The effects of Y pre-alloying on the in-situ dispersoids of ODS Co Cr Fe Mn Ni high-entropy alloy

Oxide dispersion strengthened CoCrFeMnNi high-entropy alloys(ODS-HEAs)were prepared using two different powder preparation methods classified by yttrium addition strategy to investigate the effects of in-situ and ex-situ oxide dispersoid formation on the microstructure and mechanical properties.Systematic micro structural analysis was carried out by X-ray diffraction(XRD),electron backscattered diffraction(EBSD),high-resolution transmission electron microscopy(HRTEM),atom probe tomography(APT),and small-angle neutron scattering(SANS).Cryo-milled powder analysis,grain structure evolution after spark plasma sintering,dispersoid characteristics,and matrix/dispersoid interface structure analysis of the insitu and ex-situ dispersoids within the high-entropy alloy(HEA)matrix were performed.The in-situ dispersoid formation was dominantly observed in the Y-alloyed ODS-HEA through the construction of a coherent interface relationship with complex chemical composition,leading to an increase in the Zener pinning forces on the grain boundary movement.ODS-HEA with in-situ oxide dispersoids enhanced the formation of ultrafine-grained structures with an average diameter of 330 nm at a sintering temperature of 1173 K.This study shows that the Y pre-alloying method is efficient in achieving fine coherent dispersoids with an ultra fine-grained structure,resulting in an enhancement of the tensile strength of the CoCrFeMnNi HEA.

SOME DEVELOPMENTS IN RAPIDLY SOLIDIFIED ALUMINUM ALLOYS FOR ELEVATED TEMPERATURE APPLICATIONS

Therecent developments in elevated temperature ( ET) aluminum alloys prepared by therapidsolidification / powder metallurgy ( RS P/ M) process were reviewed briefly. TheRS P/ METaluminum alloyscan beclassified as(a) the aluminum transition metaltype, such as Al Fe, Al Cr, Al Ti, Al Zrsystem alloys,etc.,and (b) thealuminum rareearth elementtype,such as Al Y, Al Nd system alloys,etc. Among them ,the Al Fe and Al Ti system alloysarethe most attractive, which possessthe potentialto replacethetitanium alloy partson aircraft,engines,etc.,fortheuseattemperaturesrangingfrom 200 315℃. Theproblemsin applicationsfor RS P/ M ETaluminum alloys werealso discussed .

Experimental Simulation of Hot Rolling of Aluminium Alloy Al-5Mg

Multipass hot-rolling process of a newly developed aluminum alloy was simulated by nonisothermal axisymmetric compression test. High temperature compression behavior and the microstructures of the alloy after deformation were investigated. The flow stress increases with the decreasing of deformation temperature or the increasing of pass times. Dynamic recovery was the operative deformation mechanism during hot deformation. Dense dislocations had been pinned by Al3Zr dispersoids during hot deformation, the Al3Zr dispersoids inhibited recrystallization and grain growth.

INFLUENCE OF COMPOSITION AND DISPERSOID ON FATIGUE FRACTURE BEHAVIOUR OF Al-Mg-Si ALLOYS

A study was made of the fatigue fracture behaviour under different aging conditions of two Al-Mg-Si alloys with different chemical compositions and dispersoid contents.The dispersoid phase can alter the mode of the deformation uniformity of alloys.The dispersoid it- self may decohere from the dispersoid/matrix interface under cyclic stress to form small dimples.

Effects of dispersoid preforming via multistep sintering of oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy

Dispersoid formation and microstructural evolution in an oxide dispersion-strengthened CoCrFeMnNi high-entropy alloy(HEA)using a newly designed multistep sintering process are investigated.The proposed multistep sintering consists of a dispersoid preforming heat treatment of as-milled 0.1 wt%Y_(2)O_(3)-CoCrFeMnNi high-entropy alloy powders at 800℃,followed by sintering at 800–1000℃ under uniaxial pressure.In the conventional single-step sintered bulk,the coarsened BCC Y_(2)O_(3)dispersoids mainly form with an incoherent interface with the HEA matrix.In contrast,finer FCC Y_(2)O_(3)dispersoids,an atypical form of Y_(2)O_(3),are formed in the matrix region after multistep sintering.Nucleation of FCC Y_(2)O_(3)disper-soids is initiated on the favorable facet,the{111}plane of the austenitic matrix,with the formation of a semi-coherent interface with the matrix during the dispersoid preforming heat treatment and it maintains its refined size even after sintering.It is found that dispersoid preforming prior to sintering appears promising to control the finer dispersoid formation and refined grain structure.

Microstructure evolution of spray deposited and as-cast 2195 Al-Li alloys during homogenization

In this paper,a comparative study on the spray deposited and as-cast 2195 alloy was carried out to reveal their microstructure evolutions and differences during the homogenization process.The dissolution of the secondary particles and the diffusion of solute were studied based on microstructure characterization and kinetics analysis.The precipitation behavior of Al3Zr dispersoids and its influence on recrystallization were investigated by using TEM and EBSD characterization.It was found that the large-size particles at triangular grain boundaries dissolve slower than the intragranular phases and other grain boundary phases.The required homogenization time depends on the dissolution processes of the large-size phases at grain boundaries.The size of grain boundary phases in the spray deposited alloy is much smaller than that in the as-cast alloy,so the homogenization time required for the spray deposited alloy is significantly shorter.Two-stage and ramp heating homogenization processes can promote the precipitation of Al3Zr dispersoids in the two alloys.In the spray deposited alloy,the dispersoids tend to precipitate at the positions of the T1 plates dissolved,which causes a non-uniform distribution and decreases the recrystallization resistance of the alloy.However,the distribution of the dispersoids in the as-cast alloy is more uniform after the homogenization,which brings a stronger inhibition on the recrystallization.According to the microstructural characterization and kinetics analysis results,it can be concluded that the homogenization with a slow ramp heating is suitable for the two 2195 alloys,and a shorter holding time can be used for spray deposited alloy,e.g.12 h at 500℃,while the holding time for the as-cast alloy is no less than 35 h at 500℃.

Superior mechanical properties and strengthening mechanisms of lightweight AlxCrNbVMo refractory high-entropy alloys(x=0,0.5,1.0)fabricated by the powder metallurgy process

Light and strong AlxCrNbVMo(x=0,0.5,and 1.0)refractory high-entropy alloys(RHEAs)were designed and fabricated via a the powder metallurgical process.The microstructure of the AlxCrNbVMo alloys consisted of a single BCC crystalline structure with a sub-micron grain size of 2-3μm,and small amounts(<4 vol.%)of fine oxide dispersoids.This homogeneous microstructure,without chemical segregation or micropores was achieved via high-energy ball milling and spark-plasma sintering.The alloys exhibited superior mechanical properties at 25 and 1000℃compared to those of other RHEAs.Here,CrNbVMo alloy showed a yield strength of 2743 MPa at room temperature.Surprisingly,the yield strength of the CrNbVMo alloy at 1000℃was 1513 MPa.The specific yield strength of the CrNbVMo alloy was increased by 27%and 87%at 25 and 1000℃,respectively,compared to the AlMo_(0.5) NbTa_(0.5)TiZr RHEA,which exhibited so far the highest specific yield strength among the cast RHEAs.The addition of Al to CrNbVMo alloy was advantageous in reducing its reduce density to below 8.0 g/cm^(3),while the elastic modulus decreased due to the much lower elastic modulus of Al compared to that of the CrNbVMo alloy.Quantitative analysis of the strengthening contributions,showed that the solid solution strengthening,arising from a large misfit effect due to the size and modulus,and the high shear modulus of matrix,was revealed to predominant strengthening mechanism,accounting for over 50%of the yield strength of the AlxCrNbVMo RHEAs.

Dispersoid Formation and Recrystallization Behavior in an Al-Mg-Si-Mn Alloy

The nucleation and precipitation of Mn-containing dispersoids in an Al-Mg-Si-Mn alloy （6082） have been studied by optical microscopy,EPMA （electron probe microanalysis） and TEM （transmission electron microscopy）.The influence of Mn-containing dispersoids on the recrystallization behavior was also investigated.The size and distribution of dispersoids were strongly affected by both the homogenization process and the alloying element distribution formed in the direct chill cast procedure.The Mn-containing dispersoids were observed to nucleate preferentially on the β-Mg 2 Si phase and to be aligned along the 〈100〉 direction of the matrix.After cold deformation,the morphology of dispersoids greatly influences the recrystallization and grain growth behavior in the annealing process.

Size effect in the Al3Sc dispersoid-mediated precipitation and mechanical/electrical properties of Al-Mg-Si-Sc alloys

A significant size effect is found in the Al3 Sc dispersoid-mediated precipitation in an Al-Mg-Si-Sc alloy.When the Al3 Sc dispersoid size smaller than about 40 nm,β " precipitates nucleate directly on the coherent dispersoids and grow by sacrificing the latter.While the dispersoid size greater than^40 nm,Q' and U2 phases are additionally produced that nucleate on the dislocations induced by the semi-/incoherent dispersoids.Mechanical and electrical properties are highly sensitive to the Al3 Sc dispersoid-tuned precipitation.The co-precipitation of β",Q' and U2 phases leads to an obvious improvement in hardness and simultaneously in electrical conductivity.

Improvement of elevated-temperature strength and recrystallization resistance via Mn-containing dispersoid strengthening in Al-Mg-Si 6082 alloys

The precipitation behavior of Mn-containing dispersoids in Al-Mg-Si 6082 alloys with different Mn contents(0,0.5 and 1.0 wt%)during various heat treatments(300–500℃)was investigated.The effects of dispersoids on elevated-temperature strength and recrystallization resistance during hot-rolling and post-rolling annealing were evaluated.The results showed that the dispersoids in the Mn-containing alloys(0.5 and 1.0%)began to precipitate at 350℃and reached the optimum conditions after 2–4 h at 400℃.However,the dispersoids coarsened with increasing holding time at temperatures above450℃.After the peak precipitation treatment at 400℃for 2 h,the yield strength at 300℃increased from 28 MPa(base alloy free of Mn)to 55 MPa(alloy with 0.5%Mn)and 70 MPa(alloy with 1%Mn),respectively,demonstrating a significant dispersoid strengthening effect at elevated temperature.In addition,the dispersoids were thermally stable at 300℃for up to 1000 h holding owing to its relative high precipitation temperature(350–400℃),leading to the superior constant mechanical performance at elevated temperature during the long service life.During hot rolling and post-rolling annealing,the presence of a large amount of dispersoids results in the higher Zener drag PZcompared with base alloy and then significantly improved the recrystallization resistance.The alloy containing 0.5%Mn exhibited the highest recrystallization resistance among three experimental alloys studied during the post-rolling process,likely resulted from the lower coarsening rate of dispersoids and the lower dispersoids free zone.

Effects of Mn content on recrystallization resistance of AA6082 aluminum alloys during post-deformation annealing

The microstructural evolutions under as-homogenized and as-deformed conditions and after the postdeformation annealing of AA6082 aluminum alloys with different Mn content(0.05 wt.%-1 wt.%)were studied by optical,scanning electron,and transmission electron microscopies.The results showed that the presence of a large amount ofα-Al(Mn,Fe)Si dispersoids induced by Mn addition significantly improved the recrystallization resistance.In the base alloy free of Mn,static recrystallization occurred after 2 h of annealing,and grain growth commenced after 4 h of annealing,whereas in Mn-containing alloys,the recovered grain structure was well-retained after even 8 h of annealing.The alloy with 0.5%Mn exhibited the best recrystallization resistance,and a further increase of the Mn levels to 1%resulted in a gradual reduction of the recrystallization resistance,the reason for which was that recrystallization occurred only in the dispersoid-free zones(DFZs)and the increased DFZ fraction with Mn content led to an increase in the recrystallization fraction.The variation in the dispersoid number density and a coarsening of dispersoids during annealing have a limited influence on the static recrystallization in Mn-containing alloys.

Joint effect of quasicrystalline icosahedral and L1_(2)-strucutred phases precipitation on the grain structure and mechanical properties of aluminum-based alloys

Dispersoid hardening is a key factor in increasing the recrystallization resistance and mechanical strength of non-heat treatable aluminum-based alloys.Mn and Zr are the main elements that form dispersoids in commercial Al-based alloys.In this work,the annealing-induced precipitation behavior,the grain structure,and the mechanical properties of Al-3.0 Mg-1.1 Mn and Al-3.0 Mg-1.1 Mn-0.25 Zr alloys were studied.The microstructure and the mechanical properties were significantly affected by annealing regimes after casting for both alloys.The research demonstrated a possibility to form high-density distributed quasicrystalline-structured I-phase precipitates with a mean size of 29 nm during low-temperature annealing of as-cast alloys.Fine manganese-bearing precipitates of I-phase increased recrystallization resistance and significantly enhanced the mechanical strength of the alloys studied.The estimated strengthening effect owing to I-phase precipitation was 150 MPa.Due to the formation of L1_(2)-structured Al3Zr dispersoids with a mean size of 5.7 nm,additional alloying with Zr increased yield strength by about 90 MPa.The L1_(2)-phase strengthening effect was estimated through the dislocation bypass looping and shearing mechanisms.