanically-metastable HEAs with a complex heterogeneous structure through cold working,followed by intermediate-temperature-annealing.Our heterogeneity-designed 873 K-annealed HEA displayed a unique combination of high strength and high ductility(yield strength is?711 MPa,tensile strength?928 MPa,and uniform elongation?30.3%),which exceeded,to our knowledge,strength-ductility properties reported on HEAs so far.Our results demonstrated that heterogeneous structures can be effectively introduced in mechanically-metastable HEAs,which have been proven to be difficult to replicate synthetically previously.The method of cold rolling and then annealing at intermediate temperatures applied in this study can be used to synthesize many heterogeneous-structured metastable HEAs as well as metastable conventional alloys,and may be easily adapted to the current industrial process and,hence,has the great potential for large-scale applications.We hope that the results of this thesis would be helpful to understand the mechanical-stability and martensitic transformation,and to in-deeply understand the deformation mechanisms and strain-hardening behaviors of fcc MEAs and HEAs at room and low temperatures,and therefore,for the development of TRIP HEAs and conventional alloys with outstanding combination of high strength and high ductility.