Effects of yttrium and strontium additions on as-cast microstructure of Mg-14Li-1Al alloys

Mg-14Li-1Al （LA141）, LA141-0.3Y, LA141-0.3Sr, and LA141-0.3Y-0.3Sr alloys were prepared in an induction furnace in the argon atmosphere. The microstructures of these alloys were investigated through scanning electron microscope （SEM）, X-ray diffractometer （XRD） and energy dispersive spectrometer （EDS）. The results show that yttrium and/or strontium additions produce a strong grain refining effect in LA141 alloy. The mean grain sizes of the alloys with addition of Y and/or Sr are reduced remarkably from 600 to 500, 260, 230 μm, respectively. Al 2 Y, Al 4 Sr and Mg 17 Sr 2 phases with different morphologies are verified and exist inside the grain or at the grain boundaries, thus possibly act as heterogeneous nucleation sites and pin up grain boundaries, which restrain the grain growth.

Relationship Between the Processing Parameters and the Properties of Semi-solid Processed Al Alloys

Relationship between the processing parameters and the properties of semi-solid processed Al alloys were studied and microstructure and mechanical properties of semi-solid processed Al parts for automobile application as a function of processing parameters were compared with those of die-cast parts and forged parts. In addition, the locations for the gate during the semi-solid processing were varied to elucidate the distribution of micro-porosities and resulting mechanical properties and the T6 heat tre...

An Experimental Study of Thermophysical Properties for Quinary High-Entropy NiFeCoCrCu/Al Alloys

Two quinary high-entropy alloys （HEAs） with equiatomic concentrations formed by doping either Cu or A1 elements into the quaternary NiFeCoCr alloy are produced by arc melting and spray casting techniques. Their entropy of fusion, thermal expansion coefficient and thermal diffusivity are experimentally investigated with differential scanning cMorimetry, dilatometry and laser flash methods. The NiFeCoCrCu HEAs contain a face- centered cubic high-entropy phase plus a minor interdendritic （Cu） phase and display a lower entropy of fasion and the Vickers hardness. The NiFeCoCrAl HEAs consist of two body-centered cubie high-entropy phases with coarse dendritic structures and show higher entropy of fusion and the Vickers hardness. Both the thermal expansion coefficient and the thermal diffusivity of the former Cu-doped alloy are signitieantly larger than those of the latter At-doped M1oy. Although the temperature dependence of thermal diffusivity is similar for both HEAs, it is peculiar that the thermal expansion curve of the NiFeCoCrAl alloy exhibits an inflexion at temperatures of 860-912 K.

Surface oxidation study of molten Mg–Al alloys by oxide/metal/oxide sandwich method

The dynamic oxidation of molten Mg–Al alloy was investigated via the oxide/metal/oxide(OMO)sandwich method.The characteristics of sandwiches were explored using optical microscopy,scanning electron microscopy,X-ray energy dispersive spectroscopy,and X-ray diffraction analyses.The results showed the formation of porous oxide films with varying thicknesses from 0.43 to 16.7 mm.Both the measurements and calculations confirmed the literature findings that the oxidation product consists mainly of MgO and Mg Al_(2)O_(4)compounds.The increase in thickness and amount of folds formed on the oxide films implies the significant effect of aluminum in reducing the oxidation resistance of magnesium.

Effect of Nb and alloying elements on interface reaction between high Nb-containing TiAl alloys and ZrO_2-based ceramic moulds

In the present study, Ti-45Al-(6, 7, 8)Nb(at%) and Ti-45Al-8Nb-0.5(Mn, Si, Y, B) alloys were prepared by arc melting and casting into Zr O2(Y2O3 stabilized) ceramic moulds to study the effect of alloying elements Nb and Mn, Si, Y, B on the interfacial reaction between casting Ti Al alloys and ceramic moulds by SEM, and the elements' distribution in the interface reaction layer by line scanning. The results showed that with an increase in Nb content, the interfacial reaction weakened and the thickness of the reaction layer decreased gradually. The interface reaction thickness of the alloys with Nb content of 6, 7, 8at% were 60, 34 and 26 μm, respectively. Clearly, the addition of 8at% Nb to Ti-45 Al is the best for the thickness of the reaction layer. The addition of Nb would form a Nb-rich film in the reaction layer, which could reduce the solubility of oxygen in the interface, and suppress further diffusion of oxygen to the matrix. If the same content of Mn, Si, Y, or B alloying elements were added respectively to Ti-45Al-8Nb, the thickness of the interface reaction layer from large to small was as follows: Mn>Si>Y>B. The interface reaction thickness increased after 0.5at% Mn added, had no obvious change after 0.5at% Si addition, and decreased after adding 0.5at% Y or B. The introduced elements, which formed a protective film or/and promoted the formation of a dense aluminum oxide layer, would be of benefit to the resistance of interfacial reaction.

Diffusion of Hydrogen along the Grain Boundaries in Ni_3Al Alloys

The diffusivity of hydrogen in two Ni3Al alloys (No.1 and No.2) has been measured in the temperature range of 100 degreesC to 420 degreesC using an ultrahigh vacuum gaseous permeation technique. The diffusivity data fall into two segments, in which the hydrogen diffusivity adheres to the Arrhenius form, respectively. From the hydrogen diffusivity, it is conjectured that the hydrogen diffusivity reflects the hydrogen transportation along the grain boundaries at lower temperature and the hydrogen transportation in the lattice at higher temperature. The intergranular fracture of Lit-type intermetallics induced by hydrogen at relative low temperature results from hydrogen transportation along the grain boundaries and not in the lattice.

Integrated Computational Materials Engineering for the Development and Design of High Modulus Al Alloys

Integrated computational materials engineering(ICME)is to integrate multi-scale computational simulations and key experimental methods such as macroscopic,mesoscopic,and microscopic into the whole process of Al alloys design and development,which enables the design and development of Al alloys to upgrade from traditional empirical to the integration of compositionprocess-structure-mechanical property,thus greatly accelerating its development speed and reducing its development cost.This study combines calculation of phase diagram(CALPHAD),Finite element calculations,first principle calculations,and microstructure characterization methods to predict and regulate the formation and structure of composite precipitates from the design of highmodulus Al alloy compositions and optimize the casting process parameters to inhibit the formation of micropore defects in the casting process,and the final tensile strength of Al alloys reaches420 MPa and Young's modulus reaches more than 88 GPa,which achieves the design goal of the high strength and modulus Al alloys,and establishes a new mode of the design and development of the strength/modulus Al alloys.

Effect of Y on microstructure evolution and mechanical properties of Mg−4Li−3Al alloys

To obtain magnesium alloys with a low density and improved mechanical properties,Y element was added into Mg−4Li−3Al(wt.%)alloys,and the effect of Y content on microstructure evolution and mechanical properties was investigated by using optical microscopy,scanning electron microscopy and tensile tests.The results show that mechanical properties of as-cast Mg−4Li−3Al alloys with Y addition are significantly improved as a result of hot extrusion.The best comprehensive mechanical properties are obtained in hot-extruded Mg−4Li−3Al−1.5Y alloy,which possesses high ultimate tensile strength(UTS=248 MPa)and elongation(δ=27%).The improvement of mechanical properties of hot-extruded Mg−4Li−3Al−1.5Y alloy was mainly attributed to combined effects of grain refinement,solid solution strengthening and precipitation strengthening.

Deformation mechanism and forming properties of 6061Al alloys during compression in semi-solid state

The 6061 semi-solid aluminium alloy feedstocks prepared by near-liquidus casting were compressed in semi-solid state by means of Gleeble-3500 thermal-mechanical simulator.The relationship between the true stress and the true strain at different temperatures and strain rates was studied with the deformation degree of 70%.The microstructures during the deformation process were characterized.The deformation mechanism and thixo-forming properties of the semi-solid alloys were analyzed.The results show that the homogeneous and non-dendrite microstructures of semi-solid 6061Al alloy manufactured by near-liquidus casting technology could be transformed into semi-solid state with the microstructure suitable for thixo-forming which are composed of near-spherical grains and liquid phase with eutectic composition through reheating process.The deformation temperature and strain rate affect the peak stress significantly rather than steady flow stress.The resistance to deformation in semi-solid state decreases with the increase of the deformation temperature and decrease of the strain rate.At steady thixotropic deformation stage, the thixotropic property is uniform, and the main deformation mechanism is the rotating or sliding between the solid particles and the plastic deformation of the solid particles.

Wear behavior of Ti6Al4V biomedical alloys processed by selective laser melting, hot pressing and conventional casting

The aim of this work was to study the influence of the processing route on the microstructural constituents,hardness andtribological(wear and friction)behavior of Ti6Al4V biomedical alloy.In this sense,three different processing routes were studied:conventional casting,hot pressing and selective laser melting.A comprehensive metallurgical,mechanical and tribologicalcharacterization was performed by X-ray diffraction analysis,Vickers hardness tests and reciprocating ball-on-plate wear tests ofTi6Al4V/Al2O3sliding pairs.The results showed a great influence of the processing route on the microstructural constituents andconsequent differences on hardness and wear performance.The highest hardness and wear resistance were obtained for Ti6Al4Valloy produced by selective laser melting,due to a markedly different cooling rate that leads to significantly different microstructurewhen compared to hot pressing and casting.This study assesses and confirms that selective laser melting is potential to producecustomized Ti6Al4V implants with improved wear performance.

Crystallographic study of Al_3Zr and Al_3Nb as grain refiners for Al alloys

The potency of Al3Zr and Al3Nb as grain refiners for Al alloys was investigated from a crystallographic point of view using the edge-to-edge matching （E2EM） model. The results show that both Al3Zr and Al3Nb have small values of interatomic spacing misfit and interplanar spacing mismatch with respect to Al. Furthermore, energetically favourable orientation relationships predicted by the model exist between Al and each of these two intermetallic phases. In the light of the edge-to-edge matching model predictions, it is suggested that both Al3Zr and Al3Nb are potent heterogeneous nucleation refiners for Al grains from the crystallographic point of view. The present crystallographic analysis provides a more reasonable explanation for the significant grain refinement obtained in the peritectic Al-Zr and Al-Nb alloys and also provides fresh insight into the understanding of the grain refinement mechanism of Al alloys.

Optimum traveling and rotation speeds in friction stir welding for dissimilar Al alloys

Aluminum alloys are subjected to large deformation and decreased strength due to the high expansion modulus caused by heat effects during friction stir welding （FSW）.The optimum conditions for friction stir welding of 5052-O and 6061-T6 Al alloys were determined.The optimum traveling and rotation speeds were identified to be 61mm/min and 1600r/min using various mechanical characteristic evaluation methods.

Correlation of the Mechanical Properties of Polycrystalline Ni_3Al Alloys with Their Electronic Structure

The electronic structure of Ni_3Al alloys with different B contents has been investigated by measuring the positron lifetime spectra.The segregation of B atoms to defects could form strong covalent bondings with Ni and Al atoms and make the electronic structure in those locations similar to that in bulk,thus strengthen their cohesion.The interaction of B atoms,which were solid-solutioned in the crystal lattice of Ni_3Al in a manner of occupying interstitial sites,with Ni and Al atoms resulted in the increase of the density of valent electrons,thus increased the bonding cohesion in bulk.The im- provement of the mechanical properties of Ni_3Al alloys by B doping was due to both“grain bounda- ry effect”and“bulk effect”of B,which correlated with their electronic structure.

CERIUM-INDUCED GRAIN BOUNDARY STRENGTHENINGOF Ni_3Al ALLOYS

The infiuence of cerium on grnin boundaries of polycrpstalline N3Al alloys has beenstudied. The ja6mcability of a Ce-doped N3Al alloy strongly depends on the certumcontent. The alloy exhibits very poor fabrica6ility and cannot be fabrtcated into goodsheet8 by cold rolling when the concentmtion of cerium is <0. 0088wt% or>0.044wt%.A 0.011wt%Ce-doped N3Al alloy exhibits relatively good fabricability and may be fab-ricated into useful sheets by repeated cold rolling with 1OOa C annealing. The ductilityof the alloy strongly depends on the cerium distribution. Wthout the grain boundaryprecipitation of a second phase containing cer1um) the higher the segmpation level ofcerzum, the 6ettcr the tensile properties of the alloys, and in turn the more enhancedthe grain boundary cohesion.

EFFECTS OF Cr AND Nb ON MICROSTRUCTURES AND MECHANICAL PROPERTIES OF Fe_3Al ALLOYS

The effects of Cr and Nb on tensile properties at room and elevated temperatures and also on the microstructure have been investigated.An improvement in ductiliy plasicity and the cleavage strength of Fe_3AI may result from adding Cr;the improvement is more pronounced at over 400℃.The formation of Fe_2Nb containing A1 precipitates in the Fe_3AI+Nb alloy provide significant strengthening effect.Additions of Cr and Nb in Fe_3AI can cause a signifi- cant improvement in the yield strength at 550℃.

MICROSTRUCTURE STUDY OF CONSTITUENT PHASES IN 2024 SERIES Al ALLOYS

X-ray microanalysis,convergent beam electron diffraction(CBD)and selected area electron diffraction(SAD)studies on the structures and compositions of the constituent phases in 2024 series Al alloys have been conducted.Partial substitution of alloying elements is found to occur in all the constituent phases,which cause small deviations from the stoichiometric com- positions reported in these ternary compounds.The dominant phase is α-Al_(12)(FeMn)_3Si which has a body center cubic crystal structure with the Im■ space group and a=1.25 nm.The next dominant phase is Cu_2FeAl_7 which has a primitive tetragonal crystal structure with the P4/mnc space group and a=0.6336 nm,c=1.487 nm.The minor phase is α'-Al_(12)Fe_3Si hav- ing α primitive cubic crystal structure with the Pm■ space group and α=1.27 nm.

MECHANISMS AND PREDICTION OF FRACTURE TOUGHNESS ANISOTROPY OF HIGH STRENGTH Al ALLOYS

Contributions of weak grain boundary,cracking path deflection and grain boundary delamination to fracture toughness anisotropy of high strength Al alloys were evaluated based upon approaches of fracture mechanics in conjunction with physical cracking mechanisms. The predicted results are close to those experimentally determined in the literature and in this work.The strong anisotropy of fracture toughness of high strength Al alloys is therefore attri- buted mainly to weak grain boundary cracking,cracking path deflection and grain boundary detamination.With the methods of this work,short-transverse fracture toughness values of some semi-products can he estimated from in-plane toughness values and corresponding frac- ture characteristics when it is difficult to be determined experimentally.

THE EXPERIMENTAL STUDIES OF NiO CLUSTER FORMATION INNi_3Al ALLOYS BY AP-FIM

The purpose of the present work is to study the NiO cluster formation in Ni3Al alloys by field ion microscope and atom probe (AP-FIM). A polycrystal Ni3Al (B-doped) was heat-treated in atmospheres, the surface adsorption of air (hydrogen, oxygen) moisture etc.) occured on the Ni3Al surface and then these absorbents diffused into the interior of alloy through groin boundaries. AP-FIM studies found that the NiO and AlO clusters appeared in the local regions and amount of NiO clusters is much more than that of AlO. Moreover the hydrogen was simultaneously detected in the identical region.These results provided an experimental evidence that the formation of NiO and AlO clusters is the result of reaction of Ni (or Al) with residual moisture in Ni3Al, i.e. Ni+H2 O→NiO+2H. But the samples of B-free Ni3Al and B-doped single crystal Ni3Al have low concentration of NiO and H. This result shows that the diffusion of H2O molecule was promoted by genie boundaries containing boron. In addition, the boron suppresses environmental emvironmental was discussed, which suggested that the formation of Ni-O bonding has influence on bonding character of Ni-Al atoms and benefits the ductility of alloy.

SUPERPLASTICITY IN LARGE-GRAINED Fe_3Al ALLOYS

The superplasticity behavior of Fe-28Al, Fe-28Al-2Ti, Fe-28Al-4Ti (all composition reported in this paper are in atomic percent) alloys has been investigated by tensile testing.optical microscopy and transmission electron mocroscopy.Tensile test were performed at 700 to 900℃ under a strain rate range of about 10-5/s to 10-2/s. Maximum strain rate sensitivity index m was found to be 0.5 and the largest elongation reached 620%. The flow activation energy was measured to be 263kJ/mol for Fe-28Al and 191kJ/ mol for Fe-28Al-2Ti, which are much lower than the creep activation energy generally observed in Fe3Al alloys. After deformation grain size became much finer from about 100 μm to 20-30μm.As combined with TEM observations, we suggested that a continuous recrystallization process took place and superplasticity may arise from this process.

Effect of alloying elements on thermal stability of nanocrystalline Al alloys

The effect of incorporating limited-diffusivity elements such as Fe and Ti on thermal stability of the nanocrystalline Al alloy was investigated.Al−10wt.%Fe and Al−10wt.%Fe−5wt.%Ti alloys were fabricated.The initial mixtures of powders were milled for 100 h in vacuum.The bulk samples were fabricated from the milled powders in a high frequency induction heat sintering(HFIHS)system.The milled powders and the bulk sintered samples were characterized by X-ray diffraction(XRD),Vickers microhardness,field emission scanning electron microscopy(FESEM-EDS)and transmission electron microscopy(TEM).The observations indicated that Fe and Ti were completely dispersed in the matrix to form a supersaturated solid solution(SSSS)with Al.Additionally,the inclusion of alloying elements led to an increase in hardness and yield strength of the alloy by 127%and 152%,respectively.The elevated temperature compression tests were carried out to evaluate the thermal stability of the alloys.The Al−10wt.%Fe−5wt.%Ti alloy revealed the optimum thermally stable behavior of the three alloys studied.The incorporation of Fe and Ti improved the thermal stability of the developed alloys through inhibiting the grain growth,hindering dissolution and growth of second phases(such as Al13Fe4 and Al13Ti),and forming a stable solid solution.