Effect Of Precipitation Strengthening On The Mechanical Properties Of Ni₂CrFeAl_0.3 High-entropy Alloy

High entropy alloy（HEA）is a new type of multi-principal alloy,due to its unique alloy design concept,the variety of alloys has been greatly expanded,and the multi-principal characteristic makes the property optimization have more possibilities.The trade-off between strength and plasticity of materials is a long-term issue.It is known that the alloys with face-centered cubic（FCC）structures generally have good ductility but low strength,and precipitation strengthening is an effective way to enhance the strength of alloys,which can greatly improve the strength of materials without losing large plasticity.This thesis investigated the effect of heterogeneous structures and precipitations on the mechanical property,microstructure and deformation mechanisms of Ni₂CrFeAl_0.3 HEAs through different heat treatments,and the main contents were presented as follows:（1）The HEAs with fully recrystallized single-phase FCC structure were obtained by cold-rolling followed by recrystallization treatment,and quasi-static tensile tests were performed on samples with different alloy statuses.It was found that the yield strength of the aging-treated samples was increased by 56.4%compared to the recrystallized samples,and also the former retained 43%of plasticity,thus achieving a good combination of strength and ductility.Through microscopic characterizations of the alloys,it was revealed that such excellent mechanical properties were mainly due to the precipitation of multiple secondary phases,including discontinuously-precipitated L12 phase（DP）,continuously-precipitated L12 phase（CP1）,and continuously-precipitated non-coherent phase（CP2）,in which the precipitations have exerted different strengthening contributions to the strength of alloys.After theoretical calculations,it is confirmed that precipitation strengthening indeed plays a major role in the strengthening effect of the alloys.（2）The cold-rolled samples were directly aged without recrystallization,and the strength of the cold-rolled plus aged samples reached 1203 MPa as well as the retention of 10.6%plasticity by quasi-static tensile testing.Through microscopic characterizations of the alloys,it was found that high-density slip bands existed inside the samples,and meanwhile the non-coherent precipitation phase（CP2）with different sizes was precipitated in the FCC matrix.Moreover,the strengthening mechanism is also related with the CP2 phase,i.e.,the large-sized CP2 phase being a dislocation bypassing strengthening and the small-sized CP2 phase being a dislocation shearing strengthening.（3）The FCC structure usually exhibits a low stacking fault energy,and therefore twin structures tend to be formed in the HEAs.Similarly,it was observed that there were numerous annealed twins and deformed twins inside the alloys,and the twins could play a dynamic Hall-Petch strengthening effect by absorbing the dislocation structures.Further observations showed that a large number of lamellar dislocations,lamellar dislocation networks and L-C self-locking structures were also presented inside the alloys.The lamellar dislocation structures with different slip planes were intersected and finally formed L-C self-locking structures at the joints.It is known that the L-C self-locking structures can pin dislocations from four directions,leading to dislocation accumulation strengthening.In addition,a large discrepancy between the experimental and theoretical calculation values for strengthening contributions was presented,which may be caused by the failure to the consideration of statistical storage dislocations and the ignoration of the contribution from multiple strengthening structures within the alloys.