Design,microstructure And Properties Of CoNiFe-base Ternary Medium Entropy Alloy With Face Centered Cubic

Multi-principal component alloys break through the traditional alloy design concept and have excellent mechanical and thermal electromagnetic properties,which have attracted considerable attention.However,at present,the types of multi-principal component alloys are limited and lack of related theories.Most of them,especially the high entropy alloys,have poor plastic workability,which greatly constrains the industrial applications of multi-principal component alloys.In this thesis,the FCC based CoNiFe-base multi-principal component alloys were designed and prepared,based on the thermodynamics(using Thermo-Calc software)combined with the first principles calculations.The effects of the thermomechanical treatments on the CoNiFe-based multi-principle component alloys were then systematically inspected in terms of the microstructural evolution,mechanical properties and corrosion resistance.In the meantime,grain boundary engineering was employed to control the fraction of special grain boundaries in order to improve the corrosion resistance.On this basis,severe plastic deformation was then utilized to prepare the CoNiFe Mo0.2V0.5 medium entropy alloy wires,of which the effect of plastic deformation on the microstructures,textures and mechanical properties was investigated.Additionaly,two eutectic medium entropy alloys,CoNiFe Zr0.3and Fe Co Ni Nb0.45,with dual-phase structures were designed by using JMat Pro thermodynamic software,and how the eutectic microstructure influences the mechanical properties particularly the elevated temperature properties were explored.And finally,a CoNiFe-base medium entropy alloy strengthened through precipitation strengthening mechanism was designed using Thermo-Calc thermodynamic software,the precipitation kinetics and its effect on the mechanical properties were investigated,which could provide some guidance on the efficient design of multi-principle component alloys with excellent mechanical properties.The main results are as follows:During multiple forging process,a large number of deformed microstructures were produced in CoNiFe-based medium entropy alloy,resulting in high strength but low ducility.Upon annealing 900°C,the ductility and good strain hardening behavior were recovered along with high strength,for the reason of the synergistic effect of massive amount of annealing twins and deformation bands.Based on the idea of grain boundary characteristic distribution optimization(grain boundary engineering),the corrosion resistance of cold rolled CoNiFe medium entropy alloy was improved by adjusting the heat treatment temperature and time,through which the fraction of special grain boundaries was increased.It is found that with the increase of heat treatment temperature and heat treatment time,the proportion of random high angle grain boundary decreases,while that of low angle grain boundary increases obviously,especially the low ?CSL grain boundary(3 ??? 29).The ?3n type grain boundary plays an important role in the special grain boundary.With the growth of the fraction of low ?CSL grain boundary,the self corrosion potential of alloy increases,and the self corrosion current density drops,indicating the enhanced corrosion resistance.The microstructure and mechanical properties of CoNiFe Mo0.2V0.5 wires vary a lot when prepared by different methods.The cyclic loading-unloading experiments of the cold rolled CoNiFe Mo0.2V0.5wires show that with the number of cycic loading-unloading increasing,the density of {001} < 110 >texture and ultimate tensile strength increase,whilst the ductility declines.In addition,the loading modulus and average hysteresis modulus also slightly decreases with the loading-unlading process,whilst the unloading modulus raises.From the tensile tests of CoNiFe Mo0.2V0.5 wires with different drawing deformations,it is found that with the increment of drawing deformation,the density of the deformation texture B {112} < 111 > enhances dramatically,and dislocation density increases almost linearly,which leads to the distinct improvement of tensile strength,but reduced plasticity.The above-mentioned results imply that severe plastic deformation is able to raise the strength of the medium entropy alloys.The CoNiFe Nb0.45 eutectic medium entropy alloy with eutectic microstructure exhibits excellent mechanical properties at both room temperature and elevated temperatures.At 700°C,the eutectic alloy still maintains high compressive strength(1.2 GPa)with remarkable ductility(40%).On the other hand,the alloy shows good corrosion resistance in 3.5 wt% Na Cl at room temperature,and the corrosion rate is only 0.003 mm/y.Based on CoNiFe medium entropy alloy,a high strength precipitation strengthened Fe43.4Co25.9Ni25.8Ti4.9 medium entropy alloy was designed by Genetic algorithm in combination with thermodynamic calculations.Upon ageing in either FCC+Ni3Ti two-phase field(high temperature ageing treatemnt)or BCC+FCC+Ni3Ti three-phase field(intermediate temperature ageing treatment),the alloy exhibits obvious precipitation hardening response,which stems from the precipitation of Ni3 Ti.It is interesting that the alloy experiences eutetoid transformation during ageing within BCC+FCC+Ni3Ti three-phase field,and the eutectoid structure consists of BCC and Ni3 Ti lamellae,similar with the pearlitic structure in steels.The lamellar structure nucleates on the prior FCC grain boundaries and grows into FCC grains.Such a pearlitic structure plays a key role in improving the hardness of the medium entropy alloy.