Ti、Zr和Hf元素协同优化Nb-Si基合金显微组织、相组成和室温断裂韧性

采用真空电弧熔炼制备Nb-16Si-xTi-yZr-zHf (x=18, 22;y=0, 4;z=0, 4;摩尔分数,%)合金并研究Ti、Zr与Hf元素对Nb-Si合金物相组成、显微组织、断裂韧性以及裂纹扩展行为的影响。结果表明:单独添加4%Zr元素能促进(Nb,X)3Si向Nb固溶体(Nbss)/γ-(Nb,X)5Si3共析反应的发生;同时添加Ti、Zr、Hf元素能进一步促进共析反应的发生。裂纹倾向于在(Nb,X)3Si中扩展,当裂纹途径Nb固溶体时会发生偏转。细密的Nbss/γ-(Nb,X)5Si3共晶组织以及层状Nbss/γ-(Nb,X)5Si3共晶组织可以使裂纹产生桥接与分支,阻碍裂纹的扩展。Nb-16Si-22Ti-4Zr-4Hf的合金化元素含量最高,因此,其室温断裂韧性最好(11.62 MPa·m1/2),相较于Nb-16Si-18Ti提高87.7%,性能的提升主要归因于存在层状共晶组织。

异质结构CoCrFeNiW高熵合金的拉伸性能和强化行为

采用真空电弧熔炼方法制备Co_(30)Cr_(30)(FeNi)_(40-x)W_(x)(x=0~8%(摩尔分数),分别简化为HWO~HW8)高熵合金。研究铸态和退火态合金的显微组织和拉伸性能。结果表明,HW2和HW4具有单一的FCC相。随着W含量和退火温度的增加,细小粒状μ相的面积分数增加且分散在FCC基体中。软FCC基体和硬μ相构成应变不相容的异质结构。随着W含量从0增加到8%(摩尔分数),屈服强度和抗拉强度分别从278和629 MPa提高到530和839 MPa,应变维持在33%。退火后的HW8表现出优异的屈服强度(810 MPa)和抗拉强度(1087 MPa)。屈服强度的提高归因于固溶、沉淀和背应力强化。异质结构中产生的背应力强化作用诱导高硬化行为,在提高抗拉强度和塑性方面发挥着主导作用。

在两步CeO_(2)修饰的多孔YSZ-Al_(2)O_(3)管上制备高渗透性和热稳定性Pd膜

采用化学镀技术在经过不同修饰方式后的多孔YSZ-Al_(2)O_(3)管上沉积Pd膜。采用SEM、AFM、XRD和气体渗透测试方法研究不同修饰方式对多孔YSZ-Al_(2)O_(3)管表面质量及Pd复合膜渗透性能的影响。结果表明,经两步CeO_(2)修饰后,多孔YSZ-Al_(2)O_(3)管表面具有更小的孔径分布和粗糙度。经两步CeO_(2)修饰后的多孔管上沉积的Pd膜在500℃、700 kPa压差下具有更高的氢渗透流量(0.549 mol·m^(-2)·s^(-1))和H_(2)/N2选择性(14241)。不同热循环测试和1000h持久渗透测试结果表明,在经两步CeO_(2)修饰后的多孔管上沉积的Pd膜具有较高的渗透稳定性。

Effects of La on microstructure and mechanical properties of NbMoTiVSi0.2 refractory high entropy alloys

To study the effects of La on the microstructure and mechanical properties of refractory high entropy alloys,NbMoTiVSi0.2 alloys with different La contents were prepared.Phase constitution,microstructure evolution,compressive properties and related mechanisms were systematically studied.Results show that the alloys with La addition are composed of BCC solid solution,eutectic structure,MSi2 disilicide phase and La-containing precipitates.Eutectic structure and most of La precipitates are formed at the grain boundaries.Disilicide phase is formed in the grains.La can change the grain morphologies from dendritic structure to near-equiaxed structure,and the average grain size decreases from 180 to 20μm with the increase of La content from 0 to 0.5 at.%.Compressive testing shows that the ultimate strength and the yield strength increase with the increase of La content,which is resulted from the grain boundary strengthening.However,they cannot be greatly improved because of the formation of MSi2 disilicide phase with low strength.The ductility decreases with the increase of La content,which is due to the La precipitates and brittle MSi2 disilicide phase.

Dynamic recrystallization and silicide precipitation behavior of titanium matrix composites under different strains

In order to elucidate the microstructure evolution and silicide precipitation behavior during high-temperature deformation,TiB reinforced titanium matrix composites were subjected to isothermal hot compression at 950℃,strain rate of 0.05 s^(−1) and employing different strains of 0.04,0.40,0.70 and 1.00.The results show that with the increase of strain,a decrease in the content,dynamic recrystallization of theαphase and the vertical distribution of TiB along the compression axis lead to stress stability.Meantime,continuous dynamic recrystallization reduces the orientation difference of the primaryαphase,which weakens the texture strength of the matrix.The recrystallization mechanisms are strain-induced grain boundary migration and particle stimulated nucleation by TiB.The silicide of Ti_(6)Si_(3) is mainly distributed at the interface of TiB andαphase.The precipitation of silicide is affected by element diffusion,and TiB whisker accelerates the precipitation behavior of silicide by hindering the movement of dislocations and providing nucleation particles.

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.

A high-Nb TiAl alloy with highly refined microstructure and excellent mechanical properties fabricated by electromagnetic continuous casting

In the present research, microstructure refinement of a high-Nb TiAl alloy （Ti-48Al-8Nb-0.15B） was realized by means of the electromagnetic continuous casting （EMCC） technique. The microstructure of an ingot obtained by EMCC was analyzed using scanning electron microscopy （SEM）. As compared with the raw as-cast alloy, the obtained EMCC alloy presented a much finer microstructure with lamellar colonies with a mean size of about 50-70 μm because the electromagnetic stirring broke initial dendrites and enhanced the heterogeneous nucleation. As the grains were refined, the properties of the TiAl alloy were improved significantly. This implies that the EMCC technique could offer the possibility of application for high-Nb TiAl alloys with a refined microstructure and excellent properties to be used as a structural material.

Microstructure evolution and its effect on mechanical properties of cast Ti48Al6NbxSi alloys

In order to improve mechanical properties of TiAlNb alloys,different contents of silicon were added into Ti48Al6Nb alloy.The Ti48Al6NbxSi (x=0,0.1,0.2,0.3,0.4 and 0.5,at.%) alloys were prepared by vacuum arc melting.The phase constitution,microstructure evolution and mechanical properties of the alloys were studied.Results show that the Ti48Al6NbxSi alloys consist of γ-TiAl phase,α2-Ti3Al phase and B2 phase,and Ti5Si3 silicide phase is formed when the addition of silicon is higher than 0.3at.%.The addition of silicon leads to the decrease in γ phase and increase in α2 phase.The formation of silicide decreases the amount of Nb dissolved in the TiAl matrix,and therefore decreases B2 phase.Compressive tests show that the ultimate strength of the alloys increases from 2,063 MPa to 2,281 MPa with an increase in silicon from 0 to 0.5at.%,while the fracture strain decreases from 34.7% to 30.8%.The increase of compressive strength and decrease of fracture strain can be attributed to the decrease of B2 phase and the formation of Ti5Si3 phase by the addition of silicon.The strengthening mechanism is changed from solid solution strengthening when the addition of silicon is less than 0.3at.% to combination of solid solution strengthening and secondary phase strengthening when the addition of silicon is higher than 0.3at.%.

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.

Dependency of microstructure and microhardness on withdrawal rate of Ti-43Al-2Cr-2Nb alloy prepared by electromagnetic cold crucible directional solidification

The intermetallic Ti-43Al-2Cr-2Nb(at.%)alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates(V)ranging from 0.2 to 1.0 mm·min^(-1),at a constant temperature gradients(G=18 K·mm^(-1)).Macrostructures of the alloy were observed by optical microscopy.Microstructures of the alloy were characterized by scanning electron microscopy(SEM)in back-scattered electron mode and transmission electron microscopy.Results showed that morphologies of macrostructure depend greatly on the applied withdrawal rate.Continuous columnar grains can be obtained under slow withdrawal rates ranging from 0.2 to 0.6 mm·min^(-1).The microstructure of the alloy was composed ofα_(2)/γlamellar structures and a small number of mixtures of B2 phases and blockyγphases.The columnar grain size(d)and interlamellar spacing(λ)decrease with an increasing withdrawal rate.The effect of withdrawal rate on microhardness was also investigated.The microhardness of the directional y solidified Ti-43Al-2Cr-2Nb alloy increases with an increase in withdrawal rate.This is mainly attributed to the increase of B2 andα_(2) phases as well as the refinement of lamellae.

Effect of boron on microstructure and mechanical properties of cast Ti-44Al6-Nb ingots

In order to improve the mechanical properties of Ti Al alloys, especially the ductility at room temperature, and to study the effect of boron(B) on Ti Al alloys, different contents(0, 0.1, 0.3, 0.6, 0.9, 1.2, at.%) of B were added into Ti-44Al-6Nb alloys to prepare ingots. The surface quality, macrostructure, microstructure, compressive properties and fracture surface of the ingots were studied. The results show that B has little influence on the surface quality except that there are some dark spots on the surface when the content of B is 0.9%. B can refine the grains. The average grain size decrease from about 0.8 mm to 0.088 mm with increasing B content. Meanwhile, the grain morphology of these ingots changes from big equiaxed grains with lamellars to fine equiaxed grains. When the content of B is 1.2%, the primary Ti B2 phase forms in the liquid phase and increases the nucleation rate, leading to further refinement of the grains. The compressive testing results show that B can increase the strength and the ductility, the compressive strength and compressibility can reach 2,037.8 MPa and 26.7% from 1,156.2 MPa and 10.2% when the boron content is 0.6%, which is resulted from grain refining and grain boundary strengthening. It is found that the compressive strength and the compressibility are relatively stable when the B content is more than 0.3%.

Microstructure and mechanical properties of Ni3Al intermetallics prepared by directional solidification electromagnetic cold crucible technique

The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.

Effect of V and Sn on microstructure and mechanical properties of gray cast iron

The cylinder liner is one of the important parts of a diesel engine.Gray cast iron is the main material for manufacturing cylinder liners due to its good casting performance,convenient processing performance,good wear resistance and low cost.In the present work,the effects of vanadium(V)and tin(Sn)on the microstructure and properties of gray cast iron were studied.Results show that increasing the contents of V and Sn can not only refine the graphite,but also reduce the pearlite lamellar space.The graphite size and lamellar spacing of pearlite are firstly reduced and then increased.Pearlite quantity reaches over 98%after adding V and Sn.Adding V and Sn can promote the precipitation and solid solution strengthening of gray cast iron,so as to improve the mechanical properties.The Brinell hardness reaches the peak of 424 HB at the contents of 0.21wt.%V and 0.06wt.%Sn,and the sample containing 0.11wt.%V and 0.08wt.%Sn shows the highest compressive strength and tensile strength of 1,699 MPa and 515 MPa,respectively.The main strengthening mechanism comes from the solid solution strengthening and fine grain strengthening of V and Sn.

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.

High-temperature deformation resistance and creep resistance of a TiAl-based alloy fabricated by cold crucible directional solidification technology

In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys,cold crucible directional solidification(CCDS)technology was employed.Aβ-type TiAl-based alloy with the nominal composition of Ti44Al6Nb1Cr2V was prepared using the optimized CCDS parameters of 45 kW input power and 0.5 mm·min^-1 solidification rate.Thermo-compression testing was utilized to evaluate the hightemperature deformation resistance and creep resistance of the CCDS Ti44Al6Nb1Cr2V alloy.Results show that the CCDS Ti44Al6Nb1Cr2V alloy billets contain aligned columnar grains and a high percentage of small-angle lamellae.Thermo-compression testing results in the radial direction of the CCDS alloy show a much higher peak stress than other reported results in similar conditions.The much higher hardening exponent and deformation activation energy are obtained,corresponding to the excellent high-temperature deformation resistance and creep resistance,which are because of the hard-oriented grains,weaker stress-strain coordination capability of lamella structure and relatively more hysteretic dynamic recrystallization.Thermo-compression testing results in the longitudinal direction of the CCDS Ti44Al6Nb1Cr2V alloy show the much higher peak stress than that in the radial direction,indicating the better high-temperature deformation resistance and creep resistance attributed to the hard-oriented lamellae in this condition.

Strengthening CoCrFeNi high-entropy alloy by Laves and boride phases

To strengthen the face-centered-cubic(FCC)type CoCrFeNi high-entropy alloy(HEA)by in-situ reinforced phase,(CoCrFeNi)_(100-x)(NbB_(2))_(x)(x=0,2,4,6,8,at.%)alloys were prepared.Phase constitution,microstructure,tensile mechanical properties of the alloys were studied,and the mechanisms were discussed.Results show that the microstructure of all the reinforced alloys consists of the matrix FCC phase,Laves phase,and(Cr_(3)Fe)B_(x) phase.The eutectic structure and(Cr_(3)Fe)B_(x) phases are formed in the interdendritic region,and the eutectic structure is composed of Laves and FCC phases.When x increases from 0 to 8,i.e.,with increase of Nb and B elements,the volume fraction of Laves and(Cr_(3)Fe)B_(x) phases increases gradually from 0 to 5.84%and 8.3%,respectively.Tensile testing results show that the ultimate strength of the alloys increases gradually from 409 MPa to 658 MPa,while the fracture strain decreases from 75%to 1.6%.Fracture analysis shows that the crack originates from the(Cr_(3)Fe)B_(x) phase.The CoCrFeNi alloys are mainly strengthened by the second phase(Laves phase and boride phase).

Effect of TiO_(2)nano-ceramic particles on microstructure and mechanical properties of Al_(0.4)CoCrFe_(2)Ni_(2)high-entropy alloy

Al_(0.4)CoCrFe_(2)Ni_(2)high-entropy alloys with different additions of TiO_(2) nanoceramic particles(0,1.25vol.%,2.5vol.%,3.75vol.%and 5vol.%,respectively)were prepared by using the vacuum arc melting method.The effects of TiO_(2) addition on the crystal structure,microstructures and mechanical properties of the alloy were investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),and tensile testing.The microstructure analysis shows that the TiO_(2)nano-ceramic particles added in the alloy are decomposed,and a small amount of Al_(2)O_(3)and a great number of intermetallic compounds(γ'phases)with simple cube structure are formed.Theγ'phases are enriched at inter-dendrite,which increases the resistance of dislocation movement during the deformation of the alloy,thus balancing the problem of high plasticity and low strength of the alloy.When the addition of TiO_(2)is 2.5vol.%,the strength of the high-entropy alloy reaches the maximum of 489 MPa,which is 11.1%higher than the matrix alloy composed of single FCC phase.

Editorial

Compared with traditional titanium alloys,TiAl alloys have higher modulus and high temperature bearing capacity.Their densities are only 3.85-4.20 g·cm^-3,which is far lower than that of nickel based superalloys,and their specific strength is close to or even higher than that of nickel based superalloys.Therefore,TiAl alloys have become one of the potential materials in aerospace,automobile and other fields.In addition to the application in the field of rotating parts,such as aerospace engine blades,discs and aviation bearings,TiAl alloys are attempted to be used in the fields of non-rotating parts,such as exhaust valves,fuselages.

Introduction of rare-earth element Sc in alloy design to modify wear features of dual-phase high-entropy alloy

Tailoring the alloy composition,which induces the hard secondary phase to increase hardness and strength to improve the wear features,is a feasible approach for developing wear-resistant metal materials.Here,a group of(AlCoCrFeNi)_(100–x)Sc_(x)(x=0–2.0,at%)high-entropy alloys(HEAs)are designed and the phase compositions and wear behaviors are explored.Sc-doped HEA series contain the primary body-centered cubic(BCC)and eutectic phases,in which the eutectic phase is composed of the alternately grown BCC and Laves phases.Sc addition promotes the diffusion of Ni atoms from BCC phase to form the Sc-rich Laves phase at the grain boundaries.Vickers hardness increases due to solid solution strengthening and second phase strengthening.And the second phase strengthening plays a more significant role relative to solid solution strengthening.Laves phase and the oxides caused by wear heating prevent the direct contact between friction pair and HEAs,thus inducing a decreased wear rate from 6.82×10^(−5) to 3.47×10^(−5)m^(3)·N^(−1)·m^(−1).Moreover,the wear mechanism changes from adhesive wear,abrasive wear and oxidative wear to abrasive wear and oxidative wear.

A Hf-doped dual-phase high-entropy alloy: phase evolution and wear features

Initially defined high entropy alloys(HEAs)usually exhibit a single-phase solid-solution structure.However,two and/or more types of phases in HE As possibly induce the desired microstructure features,which contribute to improving the wear properties of HE As.Here,we prepare a series of(AlCoCrFeNi)_(100-x)Hf_(x)(x=0,2,4and 6;at%) HEAs and concern their phase compositions,micro structures and wear properties.Hf leads to the formation of(Ni,Co)_(2)Hf-type Laves phase and tailors the microstructure from a body-centered cubic(BCC) singlephase structure to a hypoeutectic structure.An increased hardness from~HV 512.3 to~HV 734.1 is due to solid-solution strengthening,grain refinement strengthening and precipitated phase strengthening.And a few oxides(Al_(2)O_(3)+Cr_(2)O_(3)) caused by the wear heating contribute to an 85.5% decrease in wear rate of the HEA system from6.71×10^(-5) to 0.97×10^(-5) m^(3)·N^(-1)·m^(-1).In addition,Hf addition changes the wear mechanism from abrasive wear,mild oxidative wear and adhesive wear to oxidative wear and adhesive wear.