Structural Stability And Deformation Behavior Of FCC-based High Entropy Alloys

The design of conventional alloys was implemented by alloying different elements into a single dominant constituent,consequently enhancing the mechanical and physical properties.High-entropy alloys?HEA?deviate from those concepts,which contain multiple principal components with equal or near equal atomic ratio.The constituent elements contribute to a high configurational entropy,which stabilizes the system as a solid solution phase by suppressing the formation of intermetallic compounds.Different atomic species with chemical disorder also lead to a severe lattice distortion,which responds for the slow diffusion kinetics and the high resistance to dislocation gliding.The unique combination of mechanical and physical properties derived from the phase stabilization gives great potential for both high temperature and cryogenic applications.Among the reported single-phase HEAs,CoCrFeNi?Mn?alloys are the most stable system as a single-phase face-centered-cubic?FCC?structure.However,obvious phase decomposition was recently observed after prolonged anneals at intermediate temperatures.Moreover,the disturbance from chemical doping/alloying can also lead to breakdown of the entropy-enhanced phase stability and thereby significantly change the mechanical properties.Instead of focusing on the phase stabilization and single-phase formation,the second generation of HEAs devotes to obtain multiple phases to explore more potential performance by breaking the strict limitation of the element composition.Though high entropy alloys are reported to be thermodynamically metastable,but the details about the phase decomposition and mechanical behaviors have not been completely revealed.Firstly,due to the sluggish diffusion effect and the small sizes of precipitates,the phase separation process and the crystallographic structure have not been well clarified.Furthermore,simply thermally inducing intermetallic phases is one of the effective way to improve mechanical properties,but relevant studies are still not proposed.Secondly,the precipitation hardening plays an important role in high temperature structural applications.But few efforts were focused on the kinetics of the precipitates nucleation and growth by employing the high resolution atomic structural observation.Thus,microscopic characterization can be the essential way to explore the underlying mechanisms of the phase decomposition in the single-phase HEA.Lastly,the low stacking fault energy?SFE?expected in the HEA increases the strength and toughness with decrease of temperature.But there is no direct experimental evidence to estimate the SFE values and the chemical alloying effect in HEAs.Meantime,deformation behavior switch from twining to the dislocation slipping is poorly known.In addition,dislocation nucleation mechanism in the HEA still needs further exploration.In this thesis,we fabricated the CoCrFeMnNi,Al_0.3CoCrFeNi and Mo_xCoCrFeNi?x=0,0.1,0.15,0.2?alloys using the arc-melting and powder metallurgy P/M methods to explore the phase metastability and mechanical behavior in the FCC-based high entropy alloys.?1?.In order to investigate the decomposition process,severely deformed CoCrFeMnNi alloy annealed in the temperature range of 450?-1200?.During the characterization on the recovery and recrystallization process,we found that the structure is stabilized as a single FCC phase under high temperature annealing,while intermediate temperature with600?-700?annealing lead to the sigma phase decomposition.The precipitates have a complex crystal structure and chemistry,indicating complicate atomic interactions.By controlling the phase stability to optimize the precipitation size and distributions,high strength and good ductility HEA was produced.?2?.Combining the alloying effect with the controllable thermo-mechanical procedure,the primary stage of the phase separation is observed and the nano-scale nucleation is revealed in the CoCrFeMnNi and Al_0.3CoCrFeNi alloys.Structure type of the nanoparticles in CoCrFeMnNi alloy are identified as a HCP phase instead of?phase.The orientation relationship is identified as[100]HCP?[110]FCC,?01-1?HCP??111?FCC.While,B2 nanoparticles are separated out in Al0.3CoCrFeNi alloy.The orientation relationship is identified as[011]FCC?[111]BCC,?1-1-1?FCC??110?BCC.We found that B2 phase nucleates along the TBs,which is associated with the reaction of the dislocations and TBs.The growth of nanoparticle has a preferred orientation relationship to minimize the total interfacial free energy.?3?.Refractory element Mo was added by using powder extrusion.A single-phase solid solution was obtained without any phase decomposition.After investigating the dislocation core structures,we found that the additional Mo atoms obviously hinder the dislocation dissociation and improve the stacking fault energy.Consequently the deformation behavior is changed from the twining to the dislocation slipping.Furthermore,nanoindentation measurement reveals as the cooperative motion of several atoms manifests with partial dislocation nucleation.Our work provides some deep insights in the understanding of the unusual mechanical behavior and phase stability of single-phase FCC HEAs.The detail atomic occupation of the alloying atoms and the origin of the extra-high strengthen in the HEAs still need further investigations.