Study On Microstructure And Mechanical Properties Of Co-free CrFeNiNb_x Multi-principal Alloy

Multi-principal alloys,also known as high-entropy alloys,which are consisted of multiple elements in equal or nearly equal atomic ratios,have attracted widespread attention in recent years.In this paper,through alloying Nb in the single-phase Co-free face-centered cubic(FCC)Cr Fe Ni multi-principal alloy,the structure and properties of the CrFeNiNb_xmulti-principal alloys with the change of Nb content were systematically studied.At the same time,the effect of removing Co on the eutectic structure of the alloys was studied.And a new design idea of eutectic multi-principal alloy based on the difference of element content between two phases in eutectic multi-principal alloy was proposed.After adding Nb element to the alloy,the structure becomes a FCC+Laves two-phase dendrite-interdendritic structure,and the Nb element segregated in the Laves phase.The volume fraction of Laves phase increased,when more Nb element was added.At the same time,the structure changed from dendrite-dendritic structure to hypoeutectic structure,complete eutectic structure and hypereutectic structure.Furthermore,the yield strength of the alloy also increased,while the plasticity slightly reduced.Compared with the multi-principal alloy containing Co,the content of Nb in the FCC phase of the CrFeNiNb_xmulti-principal alloy was lower,indicating that Co element can promote the solid solution of Nb in the FCC phase.Through a special alloy design method,the CrFeNiNb_0.31fully eutectic multi-principal alloy was designed.Compared with the Co CrFeNiNb_0.45eutectic multi-principal alloy,the yield strength of this alloy decreased while the plasticity increased.During the eutectic reaction,the width of the FCC layer increased while the width of the Laves layer unchanged.Finally,the FCC phase volume fraction increased.When the CrFeNiNb_0.31eutectic multi-principal alloy was annealed,the?phase extension phase boundary precipitated during annealing at 750-900?,but the?phase was not founded when annealing at 1100?.At the same time,the discontinuous grain boundary coarsening of the alloy occured in the temperature range of 600?-900?while the mismatch and fracture increased in the alloy,and the eutectic cell boundary was coarsened.When the temperature reached 1100?,the structural change turned into intragranular continuous coarsening leading to spheroidization of the Laves phase,and the influence of the Hall-Petch effect reduced.Finally,the plasticity of CrFeNiNb_0.31eutectic multi-principal alloy decreased significantly after annealing at 750-900?,while the yield strength decreased and the plasticity increased after annealing at 1100?.The addition of Co element is beneficial to the phase stability and structural stability of the eutectic multi-principal alloy.