Effects Of Interstitial Atoms On The Mechanical And Damping Properties Of Refractory High-entropy Alloys

Refractory high-entropy alloys(RHEAs)are usually defined as alloys consisting of group IVB,VB,and VIB refractory elements in equal or near equal atomic percentage.They have attracted considerable attention for aerospace applications owing to their uniquie properties,such as high yield strengths at room temperature and large softening resistance at elevated temperatures.Nevertheless,RHEAs are normally brittle at room temperature,resulting in insufficent processing capability.Moreover,the metallic elements in RHEAs are easily contaminated by impurities such as carbon,boron,oxygen and nitrogen,which makes it difficult to control the impurity content and thus leads to a serious challenge in the synthesis and processing of RHEAs.In addition,the production costs of RHEAs are extremely high,and as a result,RHEAs with only single kind of properties are hard to compete with traditional metals and alloys.These shortcomings greatly limit practical applications and research efforts of RHEAs.In light of these challenges,effects of interstitial atoms on the performance of Ta-Nb-Hf-Zr-Ti RHEAs were studied in this dissertation.First,effects of configurational entropy on the room-temperature tensile properties of Ta-Nb-Hf-Zr-Ti equiatomic alloy system was investigated,and it was concluded that the yield strength increased with the increase of configurational entropy.Also,it was found that the lattice distortion in this alloy system could be simply described by the atomic size mismatch,and the lattice friction stresses scale linearly with the lattice distortion.The normalized dislocation core width of the refractory medium/high-entropy alloys was estimated to be~0.95b(b is the Burgers vector),much smaller than that of the pure Nb and NbTi alloy(i.e.,1.14b and 1.08b,respectively),suggesting that the high strength of RHEAs is probably resulted intrinsically from their high lattice friction stress.Furthermore,the TiZrHfNb RHEA exhibited the best tensile properties at room temperature among all the alloys studied.Second,effects of interstitial atoms(i.e.,carbon,boron,oxygen and nitrogen)on the microstructure and mechanical properties of Ti-Zr-Hf-Nb RHEAs were systematically studied.It was found that brittle ceramic particles were formed in the alloys with addition of 0.5 at.%carbon and boron,dramatically reducing the tensile plasticity of the alloyed RHEAs.However,the solubility limit of oxygen and nitrogen was found to be larger than 3.0 at.%,and the yield strength of the alloyed RHEAs increases with the increase of the content of these interstitial atoms.It is worth mentioning that,addition of 2.0 at.%oxygen into the TiZrHfNb RHEA could simultaneously enhances the strength and ductility,i.e.,the elongation has nearly doubled,increasing from 14.21±1.09%for the TiZrHfNb RHEA to 27.66±1.13%for the(TiZrHfNb)98O2 RHEA,accompanied by a significant work-hardening effect.Moreover,atomic-scale deformation mechanism of the anomalous interstitial-strengthening behavior in the(TiZrHfNb)98O2 RHEA was revealed.With addtion of 2.0 at.%oxygen,it was found that oxygen could assume the constitutive form of ordered oxygen complexes,a state between regular random interstitials and oxide particles.Such ordered oxygen complexes could pin dislocations and subsequently promote their cross slip,leading to the change of the plastic deformation mode from planar slip to wavy slip.This unique interaction between ordered oxygen complexes and dislocations leads to the associated promotion of dislocation nucleation and propagation,which then greatly promotes work hardening,and eventually brings in the high ductility.Lastly,this dissertation also proposed an approach of entropy-stabilization engineering to designing Snoek-type high-temperature high-damping alloys.A series of novel Snoek-type high-damping RHEAs were developed via doping 2.0 at.%oxygen or nitrogen in various typical RHEAs.Such newly designed RHEAs possess a unique combination of mechanical and damping properties,i.e.,high-temperature damping capacities,and high tensile yield strength and ductility.This finding has important implications not only for developing novel high damping materials,but also for broadening application range of RHEAs.