Precipitation Manipulation And Its Effects On Mechanical Properties Of FeCoNiCr-based High-Entropy Alloys

FCC precipitate-strengthened high entropy alloys(HEAs)have exhibited outstanding room-temperature mechanical properties due to their stable multicomponent matrix as well as high-density and dispersed coherent precipitates,which are widely recognized as a candidate for structural application at high temperatures.However,precipitation of nanosized strengthening phases in precipitate-strengthened HEAs is usually accompanied by co-precipitation of large sized brittle phases.The brittle phases can induce localized stress concentration during plastic deformation,resulting in plasticity instability.Moreover,as a new family of high-temperature materials,systematic research on the high-temperature mechanical properties,especially the high-temperature creep behavior of precipitate-strengthened HEAs is insufficient,which limits their further development and application.Therefore,this study focused on alloying effects on precipitation behavior and mechanical properties at both room tempeature and elevated temperatures of FeCoNiCr-based HEAs,and attemped to reveal the underlying deformation mechanisms.On that basis,we further explored a new design concept and established an approriate heat-treatment process for producing hierachical precipitate-strengthened HEAs.The main conclusions are drawn as follows:First,we studied not only effects of minor addition of Nb,Ta,Mo and B,but also those of the main constituent of Ni,Al and Ti contents,on the precipitate types,sizes and volume fractions of the FeCoNiCrAlTi HEAs in which nanoscale L12Ni3(Al,Ti)and brittle Heusler-Ni2AlTi phases co-precipitated.It was found that doping of Nb,Ta and Mo cannot suppress the precipitation of the Heusler phase,but instead,destablized the matrix.Although adding a high content of B reduced the quantity of the Heusler phase,more borides formed.Interestingly,adjusting the content of Ni and Al effectively prevented the Heusler phase from precipitation,and increment of Ti significantly increased the volume fraction of the L12 phase,leading to a significant improvement in the strength.In comparison,the strenghtening effect of the Heusler phase is much weaker,and more critically,the Heusler phase tends to deteriorate the ductility during loading.With the properly optimized content of Ni,Ti and Al,we developed(FeCoCr)Ni35Ti2Al6 HEAs with massive L12 nanosized particles but with no brittle Heusler phase.Second,we studied the high-temperature creep properties of FeCoNiCr-based precipitate-strengthened HEAs.The high-temperature creep properties of the optimzed(FeCoCr)Ni35Ti2Al6 HEA were significantly improved.Specifically,the creep lifetime increased by around 2.5 times while the steady creep rate reduced by nearly an order of magnitude.The coarsening rate of the L12 precipitates was estimated to be 2.68×10-29 m3/s,much lower than that in traditional commercial superalloys.Such phenomenon is a result of the slow diffusion effect of the HEA matrix.At slow strain rates,the stress exponent of the(FeCoCr)Ni35Ti2Al6 HEA was around 5,and the dislocation climbing bypass mechanism dominated the creep deformation.As the strain rate rises,power-law breakdown was observed,and the stress exponent increased to around 8.The creep deformation was dominated by the dislocation glide cutting mechanism.The activation energy of the power-law creeps was close to the activation energy of self-diffusion in single-phase FeCoNiCrMn HEA but became higher than the activation energy of self-diffusion in traiditon metals and alloys,revealing that the slow diffusion effect affected the deformation process of the power-law creep.Additionally,the creep threshold stress of the precipitate-strengthened HEAs is around 1/10 of the Orowan stress under the same testing conditions,uncovering that dislocation bypassing is realized through local climbing.Next,we adopted new methods to successfully design and synthesize multiplescale precipitate-strengthened HEAs with different hierachical structures.The first type is the precipitate-strengthened(FeCoCr)Ni35Ti5Al6 featuring gradiented sizes of the L12 phase,which was obtained through recrystallization at the near redissolution temperature of the precipitate and subsequent aging process.The strength and plasticity of the resultant alloy were improved at both room temperature and 650℃.Particularly,its overall mechanical properties are comparable to those of Inconel 718 alloy.The other type is the(FeCoCr)Ni25Ti3Al9 HEA strengthened by multicomponent B2 and L12 precipitates.The creep lifetime of the alloy exceeded 10000 h under the condition of 700℃ and 100Mpa,almost doubled as compared with that of(FeCoCr)Ni35Ti2Al6 HEA.Finally,we explored hydrogen embrittlement resistance of(FeCoCr)Ni35TixAl6(x=1,2,3,4)precipitate-strengthenedHEAs.It was found that the coherent interface between the precipitate and matrix was a reversible hydrogen trap,with a relatively low activation energy of about 14.4～16.1 kJ/mol.The entry of hydrogen can enhance the localized deformation(i.e.,the HELP mechanism)and induce localized stress concentration.When the volume fraction of the precipitate is low,localized high flow stress promote the deformation twinning and improve the plasticity.As the volume fraction of the precipitated phase increased,the critical stress of twinning was increased and the deformation twinning was suppressed.As a result,the stress concentration induced the formation of microcracks and caused the decline of plasticity.The thesis has systematically studied effect of composition and processing on the FeCoNiCr-based precipitate-strengthened HE As and deepened the understanding of creep behavior and deformation mechanism at high temperatures.A further improvement in the mechanical properties has been also achieved by manipulation of precipitation behavior.The finding can provide a guideline for the development of novel precipitate-strengthened HEAs with desirable hightemperature performance in the future.