Study On Design,microstructure And Properties Of Cast Be-Al-Sc-based Alloys

The outstanding mechanical,thermal,optical properties and high comprehensive performances including formability and fabrication characteristics enable the beryllium-aluminum(Be-Al)alloys more applications in aerospace,weapon and high-tech areas.Among the actual methods of Be-Al alloys fabrication,the casting approach is more suitable to huge size and complicated structure component owing to its considerable simplicity,low cost and net-shape capabilities,which will promote its applications in more areas.In fact,Be-Al alloy is a metallic based composite,which is a three dimensional net structure.However,the differences between intrinsic properties of two metals result in phase separation and microstructure consisting of a bi-metallic composite of pure Be dendrites encompassed by an Al matrix.The developed Be dendrites,shrinkage porosities and compositional segregation due to a wide solidification temperature range worsen its mechanical property and hinder the further applications of cast Be-Al alloys.The addition of the rare earth element scandium(Sc)is supposed to cast considerable microstructure modification and performance controlling effects on cast Be-Al alloys.Furthermore,the synergetic effects of Sc-alloying and rapid solidification are beneficial to mitigate problems embodied by the developed Be dendrites and solidification defects of cast alloys,which contribute to a better microstructure and comprehensive performance of cast Be-Al alloys as a result.Based on the Lockalloy(62Be-38Al,wt.%)alloy with high comprehensive properties,the microstructure evolution and mechanical performances of cast Be-Al alloys with scandium(Sc)addition have been systematically investigated.The mechanisms of microstructure modification and mechanical enhancement caused by Sc addition were revealed,the failure mechanism from fracture were illustrated.An optimized heat treatment process was designed and the constitutive relation of cast alloys was summarized via hot deformation behaviors study.Cast Be-Al alloys with fine microstructure,mechanical properties and castabilities were obtained through the synergetic effects of 0.4(Sc,Zr)-and 0.4Sc-alloying and improved solidification rates.Several innovative results,including:i)the addition of Sc,(Sc,Zr)and rapid solidification modified the Be dendrites greatly and restrained the solidification defects,contributing to relatively highly spherodized Be grains and better properties,ii)the standard Gibbs free energy of Be13Sc,the diffusion behaviors of Sc atoms,and the constitutive equation of cast Be-Al-0.4Sc alloy were obtained for the first time,the influences of secondary phase particles on solidification and rupturing behaviors were revealed,the optimized heat treatment parameters were summarized based on the aging behaviors of nano-scale Al3Sc precipitates,iii)the effects of solidification rate on the microstructure of Be dendrites,the distribution of Sc solutes,and the morphologies of secondary phase particles were illustrated,the high sensitivity of solidification behaviors and microstructure of cast alloy to undercooling and solidification rates of the melt were revealed.Main conclusions are summarized as follows:(1)The effects of various Sc additions on the microstructure and mechanical properties of cast Be-Al alloys were studied and the optimized addition of 0.4 wt.%Sc was obtained.The addition of 0.2 wt.%?0.4 wt.%Sc reduced the length of Be dendrites and dendritic arms and secondary dendritic arm spacing(SDAS),increased the Vickers hardness,tensile strength,elongation and the micro-mechanics of matrices.The 0.4 wt.%Sc resulted in the cellular/equiaxed Be morphology with the finest and most uniform Be grains and the highest elongation 4.7%as well as a relatively high tensile strength,whereas higher Sc additions till 3.0 wt.%transformed Be grains into developed dendrites,the tensile strength decreased after an enhancement,while substantially reduced and elongation of cast alloy in general.(2)The second phase effects on solidification and microstructure evolution were studied.Cast Be-Al-0.4Sc alloys were comprised of(Be),(Al),Bel3Sc and Al3Sc,of which the latter two phases could not be effective nucleation sites for(Be)The standard Gibbs free energy of the secondary phase Be13Sc was calculated to be G0(Be13Sc,L)=-18536+12.125T,indicating Be13Sc formed spontaneously via one-order reaction below 1350?.Cast Be-Al-0.4Sc-0.4Zr alloy contained the additional phase of Be13Zr.Corresponding nano-indentation results revealed the first pop-ins at the depths of about 10 nm for Be and Al matrices caused by the rupture of superficial oxide film,while the first pop-ins of combined secondary phase particles(SPs)were directly influenced by their intrinsic properties,crystal orientation and dislocation movements.The microhardness values of Be matrix and SPs were corrected via impression analysis.The critical shear stress ?max(6.0 GPa),position Z?max.(27.8 nm)and corresponding dislocation loop radius Rd(2.7nm)for the first pop-in of SPs were also obtained.(3)The correlations between 0.4Sc-alloying,SPs and the mechanical properties,dynamic and static fracture modes of cast alloys were clarified.Sc-containing SPs has the highest micro-hardness,elastic modulus,yield strength and the lowest plastic index 0.4Sc-alloying strengthened the ability of crack nucleation resistance but weakened that of crack development.A mixed fracture mode of cleavage fracture of Be matrix and ductile fracture of A1 matrix was found through tensile and impact fracture surface analysis for as-cast Be-Al-0.4Sc alloy.The decreased elongation was deeply influenced by the morphology,distribution,and micro-mechanics of SPs,which was:SPs located at the Be/Al interfaces did not act as micro-crack sources for the matrices,while SPs with low curvature within the Be grains induced high local strain concentration near the tips and the formation of micro-crack.(4)The precipitating behaviors of nano-scale Al3Sc precipitates during the aging processes were revealed,with an optimized heat treatment process proposed.Based on the calculated activation energy of diffusion Q and homogenization kinetics curves of Sc in cast alloy,the proper heat treatment system contained homogenizing at 620? for 2h followed by water-quenching and aging treatment at 400? for 2h followed by waterquenching.The coarsening process of Al3Sc includes four stages:(i)Sc atoms accumulate near incoherent phase boundaries and nucleate heterogeneously into secondary precipitates;the Al/Al3Sc interfacial dislocations appear with the growth and coarsening of them;(ii)secondary Al3SC precipitates grow perpendicular to the negative gradients of Sc atoms with their morphology controlled by elastic free energy;(iii)based on the Ostwald ripening mechanism,all Al3Sc precipitates continue their coarsening and the coalescence of these secondary Al3SC precipitates happens;(iv)secondary Al3Sc precipitates grow into rhoptries with the release of elastic free energy by elongating along the<110>orientation and narrowing along the<100>orientation.The above microstructure evolution due to the overall increase in the lattice mismatch between Al3Sc and Al was illustrated.The homogenization and aging treatments influenced the enthalpy needed by the A1 matrix during melting and cooling processes owing to the effects of Al3Sc particle morphology,density,distribution and supersaturated solid solution of Sc.(5)The rheological behavior and microstructure evolution of cast Be-Al-0.4Sc alloy during hot deformation process were investigated.The strain rate sensitivity factor(m)values of cast alloy were 0.0637,0.0632 and 0.0739 at 200?,300?,400?,respectively,within the temperature and strain rate ranges from 200? to 500? and 10-3 s-1 to 100 s-1,indicating that the alloy was more suitable for deformation with high strain rates.The obtained relatively high stress exponent(n)value 14.319 of the alloy implied that dislocation climb dominated the deformation mechanism at high temperatures.0.4Sc-alloying increased the hot deformation activation energy(Qd)of cast Be-Al alloy to 470.8 kJ/mol.The plastic deformation of cast alloy was thermally activated during hot depression,with the correlation between peak stress ?,temperature T,strain rate ? being expressed by constitutive equations lnZ=130.69+40.21ln[sinh(0.00426?)J and ?=e15.19[sinh(0.00426?)]14.319exp(-470800/RT).Notably,the Be and Al matrices underwent the dynamic recovery instead of dynamic recrystallization during hot deformation till the true strain of 0.9.The reasonable processing parameters of cast VIM-Be-Al-0.4Sc alloy was summarized to be T=350??400?,?=10-2.5 s-1?10-2 s-1 and T=400?-450?,?=10-1 s-1?100 s-1?(6)The advantageous alloying effects of 0.4(Sc,Zr)on cast Be-Al alloys were determined,and pilot-scale cast Be-Al-0.4Sc alloy with fine castability was produced by 0.4Sc-alloying and increased solidification rate.The addition of 0.4(Sc,Zr)reduced the SDAS of Be dendrites from?12?m of cast Be-Al-0.4Sc alloy to?7.5?m,further refined the Be grains and SPs,and increased the Vickers hardness by 12.8%.Notably,the extra addition of 0.4Zr improved the thermal stabilities of nano-scale Al3Sc precipitates.Cast Be-Al-0.4Sc alloys with solidification rates of 100?/s,102?/s,103?/s and 104?/s were prepared.It was found that increasing solidification rate only resulted in Be grain refinement,whereas 0.4Sc modified Be dendrites into equiaxed/cellular grains with weakened dendritic arms,further decreased grain size and mechanical reinforcement.0.4Sc-alloying also contributed to the decrease of porosity from 6.32%of furnace-cooled cast Be-Al alloys to 1.86%,restrained casting defects including hot cracking,shrinkage cavities and holes,while the synergetic effects of 0.4Sc-alloying and rapid solidification process(RSP)further reduced the porosity to 0.11%with marginally detected casting defects and improved the thermal and mechanical properties of conventionally cast Be-Al alloys to a level approaching the commercial cast Be-Al alloys produced in USA.The solidification behaviors and microstructures of cast be-Al alloys were highly sensitive to the undercooling and solidification rates of the melt.