Microstructures,micro-segregation and solidification path of directionally solidified Ti-45Al-5Nb alloy

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s^(-1) under the temperature gradients of 15-20 K·mm^(-1), and the primary phase is transformed into an α phase at relatively higher growth rates(V >20 μm·s^(-1)). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s^(-1) under a temperature gradient of 15 K·mm^(-1). The increase of temperature gradient to 20 K·mm^(-1) can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L+β L+β→αα+β_R β→ααα→γα+γα→α_2+γγ_R+(α_2+γ), in which β_R and γ_R mean the residual β and

Eliminating shrinkage defects and improving mechanical performance of large thin-walled ZL205A alloy castings by coupling travelling magnetic fields with sequential solidification

ZL205 A alloys with large thin-walled shape were continuously processed by coupling travelling magnetic fields(TMF)with sequential solidification,to eliminate the shrinkage defects and optimize the mechanical performance.Through experiments and simulations,the parameter optimization of TMF and the influence on feeding behavior,microstructure and properties were systematically studied.The results indicate that the magnetic force maximizes at the excitation current of 20 A and frequency of 200 Hz under the experimental conditions of this study,and increases from center to side-walls,which is more convenient to process thin-walled castings.TMF can break secondary dendritic arm and dendrites overlaps,widen feeding channels,prolong the feeding time,optimize the feeding paths,eliminate shrinkage defects and improve properties.Specifically,for as-cast state,TMF with excitation current of 20 A increases ultimate tensile strength,elongation and micro-hardness from 186 MPa,7.3%and 82.1 kg/mm^(2) to 221 MPa,11.7%and 100.5 kg/mm^(2),decreases porosity from 1.71%to 0.22%,and alters brittle fracture to ductile fracture.

Improvement of microstructure and mechanical properties of TiAl-Nb alloy by adding Fe element

In order to improve mechanical properties and optimize composition of TiAl-Nb alloys, Ti46 Al5 Nb0.1 B alloys with different contents of Fe(0, 0.3, 0.5, 0.7, 0.9, and 1.1 at.%) were prepared by melting. Macro/microstructure and compression properties of the alloys were systematically investigated. Results show that Fe element can decrease the grain size, aggravate the Al-segregation and also form the Fe-rich B2 phase in the interdendritic area. Compressive testing results indicate that the Ti46 Al5 Nb0.1 B0.3 Fe alloy shows the highest ultimate compressive strength and fracture strain, which are 1869.5 MPa and 33.53%, respectively. The improved ultimate compression strength is ascribed to the grain refinement and solid solution strengthening of Fe, and the improved fracture strain is due to the reduced lattice tetragonality of γ phase and grain refinement of the alloys. However, excessive Fe addition decreases compressive strength and fracture strain, which is caused by the severe Al-segregation.

Microstructure and properties of novel quinary multi-principal element alloys with refractory elements

Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.

Microstructural evolution of Al-Cu-Li alloys with different Li contents by coupling of near-rapid solidification and two-stage homogenization treatment

Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density.In this regard,the Al-Cu-Li alloy castings with high Li content from 1.5 wt.%to 4.5 wt.%were prepared by near-rapid solidification,followed by two-stage homogenization treatment(490℃/16 h and 530℃/16 h).The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software.The results indicate that with the increase of Li content,the grain sizes decrease,the solution ability of Cu in the matrixα-Al phase increases,while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification.Additionally,by the coupling of near-rapid solidification and two-stage homogenization,the metastable precipitated phases(Al7Cu4Li and AlCu3)can be dissolved effectively in the alloys with Li content of 1.5 wt.%-2.5 wt.%;moreover,the stable precipitated phases(Al6CuLi3 and Al2CuLi)uniformly distribute at the grain boundaries in the alloys with Li content of 3.5 wt.%-4.5 wt.%.As a result,the refined and homogenized microstructure can be obtained.

Effect of Zr on microstructure and mechanical properties of binary TiAl alloys

Ti43Al and Ti47Al alloys with different contents of zirconium were prepared by non-consumable vacuum arc melting furnace.The microstructure and mechanical properties were investigated.The results showed that Zr had no obvious effect on microstructure morphology of Ti43Al,while that of Ti47Al was modified from dendrites into equiaxed grains.The addition of Zr could refine the grains.Zr promoted the formation ofγphase significantly and the solubility values of Zr inγphase were 12.0%and 5.0%(molar fraction)in Ti43Al and Ti47Al,respectively.Zr-richγphase mainly formed throughβ→γin Ti43Al-xZr(molar fraction,%)andβ→α→γin Ti47Al-xZr(molar fraction,%).Fine-grain strengthening and solution strengthening were beneficial to improving the compressive strength while severe micro-segregation was detrimental to compressive properties.Large solubility of Zr was bad for ductility of alloys as well.The maximum compressive strengths of Ti43Al-xZr and Ti47Al-xZr were 1684.82 MPa(x=5.0%)and 2158.03 MPa(x=0.5%),respectively.The compressive strain fluctuated slightly in Ti43Al-xZr and reached the maximum value of 35.24%(x=0.5%)in Ti47Al-xZr.Both alloys showed brittle fracture.

Effect of growth rate on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy

The effect of growth rates (V=2-50 μm·s-1) on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B (at.%) alloy at a constant temperature gradient (G=18 K·mm-1) was investigated. Results indicated that β phase was the primary phase of the directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy. As the growth rate increases, the solid/liquid interface turns from cellular growth to dendric growth. The interlamellar spacing (λs) decreases with the increase of growth rate according to the relationship of λs=3.39V -0.31. The solidification segregation occurs due to the enrichment of β-stabilizing element Nb, Cr in primary β phase during solidification;moreover, the degree of the segregation increases with the growth rate, resulting in the emergence of B2 phase in lamellar colonies at high growth rates. The microhardness (Hv) grows with the growth rate based on the equation of HV=328.69V 0.072, which mainly attributes to the microstructure refinement.

Influence of laser parameters on segregation of Nb during selective laser melting of Inconel 718

A transient three-dimensional powder-scale model was established for understanding the flow field and mass transfer within the molten pool during the selective laser melting(SLM)of Inconel 718 alloy by considering some important physical phenomena,such as,a transition from powder to solid,nonlinearities produced by temperature-dependent materials’properties,and fluid flow in the calculation.The influence of laser power or scanning speed on the flow field and cooling rate was discussed in detail.The simulation results reveal that the motion of molten pool and higher cooling rate promote the mass transfer and benefit the solute distribution by increasing laser power.However,with increasing the scanning speed,the melt flow speed and cooling rate are elevated,resulting in an agglomeration of the solute elements,which is ascribed to the shorter dwelling time of liquid.Therefore,the segregation of Nb can be effectively suppressed by increasing laser power or decreasing scanning speed,which can decrease the dwelling time of liquid.

Chemical composition analysis on industrial scale ingots and castings of TiAl alloys

The chemical composition variation of the TiAl-4722 alloys was examined in a batch of the industrial scale master ingots,and in the corresponding castings prepared by conventional vacuum arc remelting(VAR)combined with induction skull melting(ISM)and investment casting processes.The content changes of major elements and interstitial elements were evaluated based on the chemical analysis at the top and bottom of the ingots and castings.Results show that the contents of C,N,H,Fe and Si have almost no change in the ingots and castings,suggesting that the chemical analysis on these elements can be based on the batch analysis.The O content keeps almost the same in different ingots,but exhibits relatively large differences in castings,which was probably influenced by the reaction between the shell mold and the molten alloy,and the spalling of face coat of the shell mold during casting.For the major elements of Al,Nb and Cr,the composition difference between the top and the bottom of the ingots is less than that of the castings.But for the O element,the trend is different,especially for the castings,suggesting that the investment casting is a homogenization process for Cr and Nb,but a differentiation process for O.The contents of major elements in castings fluctuate mainly in the same range as that in the ingots,indicating that the contents of the major elements are controllable during investment casting.

Microstructures and phase transformation in directionally solidified TiAl-Nb alloys

Intermetallic Ti-45Al-8Nb-(W,B,Y)(at.%)and Ti-46Al-5Nb alloys are directionally solidified at a constant growth rate of 30μm·s-1 using a Bridgman type apparatus.The quenched microstructures and lengths of different phase regions were observed and measured after various growing times of 0-30 min.Results show that the phase transformations in different phase regions are mainly depending on the high temperature microstructure and the supercooling degree during quenching process.After isothermal holding,the primary phase grows into the liquid phase,the dendrites change from equiaxed to columnar grains,and the length of the L+βphase region,L+β+αphase region and mushy zone varies,indicating that the entire directional solidification process can be described by a static equilibrium-nonequilibrium-dynamic equilibrium evolution process.In addition,the gap between the original growth interface and front interface shows that the actual crystal growth rate is not equal to the drawing velocity during directional solidification.

Effect of excitation current intensity on mechanical properties of ZL205A castings solidified under a traveling magnetic field

The effect of excitation current intensity on the mechanical properties of ZL205 A castings solidified under a traveling magnetic field was studied. The results of the experiment indicate that the excitation current intensity of the traveling magnetic field has a great influence on the mechanical properties of the ZL205 A castings. When the excitation current intensity is 15 A, the tensile strength and elongation of ZL205 A alloy castings increase 27.2% and 67.7%, respectively, compared with those of the same alloy solidified under gravity. The improvement of mechanical properties is attributed to the decrease of micro-porosity in the alloy. Under the traveling magnetic field, the feeding pressure in the alloy melt before solidification can be enhanced due to the electromagnetic force. Moreover, the melt flow induced by the traveling magnetic field can decrease the temperature gradient. The feeding resistance will be increased because the temperature gradient decrease. So traveling magnetic field has an optimum effect on feeding.

Microstructure and microhardness of Ti-48Al alloy prepared by rapid solidification

To improve the microstructure and microhardness,Ti-48Al(at.%)alloy was rapidly solidified by melt spinning under different cooling rates.The microstructure and microhardness of rapidly solidified Ti-48Al alloy were systematically investigated.Results show that the average lamellar colony size of the alloy reduces from 60.6μm to 11μm as the cooling rate increases from 2.3×105 to 5.1×105 K·s-1,caused by the increase of nucleation rate at a higher cooling rate.At the high cooling rate of(4.3-5.1)×105 K·s-1,theαphase is the primary phase,and a few metastableαphases are reserved,which then transform intoα2 phase and subsequently lead to the formation ofα2 equiaxed grain.The lamellar spacing also decreases with the increase of cooling rate.The relationship between lamellar spacing(d)and cooling rate(v)is d=33.6v-1.34.The microhardness increases with the increase of cooling rate because the refined lamellar spacing and grain size can improve the microhardness.

Impact of laser scanning speed on microstructure and mechanical properties of Inconel 718 alloys by selective laser melting

Inconel 718 alloys were fabricated by selective laser melting under different scanning speeds to investigate the change of the morphology of molten pool,direction of grain growth,and tensile properties.Results show that as the scanning speed increases from 1,000 to 1,450 mm·s^(-1),the ratio between depth and width of molten pool increases,yet their overlapping regimes decrease.Meanwhile,increasing scanning speed can promote the solidified structure evolve from cell to columnar dendrites,and decrease the dendrite spacing from 0.54 to 0.39 μm;the average columnar grain size also decreases from 84.42 to 73.51 μm.At different scanning speeds,the preferred orientation of grains along the building is mainly <001> direction.In addition,the tensile properties of samples under different scanning speeds present a non-monotonic transition.The maximum ultimate tensile strength and elongation can reach 1,014±19 MPa and 19.04±1.12 (%),respectively,at the scanning speed of 1,300 mm·s^(-1).

Hydrogen-induced amorphization of Zr-Cu-Ni-Al alloy

Arc melting was utilized in this study to produce Zr_(55)Cu_(30)Ni_5Al_(10) alloys under mixed atmospheres with various ratios of high-purity hydrogen to argon. The influences of hydrogen addition on the solidification structure and glass-forming ability of Zr_(55)Cu_(30)Ni_5Al_(10) alloy were determined by examining microstructures in different parts of the cast ingots. The results showed that different degrees of crystallization structures were obtained in the ascast button ingots after arc melting in high-purity Ar, and the cross-sectional solidification morphology of arcmelted ingots was found to consist of crystals with varying from the bottom up. By contrast, there were completely amorphous structures in the middle and upper areas of the as-cast button ingots fabricated by adding 10% H_2 to the high-purity Ar atmosphere. A clear solidification interface was found between the crystal and glass in the ascast button ingots, which indicates that hydrogen addition can enhance the Zr_(55)Cu_(30)Ni_5Al_(10) alloy's glass-forming ability. The precise mechanism responsible for this was also investigated.

Enhancement of comprehensive properties of Nb-Si based in-situ composites by Ho rare earth doping

This study examines the microstructure,mechanical properties(with a focus on room-temperature toughness),and oxidation resistance of Ho-doped NbDSi based in-situ composites.The base alloy consists of the coarse primary Nb_(5)Si_(3)phase and the Nb_(5)Si_(3)+Nbss(Nb solid solution)eutectic cells.Ho doping influences the solidification path.When the Ho doping is higher than0.2 at%,the alloys transform into eutectic alloys.Ho can be solid-solved in trace amounts in the Nbss phase.However,most of Ho forms a stable Ho oxide phase,which alleviates oxygen contamination problem to some extent.Moreover,the interface separation between Ho oxide and other phases reduces the plastic deformation constraint.Thus,with 0.4 at%Ho doping,the K_(Q)value is18.03 MPa·m^(1/2),which is 31.1%higher than that of the base alloy.The strength of the Ho-doped alloys does not deteriorate with an increase in toughness.However,the large network-like Ho_(2)O_(3)in the 0.8Ho alloy causes a decrease in toughness and strength.In addition,the Ho oxide phase effectively blocks the inward oxygen intrusion.With 0.8 at%Ho doping,the oxidation mass gain per unit area is 10.16 mg·cm^(2),which is 39.7%lower than that of the base alloy.

Microstructure and mechanical properties of Ti44Al6Nb1Cr2V alloy after gaseous hydrogen charging at 1373-1693 K

The hydrogenation behavior of Ti44A16Nb1Cr2V(at%)alloy at temperature range of 1373-1693 K and its effect on microstructure and room-temperature mechanical properties were studied systematically in this study.The results show that hydrogen content increases with the increase in temperature,and the maximum hydrogen content is 0.126 wt%at 1693 K.The heat of solution of hydrogen is calculated as 82.9 kJ·mol^(-1)by curve fitting,indicating that hydrogen absorptionin TiAl alloys is endothermic.Hydrogen promotes the lamellar colony size because hydrogen promotes the diffusion of elements.Hydrogen stabilizes B2phase during hydrogenation resulting in more residual B2phase in the hydrogenated alloy.The nanohardness and elastic modulus decrease after hydrogenation due to that hydrogen weakens the bonds.The Ti44A16Nb1Cr2V alloy exhibits higher plasticity and lower flow stress hydrogenation with 0.039 wt%H,and the ultimate compressive strength decreases from 1220 to 1130 MPa,while the fracture strain is enhanced by 26%.

Microstructure and mechanical properties of Ti43Al6Nb alloys with different zirconium contents

Ti43Al6Nb-xZr alloys with different additions of zirconium were prepared by vacuum arc melting furnace.The microstructure and compressive properties at room temperature(RT) were investigated.The microstructure shows dendrites with addition of 0 at%-2.5at% Zr,and the dendrites are refined with the primary dendrite arms spacing decreasing from 222.64 μm(0 at%Zr) to 92.57 μm(2.0 at% Zr).With Zr addition more than2.5 at%,the microstructure shows equiaxed grains surrounded by y phase.Zr is a y stabilizer and promotes the β/y transition,resulting in the change of microstructure morphology.Zr reaches the maximum solid solubility(about 6.5 at%) in y phase with addition of 2.5 at% Zr;moreover,γ phase increases in quantity,bringing about severe micro-segregation.With addition of Zr,the remained β phase turns into ω phase with B82 structure.Ti43Al6Nb-xZr alloys show brittle fracture.The maximum compressive strength is 2161.69 MPa with addition of 2.5at% Zr and the maximum compressive strain is 30.62%with addition of 0.5 at% Zr,improving by 9.24% and7.33%,respectively.The improvement of compressive strength results from fine-grain strengthening and solution strengthening.Severe micro-segregation is bad for compressive strength,and large solubility of Zr is detrimental to ductility.

Decomposition of Nb_(3)Si and mechanical-property improvement by adding appropriate amount of MgO in Nb-16Si-20Ti alloy

Five Nb-16Si-20Ti-xMgO alloys(x=0,0.1,0.5,1.0 and 3.0)were prepared via arc melting in this study,and the effect of MgO addition on their phase composition,microstructure evolution,and mechanical properties was examined.The results demonstrated that MgO reacted with the Nb-Si-Ti alloy while Mg atoms replaced Nb atoms in the Nbss phase.The hypoeutectic alloy was transformed into a hypereutectic alloy upon the addition of 3.0 at%MgO,and the Nb_(3)Si phases decomposed into a fine Nbss/α-Nb5Si3 eutectic structure.The highest fracture toughness was achieved for the Nb-16Si-20Ti-3MgO alloy,with an ambient fracture toughness value of 9.4 MPa·m^(-1/2)due to its largest Nbss phase content and optimal Nbss/α-Nb_(5)Si_(3)eutectic structure.Furthermore,the alloy compressive strength increased with MgO addition.The compressive strength of the Nb-16Si-20Ti-3MgO alloy was 2624.1 MPa,26.0%higher than that of Nb-16Si-20Ti,due to the formation of a solid solution of Mg atoms in the Nbss phase and reinforcement of a small amount of the Ti_(2)O phase.Finally,an increase in the content of the Nbss/α-Nb_(5)Si_(3)eutectic structure increased both the alloy strength and fracture strain.

Microstructures and properties of Nb-Si-based alloys with B addition

The Nb-16Si-18Ti-xB(at%,similarly hereinafter,x=0,1,2,3)alloys were prepared by arc melting in a water-cooled copper crucible.The influences of B addition on their microstructures and properties were based on the data of X-ray diffraction(XRD),field emission scanning electron microscopy(FESEM),and electronic universal material testing machine.It is found that the addition of B promotes the formation ofα-Nb5Si3phase and suppresses the formation of Nb3Si phase.B addition also tends Nb-16Si-18Ti alloy to form the hypereutectic structures.The content of silicide phases shows a trend of firstly decreasing and then increasing in Nb-16Si-18Ti-xB(x=0,1,2,3)alloys.The size of Nb solid solution(Nbss)phase increases in Nb-16Si-18Ti-xB(x=0,1,2,3)alloys after heat treatment at 1523 K for 10 h.The room temperature compression strength of Nb-16Si-18Ti alloy increases firstly and then decreases with B addition.The high-temperature compression strength of Nb-16Si-18Ti alloy decreases firstly and then increases with B addition.It is found that the volume and size of silicide phases have a synergistic effect on the compression strength of Nb-TiSi-based alloys.

Microstructure and mechanical properties of NbZrTi and NbHfZrTi alloys

Two medium-entropy alloys,NbZrTi and NbHfZrTi,were prepared by arc melting.Both NbZrTi and NbHfZrTi alloys are composed of simple body-centered cubic(bcc)solid solution phase and exhibit dendritic structure.After being homogenized,both NbZrTi and NbHfZrTi alloys are still composed of the single bcc solid solution phase,but the microstructure of the two alloys transforms from the dendritic structure into the polycrystalline structure.Two alloys display significantly workhardening effect during compression at room temperature and show relatively good deformation plasticity during compressive deformation at room temperature.For NbZrTi and NbHfZrTi alloys,the dynamic recrystallized grains form along the boundary during compression at the temperatures of 1073 and 1273 K.