Solidified Microstructure Characteristics And Properties Of Dy、Hf Microalloyed NiAl-V System Eutectic Composites

NiAl intermetallic compounds with long-range ordered B2 structure are expected to become a new generation of high-temperature structural materials,but their poor room temperature plasticity and low high-temperature strength limit their application in industrial fields.Through the study of NiAl-V-Dy alloys,it is found that microalloying can improve the comprehensive properties of the alloys.Therefore,in order to continue to improve the comprehensive properties of NiAl alloys,In this thesis,NiAl-V-Dy series alloys with different Hf contents are taken as the research object,and NiAl-32V（at.%）-0.05Dy（wt.%）-x Hf（at.%,x=0,0.05,0.1,0.3,1）alloy system,NiAl-39V（at.%）-0.05Dy（wt.%）-x Hf（at.%,x=0,0.05,0.1,0.3,1）Alloy system,NiAl-43V（at.%）-0.05Dy（wt.%）-x Hf（at.%,x=0,0.05,0.1,0.3,1）alloy system.XRD,optical microscope,SEM+EDS were used to analyze the phase and microstructure of the three alloys,and mechanical tests were performed using HV-50A Vickers hardness tester,CSS-8800electronic universal testing machine,and Gleeble 1500-D thermal simulator.Determination and calculation of hot deformation activation energy of 1Hf alloy.The effects of Dy and Hf combined microalloying on the microstructure and mechanical properties were investigated.The following conclusions are drawn:In microstructure observation analysis:In NiAl-32V-0.05Dy-x Hf hypoeutectic alloy,with the increase of Hf content,the nucleation and direction of primary NiAl phase are promoted to grow regularly,and gradually refine and increase.In the NiAl-39V-0.05Dy-x Hf eutectic alloy system,with the increase of Hf content,the eutectic layer structure at the cell boundary is coarser than that in the cell,and the trend is more prominent with the increase of Hf content.In the NiAl-43V-0.05Dy-x Hf hypereutectic alloy system,with the increase of Hf content,the nucleation and growth of primary V dendrites are inhibited,resulting in coarsening and reduction of the number of primary V dendrites.In the three alloy systems,with the increase of Hf content,the microstructure of the eutectic layer is gradually refined,the degree of instability at the grain boundary increases,and the area between the unit cells increases.The Hf phase,when the content reaches 1%,forms the Ni₂Al Hf phase with discontinuous distribution.In the microhardness experiment:after adding Hf element to the NiAl-32V-0.05Dy-x Hf alloy,the hardness value of the alloy increases because the proportion of NiAl phase increases in the structure.In the NiAl-39V-0.05Dy-x Hf eutectic alloy,the addition of Dy and Hf will refine the NiAl/V eutectic layer,purify the matrix,and improve the hardness.In the NiAl-43V-0.05Dy-x Hf hypereutectic alloy,the hardness value of the alloy increases after adding Hf element,because the proportion of NiAl phase increases by inhibiting the nucleation of primary V phase in the microstructure.Among the three alloy systems,the addition of Hf increases the proportion of NiAl phase in the microstructure,improves the hardness of the alloy system,and the formed Ni₂Al Hf phase is a brittle phase.In the room temperature compression experiment:the addition of Dy and Hf forms Dy NiAl and Ni₂Al Hf phases in NiAl-32V-0.05Dy,NiAl-39V-0.05Dy and NiAl-43V-0.05Dy,and the softening effect of this brittle phase is greater than that of Dy and Hf.The strengthening effect of the alloy leads to a decrease in the compressive strength of the alloy system of all components.In the high temperature compression experiment:the addition of Hf improves the microstructure of NiAl-32V-0.05Dy,NiAl-39V-0.05Dy and NiAl-43V-0.05Dy alloys,refines the grains,and greatly improves the high temperature of the alloys.strength.The reasons are the strengthening of V phase and the fine-grain strengthening caused by Dy and Hf,the solid solution strengthening effect of Hf on the NiAl matrix and the dispersion strengthening of the formed Ni₂Al Hf in the matrix,and the Ni₂Al Hf phase on the boundary between NiAl and V phases.The resulting precipitation strengthens the synergy.After calculating the activation energies of NiAl-32V-0.05Dy-1Hf,NiAl-39V-0.05Dy-1Hf and NiAl-43V-0.05Dy-1Hf alloys,it is found that they are all smaller than the activation energy of 0Hf alloy,and the addition of Hf promotes the precipitation phase The formation of the alloy hinders the movement of dislocations,makes recrystallization easier,refines the unit cell,reduces the activation energy of the alloy,and improves the high temperature strength of the alloy to a certain extent.It shows that after the joint microalloying of Dy and Hf,the solidification structure is refined,resulting in fine grain strengthening,and Dy NiAl and Ni₂Al Hf phases are formed in the structure,resulting in solid solution strengthening,dispersion strengthening and precipitation strengthening.These are the main reasons for the improvement of the high temperature strength of the alloy.