Study On The Mechanical Properties And Tribological Properties Of Electrodepositied Nanostructure Ni-based Alloys

The development of national economy and national defence industry has an increasingly urgent demand of high strength and high plasticity/ductility metal structure materials.Nanostucture materials have attracted a great deal of attention due to extremely high strength.However,the low plasticity/toughness caused by limited work hardening ability has become a bottleneck to further development and widely application of nanostructure materials.By introducing heterogeneous nanostructures,the distribution and strain gradient of the interface of the heterogeneous structure can be modified and also enhance work hardening ability of nanostructured materials.Especially,for multi-principal element alloys(MPEAs)represented by medium entropy alloys(MEAs)and high entropy alloys(HEAs),chemical composition undulations at intrinsic lattice scale and short-range orders(SROs)structure at atomic scale exist in M/HEAs.This local chemical composition undulations and SROs not only can increase the dislocation slip resistance and work hardening ability,but also futher expand the range of microstructure regulation of heterogeneous nanostructures.At the same time,nanostructuring and herterogeneous nanostructures can also improve low room temperature strength of face-center cubic(FCC)M/HEAs.Therefore,modifying the microstructure of M/HEAs to prepare M/HEAs with nanostucutres and heterogeneous nanostructures has great potential to achieve excellent strength-ductility match.Nowadays,a huge number of researches has focused on the strength and toughness of heterogeneous nanostructured materials and nanostructured M/HEAs,but the research on their friction and wear properties is still relatively limited.As a matter of fact,friction and wear properties are also the key factors of application for metal structural materials.In addition,the study of the dynamic mechanical behavior of nanostructure M/HEAs under extreme conditions of high strain rate and strong dynamic load is still in infancy,which still needs to be studied further.However,most of researches only pay attention on M/HEAs with single phase structure.Thus,there are still some research “blind spots” and gaps to be filled for dual-phase structure M/HEAs,especially M/HEAs with heterogeneous nanostructures.In the present thesis,we performed the following investigations to address upon these fundamental issues:1)As model materials,three Ni Co alloys with different heterogeneous nanostructure were prepared by pulse electrodeposition with different process parameters.For Ni-20 wt.%Co alloy,nano-twins included in ultrafine grains.For Ni-50 wt.%Co alloy,nano-twins and stacking faults formed three-dimensional network structures in ultrafine grains.For Ni-80 wt.%Co alloy,nanoscale martensite bundles,which comtained alarge number of stacking faults,formed network structures in ultrafine grains.The mechanical properties and tribological properties of three heterogeneous nanostructure Ni Co alloys were systematically studied.The results show that the hardness of Ni-50 wt.%Co alloy was the highest,while the hardness of Ni-80 wt.%Co alloy was slightly lower than that of Ni-50 wt.%Co alloy.However,Ni-80 wt.%Co had the best wear resistance.The analysis shows that the plastic deformation of three Ni Co alloys was controlled by the interaction between dislocation and interface.The high hardness of Ni-50 wt.%Co alloy was mainly caused by the three-dimensional network structure formed by nanotwins and stacking faults.Due to the hierarchical nanostructure and TRIP effect,Ni-80 wt.%Co alloy had the best wear resistance.2)Based on plused electrodepostion Ni Co alloys,equal-atomic ratio nanocrystalline(NC)FeCoNi MEA was prepared by adding ferric salts into electrolyte bath.The mechanical properties and tribological properties of nanocrystalline FeCoNi alloys were systematically studied.The results show that the plastic deformation of NC FeCoNi MEA was controlled by interaction between dislocation and grain boundaries/SROs structures.By refining grain size to nanoscale,the hardness of NC FeCoNi MEA was improved to 5.8 GPa,which was nearly three times higher than that of coarse-grain(CG)FeCoNi MEA.The wear surface of NC FeCoNi alloy formed more propective oxide layers due to nanostructuring.Above two factors significantly improved the wear resistance of NC FeCoNi MEA,which wear rate was reduced by an order of magnitude compared with that of CG FeCoNi MEA.In addition,in NC FeCoNi MEA,the hindrance of SROs structures to dislocation slip will lead to the improvement of work hardening ability.Thus,the coefficient of friction and wear rate of NC FeCoNi MEA were reduced significantly compared with nanocrystalline Ni and Ni-based alloys.3)Using FeCoNi MEA as a model material,based on the concurrent of grain growth and phase transition,the microstructure modifying of heterogeneous nanostructure in MPEAs was systematically studied.The results show that during the process of annealing at 200~500°C,a diffusion transformation(from FCC to BCC)occurred in NC FeCoNi alloy due to high diffusion ablity of element caused by high volume fraction grain boundaries.Correspondingly,heterogeneous nanostructures were formed in FeCoNi alloy.The heterogenous nanostructures was composed of nanocrystalline,nanotwins/stacking faults,nanoscale second phase,and hierarchical composition undulations in grain.The nanoindenation results show that NC FeCoNi MEA had the best comprehensive mechanical properties(high hardness and high strain hardening ability)after annealing at 300°C for 6 h and 400°C for 1 h.The analysis shows that the main strengthening mechanisms included fine-grain strengthening,phase boundaries strengthening and grain boundary stabilization caused by phase transition.The toughening mechanism mainly was the TRIP effect and dislocation accumulation caused by composition undulations.4)The dynamic mechanical behavior and microstructure evolution of NC FeCoNi MEA under high strain rate were systematically studied by using the Split Hopkinson pressure bar technique.The results show that with increasing strain rate,the dynamic compression strain gradually increased,and corresponding deformation mechanism was controlled by dislocation slip and grain rotation.With increasing strain rate,deformation transformed to disolocaition slip and was supplemented with deformation twins.The adiabatic temperature rise rate has an important effect on the transformation of deformation mechanism.In the initial stage of high strain rate deformation,the dynamic strain aging caused by the interaction between SROs structures and dislocations made the alloy exhibit negative strain rate sensitivity.However,with the SROs structures were destoried by dislocation slip,the strain rate sensitivity gradually changed from negative to positive.During the high strain rate deformation process,the dislocation accumulation and the formation of deformation twins produced a significant strengthening effect.