Designing Rules Of High Entropy Alloys And CoCrFeNiPd_xMn_y Eutectic High Entropy Alloys

High entropy alloys(HEAs)or multi-principal element alloys have emerged as a novel type of alloys in recent years.They possess excellent mechanical properties at high temperature or low temperature,outstanding tribological property,magnetic property and so on.Therefore,HEAs have become potential candidates as structural and functional materials.Even though HEAs have been intensively studied,there are no effective empirical parameters for designing HEAs with single solid-solution phase(SSP),two phases or amorphous phase up to now.Designing of HEAs are still based on a large number of tedious experiments.Moreover,HEAs are usually fabricated by casting.Because of their multi-principal elements,poor fluidity and casting performance,macro-segregation is severely in the casting ingots.As is well-known,eutectic alloys have good fluidity and excellent mechanical performance at high temperature.Besides eutectic alloys as a natural in-situ composites could possess simultaneously high strength and ductility by adjusting the constituent phases.In this sense,Lu et al.proposed the concept of eutectic high entropy alloys(EHEAs),aiming to prepare large EHEA ingots with high strength and ductility and putting them into industrial manufacture.In this thesis,efficient parameters for designing HEAs with different kinds of phases were proposed.Using the existing strategies to design EHEAs,two EHEA systems i.e.,CoCrFeNiMnPdx(x=0.2-2.0)and CoCrFeNiPdMnx(x=0.2-0.8),were prepared.Our main conclusions are as follows.(1)The standard deviation of dimensionless enthalpy of mixing was introduced to characterize the chemical bond mismatch,and both the important roles of average properties and their standard deviations were studied.Compared with the parameters?r,?d,?,?,?Hmix and ?,the square root of the standard deviation of dimensionless enthalpy of mixing(?)is able to separate roughly the three regions of HEAs(i.e.,the region of the HEAs with a single SSP,the HEAs with multi-phases and the HEAs with an amorphous phase)and thus is a useful parameter for designing of HEAs.One of the necessary conditions for the formation of HEAs with a single SSP is(?)according to the collection of HEAs of Gao et al.All the previous empirical parameters that describe solely the atom size difference,both the atom size difference and the chemical bond mismatch and solely the chemical bond mismatch fail to predict the stability of a single SSP in HEAs.(2)The ? parameter including both the average and extremum properties of constituent elements was proposed thermodynamically by analyzing the competitions between the formation of a SSP and ordering and between the formation of a SSP and segregation.Compared with the previous ? and ? parameters,the current ?parameter can separate reasonably the HEAs with a SSP from the HEAs with multi-phases and with an amorphous phase,even when the most up to date experimentally reported HEAs were collected.Even when both the effects of atomic size difference and chemical bonds are integrated,the three regions of HEAs cannot be separated reasonably if mere the average properties(e.g.,the average mixing enthalpy)are considered.The important roles of significant deviations of individual properties from the average properties therefore should be highlighted in designing HEAs.The process dependent phase-selection plays also an important role in designing HEAs with desired microstructures and properties.(3)A series of CoCrFeNiMnPdx(x=0.2-2)EHEAs were prepared by adding Pd to the most-widely studied CoCrFeNiMn HEA with a single FCC phase.Through a series of experimental characterization,the microstructures were analyzed and the eutectic phases were identified.With the increase of the composition of Pd,the FCC phase changes successively from(CoCrFeNiMn)-rich,(CoCrFeNi)-rich to(CoCrFeNiPd)-rich.The MnxPdy phase is Mn2Pd3 for the CoCrFeNiMnPd0.2 and CoCrFeNiMnPd0.6 EHEAs,Mn7Pd9 for the CoCrFeNiMnPd1.0,CoCrFeNiMnPd1.4 and CoCrFeNiMnPd2.0 EHEAs,and Mn3Pd5 for the CoCrFeNiMnPd1.8 EHEA.The unique seaweed eutectic-dendritic solidification pattern,the coarse granular eutectics around the fine lamellar eutectics and the single eutectic microstructure within a wide composition range of Pd from x=0.6 to 2.0 indicate that solidification of the present EHEAs as a result of the sluggish diffusion effect is under the local non-equilibrium condition.(4)The CoCrFeNiPdMnx(x=0,0.2,0.4,0.6,0.8)HEAs were prepared and characterized.With the increase of Mn addition,the microstructures of CoCrFeNiPdMnx HEAs change from dendrites to divorced eutectics,to hypoeutectic microstructures and finally to eutectic dendrites.For the CoCrFeNiPdMn0.2 and CoCrFeNiPdMn0.4(CoCrFeNiPdMn0.6 and CoCrFeNiPdMn0.8)EHEA,the FCC phase is a CoCrFeNiPd-rich(CoCrFeNi-rich)phase and the MnxPdy intermetallic compound is Mn3Pd5(Mn7Pd9).Addition of Mn might influence the interface energy anisotropy of both the FCC/liquid and MnxPdy/liquid interfaces,thus forming the seaweed eutectic dendrites in the Mn0.8 EHEA.With the increase of Mn addition,the hardness of FCC phase increases firstly and then decreases.For the Mn3Pd5 and Mn7Pd9 intermetallic compounds,a decrease of either spacing of primary twins or secondary twins would increase the hardness.(5)A multimodal grain size distribution,i.e.,from several hundred microns for the eutectic-dendrite,several microns for the coarse granular eutectics,several hundred nanometers for the fine lamellar eutectics and the lath-shaped microstructures in intermetallic compounds,to several nanometers for the nano-twins,was found,indicating a possible excellent combination of strength and ductility.For example,with the increase of Mn addition in the CoCrFeNiPdMnx EHEAs,the yielding strength holds constantly at about 650 MPa.The fracture strain(strength)decreases from about 50%(2.4 GPa)for the Mn0.2 EHEA to about 35%(1.9 GPa)for the Mn0.8 EHEA.Anyway,the CoCrFeNiPdMnx EHEAs have good strength and ductility.