| Multi-principal element alloys(MPEAs)or high entropy alloys(HEAs),due to their good high-temperature stability and radiation resistance,have promising applications in nuclear industry.In order to meet the low-activation requirements in fusion reactor system and other advanced nuclear energy systems,it is necessary to replace or avoid the activation elements(such as Nb,Mo,Ni,and Co etc.)commonly used in HEAs.Because of the difference in atomic size and electronegativity of the elements,it is difficult to find suitable replaceable low-activation elements to ensure that the alloy can form a good solid solution after replacement.If the activation elements are directly removed,the number of principal elements as well as the configurational entropy will decrease,the phase stability of the alloy will be changed.It is important to study the phase stability and transitions of the alloys without activation elements.In this thesis,a low-activation MPEA with composition Fe5.2Mn30Cr18 was designed and fabricated to investigate the phase stability in the alloy.The processes and mechanisms of phase transformations in the alloy under different heat treatment conditions were analyzed,and the influence of phase transformation on the mechanical properties of the alloy was clarified.The main contents and conclusions of the thesis are as follows:1)According to the basic design ideas of low activation,multi-principal element and austenite-ferrite(y+a)duplex alloy,the Fe52Mn30Cr18 alloy was designed and fabricated by induction melting.After homogenization at 1150? for 3 h and quenching,the alloy formed a y+a duplex phase structure at room temperature.2)The phase stability of the Fe5.2Mn30Cr18 duplex alloy was studied through isothermal heat treatment process at different temperatures.The phase evolution of the alloy between room temperature and 1200? was obtained.The results showed that the y and a phases in the alloy remained stable until the isothermal heat treatment at 450?.The a phase was transformed to the ? phase in the alloy at about 475?.And at about 800?,the ? phase was gradually decomposed into the ?+? phase lamellar pearlite-like structure.The ? phase decomposed above 1000? to form an a phase,and the y+a two phases coexisted in the alloy.As shown in the phase diagram,the y phase disappeared in the alloy above 1200? to form a single a phase structure.3)The characteristics and mechanisms of x and a phase formations in Fe52Mn3oCr18 alloy were studied,which provided a basis for the control of these phases in the alloy.The main characteristics of the phase transformation from a phase to ? phase near 475°C were ferromagnetic disappearance,volume shrinkage and no obvious redistribution of elements.The decomposition process of x phase to ?+? phase near 800°C was accompanied by obvious redistribution of Cr element and volume shrinkage.According to the phase transition characteristics and mechanism analysis,it is suggested that the formation of the ?phase can be suppressed by introducing alloying elements with large differences in atomic radius.4)The effect of phase transformation on the mechanical properties of the alloy was studied.The results show that the y+a duplex alloy obtained by water quenching after homogenization at 1150? didn't contain ? phase and a phase,and it had preferable mechanical properties among the samples studied in this thesis.The tensile strength and elongation of the alloy were equivalent to those of 316ss.The yield strength was about twice that of 316ss.The presence of ?phase near 475? caused hardening and embrittlement of the alloy.The transformation of the ? phase to lamellar structure above 800°C could partially alleviate the embrittlement of the alloy.After homogenization at 1000? and quenching,the embrittlement of a phase could be eliminated.By analyzing the phase stability of Fe-Mn-Cr MPEAs,the phase transformation mechanisms involving ? phase and ? phase were revealed,and the influence of phase transformation on the mechanical properties of the alloy was analyzed.These studies provide a scientific basis to improve the mechanical properties of low-activation MPEAs by heat treatment.The research results can provide important theoretical and experimental support to optimize the design of MPEAs. |
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