Effects Of Alloy Elements On Microstructure And Properties Of High Entropy Alloys

The conventional strategy for developing alloys is to select one or two elements as theprincipal components, and then to add other minor elements for improving microstructureand properties, such as iron-, copper-, and aluminum-based alloys. In2004, a novel conceptof high-entropy alloy was proposed by Yeh et al., which caused great research attention.The high-entropy alloy system includes five to thirteen principal elements at equimolar ornear-equimolar compositions. The atomic fractions of each element in high-entropy alloycannot be less than5%or more than35%, producing the high mixing entropy that preventsthe formation of intermetallic phases. Therefore, these alloys predominantly consist of amixture of simple solid solutions and have a favorable combination of compressionstrength and ductility. Many high-entropy alloys have high hardness, strength, wear andcorrosion resistance, as well as microstructure stability and anti-oxidation against heattreatment. It has been predicted that in the coming decades, three most potential researchfields would be bulk amorphous materials, composites and high-entropy alloys. In despiteof attractive mechanical and structural features, the low ductility and high brittleness ofhigh-entropy alloys seriously limit their potential applications in engineering, especially atroom temperature. Therefore, the study on improving the plasticity and mechanicalperformance is very important and meaningful. Moreover, the high-entropy alloy filmscould effectively protect the substrates due to the excellent properties of the high corrosionresistance, microstructure stability and anti-oxidation against heat treatment. Thus, thehigh-entropy alloy films also attract our attention.In order to solve the above problems, we make effort to improve the mechanical properties by taking advantage of the selection of the alloying elements and the dopingconcentration, and discuss the effects of different alloying elements on microstructure andmechanical properties of high-entropy alloy and films. In this thesis, FeCoCuNiSn_x,FeMnNiCuCoSn_x, FeNiCuMnTiSn_xhigh-entropy alloys and nitride FeCoNiCuVZrAl flimwere prepared. Their microstructure, mechanical properties, magnetic properties andcorrosion behavior were investigated. The main results are as follows:1) We got two high-entropy alloy systems with good plasticity and tensile strength,which were FeCoCuNiSn_xand FeNiCuMnTiSn_xsystems.The tensile testing of the two high-entropy alloy systems showed that the plasticity,which could reach19.8%and16.9%, were obviously better than other reportedhigh-entropy alloys.2) It was found that the content of Sn had great effects on microstructure, crystalstructure and tensile properties of high-entropy alloy systems.The FeCoCuNiSn_xalloys had good plasticity and high strength. The results showedthat the alloys had a single FCC solution when Sn content was less than0.05. The plasticityand strength of alloys increased with the increasing Sn content. When Sn content was morethan0.05, the tendency was opposite and a new Cu81Sn22phase appeared. BecauseCu81Sn22phase is hard and brittle, the tensile strain and strength of the alloys decreased.Considering the diversity of alloy component and metal material price, we preparedFeMnNiCuCoSn_xalloys by adding Mn element into FeCoCuNiSn_xalloy system. Theaddition of Mn increased the mixing entropy and reduced the cost. The results showed thatthe variation tendency of the plasticity and tensile strength of FeMnNiCuCoSn_xalloys withincreasing Sn contents was similar that of FeCoCuNiSn_xalloys. However, the maximumtensile strain and strength decreased to16.9%and476.9MPa, respectively.3) For FeNiCuMnTiSn_xalloy system, the variations of microstructure, crystalstructure and magnetic property of alloys had been discussed. FeNiCuMnTi alloy （x=0）,was composed of intermetallic compounds （Fe₂Ti, NiTi, FeTi and Fe₃Mn₇）. However,FeNiCuMnTiSn alloy （x=1） had a structure similar to TiNi₂Sn phase, which is zinc blendestructure. Moreover, the magnetic transformation underwent from the paramagnetism （x=0, 0.05,0.1） to superparamagnetism （x=0.5）, and finally to soft magnetism （x=1） at roomtemperature. We used computer simulation and found that as Sn content increased, moreTiNi2Sn were formed. When Fe atoms replaced Ni atoms as Ti₄（Ni₄Fe₄）Sn₄, the magnetismappeared.4) The formation mechanism of the amorphous FeCoNiCuVZrAl nitride films wasexplored, and the effects of N₂concentration on properties of the amorphous films werediscussed.The amorphous nitride films of FeCoNiCuVZrAl high-entropy alloys weresuccessfully deposited by using direct current magnetron sputtering. At different nitrogenflow ratios, all the deposited films had amorphous structure. And the film thicknessdecreased with the increasing nitrogen flow ratio. The relative concentrations of Fe, Co, Cu,V and Zr elements are substantially constant, compared to that of Al and Ni. Al atoms weremore like to be deposited at high N₂concentration. When N₂concentration reached30%, aperfect dense and smooth amorphous film was obtained, where the hardness and Young’modulus of the film reached the maximum values of12and166GPa, respectively. Theformation mechanism of the amorphous film had been briefly discussed at thethermodynamic point of view.In summary, we prepared a series of high-entropy alloy systems and films, andexplored the effects of alloying elements on the micostructure, crystal structure, mechanicalbehaviors and magnetic properties, which is important and meaningful for furtherdevelopment of high-entropy alloy.