High-entropy Effects On Microstructural Stability And Properties Of Nanocrystalline Alloys

Nanocrystalline(NC)materials possess nanometer-scale grains and a high volume fraction of grain boundaries(GBs),which attributes give rise to appealing physical,chemical and mechanical properties.However,NC materials generally show an increased tendency to coarsen by comparison with conventional metals with coarse grains,due to the introduction of a high density of interfaces.Many NC metals undergo grain coarsening even at room temperatures.Thus,improving thermal stability has become a key challenge in the field of NC materials.Recent studies on high-entropy alloys(HEAs)show that the increase of configurational entropy leads to long-range chemical disorder of atomic distribution and slow lattice diffusion.Therefore,the high-entropy effect provides a new opportunity to improve the thermal stability of NC structure.In light of this,high-entropy effects on microstructure stability and properties of NC alloys and the related mechanism were studied in this dissertation.Firstly,the microstructure and stability of NC HEAs after high-pressure torsion(HPT)nanocrystallization were studied.It is found that the microstructure of GdDyErHoTb rare-earth HEA and HfZrTi HEA with single HCP structure,and TaHfZrTi HEA with single BCC structure is refined after HPT,and the phase structure remains unchanged.However,the phase transition from BCC to HCP occurs in the Ta0.5HfZrTi HEA with HCP+BCC dual-phase structure after HPT,and the resulted HCP lamella plates could increase the degree of grain refinement.After different annealing heat treatments,it is found that the thermal stability of the HfZrTi NC HEA was much higher than that of pure metals(pure Ti and pure Zr).When the annealing temperature is lower than 823 K,the microstructure does not change obviously,which may be related to the high mixing entropy and the sluggish diffusion effect of different solute atoms.However,obvious phase decomposition is observed in the annealed NC Ta0.5HfZrTi and TaHfZrTi HEAs,and the corresponding nano-hardness values decrease sharply.The phase decomposition behavior will greatly damage the mechanical properties of the samples,resulting in a severe embrittlement of the annealed samples.Therefore,the thermal stability of NC HEAs involves not only the stability of grain,but also the stability of phase.Secondly,the effect of HPT nanocrystallization on the properties of HEAs was investigated.After HPT,the nano-hardness and strength of NC HEAs are increased obviously,and the corrosion resistance of the HfZrTi and TaHfZrTi HEAs in PBS solution is significantly improved,which is mainly due to the introduction of high-density of GBs and dislocations during HPT deformation.For GdDyErHoTb rare-earth HEA,the antiferromagnetic transition of the NC sample becomes smooth compared with the as-cast sample.The maximum magnetic entropy change at 5 T magnetic field is decreased from 8.6 to 6.6 J kg-1 K-1,but the working temperature range is increased from 102 to 113 K.Our analysis shows that compared with the conventional magnetocaloric materials,the strong chemical disorder makes magnetic ordering in HEAs difficult,thus giving rise to a sluggish magnetic phase transition and a resulted wide temperature range.However,the influence of the microstructural nanocrystallization on the magnetocaloric effect of GdDyErHoTb rare-earth HEA is not obvious.Based on the above work,the influence of high-entropy effects on the microstructure and properties of NC metals was studied.Ti,Ni,Co and Hf solute elements were added in the pure Nb with an equal fraction,and the total contents were 1,2,5 and 10 at.%,i.e.,1HEA,2HEA,5HEA and 10HEA.Then NC alloys were prepared by HPT deformation.For comparison,a binary Nb-Ni NC alloy with 1 at.%Ni was also prepared,i.e.1Ni.After annealing at different temperatures for 2 hours,it is found that the onset coarsening temperatures of the NC pure Nb,1Ni,5HEA and 10HEA are 873 K.After annealing at 973 K for 2 hours,the recrystallized grains in the NC pure Nb has coarsened quickly to micrometer-size.Although NC 1Ni,5HEA and 10HEA alloys have shown a similar onset growth temperature with NC pure Nb,the coarsening rate of the NC 1Ni,5HEA and 10HEA is much slower,and the grain sizes after annealing are 233±18 nm?123±8 and 133±8 nm,respectively.By comparison,the onset coarsening temperature of the NC 1HEA and 2HEA is obviously increased.In particular,the onset coarsening temperature of the NC 1HEA has reached 1023 K,and after annealing at 973 K even for 2200 hours,no obvious change in either the morphology or the size of the grains was observed,and the corresponding values of nano-hardness remain almost unchanged,thus the thermal stability of the structure is significantly improved.Nevertheless,once the coarsening of grains commences,the growth rate is inversely proportional to the alloying content,i.e.,the higher the alloying content,the slower the coarsening rate.The dramatically enhanced microstructural stability in the NC 1HEA can be attributed to the reduced grain boundary(GB)energy from co-segregation of multiple solute atoms at the GBs and GB mobility from local chemical ordering.The rate of grain coarsening is related to the pinning effect of kinetics.The higher the total alloying content,the more obvious the pinning effect.Taking NC 1 0HEA as an example,a large number of high-entropy precipitates with B2 structure appeared at the GBs after annealing,which could effectively hinder the GB migration and slow down the grain coarsening rate.It can be concluded from the above experimental results that several co-segregated solute elements with strong GB segregation tendency should be added to stabilize NC metals by utilizing high-entropy effects,and their total content should remain within the dilution limit of GB,so as to induce a variety of co-segregated elements at the GB and maximize the reduction of GB energy,and thus improve the effect of stabilizing microstructure.In summary,the microstructure,stability and the change of properties for HEAs after HPT were systematically investigated in this work,which gives a preliminary understanding for the development and application of NC HEAs.On this basis,the high-entropy effect is utilized to stabilize NC metals,and the stabilization micro-mechanism was explored,which paves a new route for designing stable NC materials.