High-temperature mechanical properties of aluminium alloys reinforced with titanium diboride (TiB_2) particles

The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements. The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys （6061 and 7015） at high temperatures were studied. Titanium diboride （TiB2） particles were used as the reinforcement. All the composites were produced by hot extrusion. The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy. TiB2 particles provide high stability of the alumin- ium alloys （6061 and 7015） in the fabrication process. An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys. Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure, and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.

AN INVESTIGATION OF HIGH-TEMPERATURE DEFORMATION STRENGTHENING AND TOUGHENING MECHANISM OF TITANIUM ALLOY

The high-temperature deformation strengthening and toughening mechanisms of titanium alloys have been investigated in this paper. The materials processed by this method produce a new tri-modal microstrvcture, which consists of 10-20% equiaxed alpha, streaky alpha and transformed beta matrix. It is found that the higher ductility of tri-modal microstructure is attributed to the equiaxed alpha's coopemtive slip and coordinated deformation with the transformed beta matrix. The streaky alpha phases not only increase the strength and creep properties, but also increase the fracture toughness. Propagating along grain boundaries between two neighboring streaky alpha phases, cracks in tri-modal microstructure make a more tortuous way, and then the materials show a higher fracture toughness. This new method is applicable to α, near α,α+β and near β titanium alloys.

Creep rupture life prediction of high-temperature titanium alloy using cross-material transfer learning

High-temperature titanium alloys are the key materials for the components in aerospace and their service life depends largely on creep deformation-induced failure.However,the prediction of creep rupture life remains a challenge due to the lack of available data with well-characterized target property.Here,we proposed two cross-materials transfer learning(TL)strategies to improve the prediction of creep rupture life of high-temperature titanium alloys.Both strategies effectively utilized the knowledge or information encoded in the large dataset(753 samples)of Fe-base,Ni-base,and Co-base superalloys to enhance the surrogate model for small dataset(88 samples)of high-temperature titanium alloys.The first strategy transferred the parameters of the convolutional neural network while the second strategy fused the two datasets.The performances of the TL models were demonstrated on different test datasets with varying sizes outside the training dataset.Our TL models improved the predictions greatly compared to the mod-els obtained by straightly applying five commonly employed algorithms on high-temperature titanium alloys.This work may stimulate the use of TL-based models to accurately predict the service properties of structural materials where the available data is small and sparse.

Alloying elements characterization in a Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-1Nd titanium alloy by carbon addition

The effects of carbon addition (0.01wt%-0.43wt%) on a Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-1Nd (wt%) alloy with a bimodal microstructure were investigated. Electron probe microanalysis was carried out to examine the partitioning behavior of carbon and the relation of carbon content to the distributions of Al and Mo in the primary αp phase (α p) and β transformed structure (β). It was found that interstitial carbon is enriched in the α p phase and its content slightly reduces with the increase of the volume fraction of α p. The measurements of carbon content in the present alloy with an α p of 15vol% showed that the carbon content in the α p phase increases with the increment of carbon addition until a maximum but keeps almost constant in the β phase. The addition of carbon reduces the solubility of Al and Mo in the α p phase and leads to the increment of Mo partitioning to the β phase. When the carbon content is over 0.17wt% (0.67at%), carbide precipitation occurs in the matrix and its volume fraction is related to the volume fraction of α p which can be explained in term of the difference of carbon solubility in the α p and β phases.

Comparative Assessment on Microstructure and Properties of in-situ TiC+Ti_(5)Si_(3)Reinforced TiAl-Sn-Zr Matrix Composites by Spark Plasma Sintering and Argon Protected Sintering

The effects of SiC particles(SiCp)on high temperature oxidation behavior of titanium matrix composites(TMCs)under different powder metallurgy processes were investigated.In situ Ti C+Ti_(5)Si_(3)reinforced titanium matrix composites were prepared by discharge plasma sintering(SPS)and argon protective sintering(APS).The results show that the two processes have a negligible effect on the composition and hardness of the samples,but the hardness of the two samples is significantly improved by adding SiCp.The apparent porosity of SPS process is obviously smaller than that of APS process,whereas,the apparent porosity increases slightly with the addition of SiCp.The oxide layer thickness and mass gain of the samples obtained by SPS process are smaller than those obtained by APS process.The oxide thickness and mass gain of both processes are further reduced by adding SiCp.The SPS composites showed the best high temperature oxidation resistance.Therefore,TMCs with Si Cp by SPS can effectively improve the high-temperature oxidation behavior of the materials.

Microstructure evolution of a new near-β titanium alloy:Ti555211 during high-temperature deformation

The isothermal compression testing of Ti555211 titanium alloys was carried out at deformation temperatures from 750 to 950 ℃ in 50 ℃ intervals,with strain rate of 0.001-1.000 s-1and height reduction of 20%-60%.The effect of processing parameters（deformation temperature,strain rate and deformation degree） on the microstructure evolution and the microstructure variables was investigated.The results show that the content of primary α phase decreases with the increase in deformation temperature.The effect of strain rate on microstructure variables of isothermally compressed Ti555211 alloy is mainly dependent on the deformation temperature.The fine second α phase partly transforms to β phase due to the deformation heat effect at the high strain rate,α phase becomes globular with temperature increasing and strain rate decreasing.In contrast,α phase is refined with temperature decreasing and strain rate increasing.The deformation degree has a little influence on the grain size and the morphology of primary α phase,but the effect of deformation degree on the morphology of second α phase is significant.The larger deformation degree is in favor of the globularization of lamellar α structure.

碳对高温钛合金组织稳定性的影响

结合实验研究,分析了碳合金化对高温钛合金显微组织、高α+β相区、时效过程中α2和硅化物析出的影响机制及其碳化物的稳定性。α+β相区再结晶,碳化物主要在β转变组织中析出以及碳化物的百分数取决于初生α(αp)体积分数;β相区热处理,碳化物的析出阻碍β晶粒的长大。碳的加入延缓αp体积分数随温度的变化速度,扩大高温钛合金的高α+β相区;同时降低αp相Al的浓度,增加β转变组织中Al和Mo的浓度,导致αp相内α2颗粒直径逐渐减小,颗粒间距增大。β相与碳化物之间的包析反应是碳化物溶解的主要动力。

钛及钛合金冷塑性加工润滑剂的研制

本文重点研究钛及钛合金冷塑性加工前的润滑载体。利用扫描电子显微镜对氟化膜进行了观察,同时对氟化膜的成分进行了能谱分析,并介绍了钛及钛合金氟化润滑处理工艺及废氟化液处理工艺。

枪钻切屑变形对孔表面质量影响研究

随着枪钻使用的日益增多,导致小深孔表面质量的检测问题也日益突出。文章以Ti6A14V棒料为加工材料,对孔径为φ17的深孔枪钻加工进行研究,通过分析切屑变形与孔表面粗糙度之间的关系,尝试通过切屑变形间接的反映出孔的表面质量,以解决深孔表面质量检测的难题。

Ti-0.3Mo-0.8Ni合金管材冷轧开裂原因研究

研究了润滑剂、冷轧道次加工率和冷轧送进量对Ti-0.3Mo-0.8Ni合金管材开裂比率的影响。结果表明,冷轧时不同的润滑剂对应着不同的金属变形温度,使用使金属变形温度保持在150～200℃之间的B润滑剂,可使Ti-0.3Mo-0.8Ni合金管材开裂比率比较低,为5%左右;而两辊冷轧道次加工率为40%,多辊冷轧道次加工率为30%和25%,冷轧送进量选为中等大小,可有效地控制Ti-0.3Mo-0.8Ni合金管材的开裂比率在较低的范围内波动,从而提高了Ti-0.3Mo-0.8Ni合金管材冷轧成品率。

基于加工表面完整性的TB6精铣参数优选

TB6高强度钛合金因其优良的比强度和热变形性能被广泛应用于航空领域,是制造直升机旋翼系统主承力结构件和起落架的主要材料。为了使零件获得较佳的疲劳性能,基于加工后的表面完整性指标对其精铣加工参数进行了优选。试验结果表明,每齿进给量f_z对加工后的表面粗糙度R_a和表面残余压应力σ_H有较大影响,随着每齿进给量f_z的不断增大,表面粗糙度R_a近似线性增大,表面残余压应力的幅值也随之增大;线速度v_c主要影响加工表面显微硬化率,显微硬化率随线速度的增大而降低;径向切宽a_e对加工表面残余应力也有较大的影响,随着a_e的增大,加工表面残余压应力的幅值逐渐降低。因此,从提高零件疲劳寿命的方面考虑,在满足例如零件表面质量要求和生产效率等各项生产要求的前提下,应尽可能降低铣削线速度,减小径向切宽,提高每齿进给量,适当选择轴向切深。

High-temperature dielectric and microwave absorption performances of TiB2/Al2O3 ceramics prepared by spark plasma sintering

TiB_(2)/Al2O3 ceramics reinforced with Mg O are prepared by spark plasma sintering(SPS).The dielectric and microwave(MW)absorption properties are discussed.The results indicate that both the commercial TiB_(2)(C-TiB_(2))content and preparing temperature play important roles in the dielectric properties.Simultaneously,TiB_(2)/Al2O3 composite shows the best MW absorption property when the C-TiB_(2)content and preparing temperature are 9 wt%and 1400°C.To further improve the MW absorption properties,the composite containing 9 wt%synthesized TiB_(2)(S-TiB_(2))has been sintered at 1400°C.Its high-temperature complex permittivity is greater than that of TiB_(2)/Al2O3 composite with 9 wt%C-TiB_(2)sintered at 1400°C and is directly proportional to the temperature.Besides,TiB_(2)/Al2O3 composite with 9 wt%S-TiB_(2)possesses a better MW absorption at 25–500°C,its effective absorption bandwidth(RL<-5 dB)can reach 4.2 GHz at 25–500°C.And the minimum reflection loss(RLmin)value reaches-43.41 dB at the temperature of 800°C with a thickness of 1.45 mm for TiB_(2)/Al2O3 composite with 9 wt%C-TiB_(2).Consequently,the satisfying absorbing layer(d<1.75 mm),flexural strength,heat stability and considerable high-temperature MW absorption ability grant TiB_(2)/Al2O3 ceramics practical applications as high-temperature microwave absorption materials(MAMs).

大型复杂结构钛合金机匣铸件的表面渗透检测

对直径为1 400mm的大型复杂钛合金机匣进行渗透检测时,使用静电喷涂法施加渗透液,可使渗透液覆盖机匣全表面,无多余渗透液淤积,清洗时间减少,背景良好,缺陷显示结果可靠。检测中结合黑光光源,使用工业内窥镜可以拓宽渗透检测过程中人眼目视可达的区域范围。选择工业内窥镜探头时,应根据腔体大小及缺陷部位,保证测量光照条件(与待检表面距离合适),才能得到较清晰的缺陷显示,从而提高判定的准确性与可靠性。

Simultaneously enhancing the hot workability and room-temperature strength of Ti-6Al-4V alloy via adding Mo and Fe

Reducing the hot working temperature and high-temperature deformation resistance of titanium alloy to improve hot rolling and hot extrusion workability of products with thin walls and complex section shapes has always been an important topic in the field of titanium alloy processing.This paper proposed a strategy of adding Mo and Fe elements to simultaneously reduce the hot working temperature and high-temperature deformation resistance of Ti-6Al-4V alloy.The effects of Mo and Fe contents on the mi-crostructure,β transus temperature(Tp),and high-temperature flow stress(HFS)of Ti-6Al-4V-xMo-xFe(x=0-5)alloys were investigated.The results showed that adding Mo and Fe can substantially reduce the Tp and HFS of the alloy,and greatly improve its room-temperature strength.Compared with com-mercial Ti-6Al-4V samples,the T_(β) of Ti-6Al-4V-2Mo-2Fe and Ti-6Al-4V-3Mo-3Fe samples was decreased by 68-98 ℃,and the HFS at 800-900 ℃ was decreased by 37.8%-46.0%.Compared with hot-rolled Ti-6Al-4V samples,the room-temperature tensile strength of hot-rolled Ti-6Al-4V-2Mo-2Fe samples was increased by about 30%,while the elongation hardly decreased.The increased strength was mainly at-tributed to fine grain strengthening and solid solution strengthening.The hot workability and room-temperature strength of Ti-6Al-4V alloy can be significantly improved by adding 2-3 wt.%Mo and Fe simultaneously.

High temperature self-lubricating Ti-Mo-Ag composites with exceptional high mechanical strength and wear resistance

Titanium alloys are of keen interest as lightweight structural materials for aerospace and automotive in-dustries.However,a longstanding problem for these materials is their poor tribological performances.Herein,we designed and fabricated a multiphase Ti-Mo-Ag composite(TMA)with heterogeneous triple-phase precipitation(TPP)structure by spark plasma sintering.A lamellarα-phase(αL)precipitates from theβ-phase under furnace cooling conditions and maintains a Burgers orientation relationship(BOR)withβ-matrix.An active eutectic transition also occurs in the titanium matrix,resulting in TiAg phase.The intersecting acicular TiAg and lamellarαL cutβgrains into fine blocks and the primary equiaxedαphase also provides many interfaces withβphase,which together effectively impede dislocation move-ment and increase strength.Compared with other titanium composites,TMA with TPP microstructure gets an excellent combination of strength(yield strength 1205 MPa)and toughness(fracture strain 27%).Furthermore,the TPP structure endows TMA with strong cracking resistance,which aids in reducing abra-sive debris at high temperatures during sliding and obtaining a low wear rate.Simultaneously,Ag parti-cles distributed at grain boundaries will easily diffuse to the wear surface,in situ forming the necessary lubricating phase Ag_(2)MoO_(4) with Mo-rich matrix debris via oxidation.TMA possesses excellent tribologi-cal properties with especially low wear rate of 8.0 x 10^(-6)mm^(3)N^(-1)m^(-1) and friction coefficient(CoF)of merely 0.20 at 600℃.Unlike other self-lubricating composites with high volume fraction of soft ceramic lubricants,which inevitably sacrifice their mechanical strength and ductility,the composite TMA pos-sesses well-balanced strength,toughness and self-lubricating properties.It holds important implications to design other metal matrix self-lubricating composites(MMSCs)used for load-bearing moving parts.

Selective laser melting of near-α titanium alloy Ti-6Al-2Zr-1Mo-1V:Parameter optimization, heat treatment and mechanical performance

This paper presents a comprehensive study conducted to optimize the selective laser melting(SLM)parameters and subsequent heat-treatment temperatures for near-α high-temperature titanium alloy Ti-6 Al-2 Zr-1 Mo-1 V(TA15),which is widely used in the aerospace industry.Based on the surface morphology and relative density analysis,the optimized process parameters were:laser power from 230 W to 380 W,scan speed from 675 mm/s to 800 mm/s,scan spacing of 0.12 mm,and layer thickness of0.03 mm.The effects of the laser power and the layer thickness on the phase constitutions,microstructure features,as well as room-temperature and high-temperature(500℃) tensile properties,were then studied to obtain an in-depth understanding of SLM-built TA15.Six typical temperatures(650,750,850,950,1000 and 1100℃) covering three representative temperature ranges,i.e.,martensite partial decomposition temperature range,martensite complete decomposition temperature range and above βtransus temperature,were subsequently selected as heat-treatment temperatures.The heat treatmentmicrostructure-mechanical property relationships of SLM-built TA15 were elucidated in detail.These results provide valuable information on the development of SLM-built TA15 alloy for industrial applications,and these findings are also beneficial to additive manufacturing of other near-α Ti alloys with desirable high-temperature properties.

Phase precipitation behavior and tensile property of a Ti-Al-Sn-Zr-Mo-Nb-W-Si titanium alloy

The characteristic of precipitation behavior of a2 phase and silicide, and the tensile properties at room temperature and 650℃after heat treatments in anovel TiAl-Sn-Zr-Mo-Nb-W-Si titanium alloy（BTi-6431 S） were investigated by microstructure analysis and mechanics performance testing. The results show that no second phase precipitates after solution treatment(980 ℃/2 h, air cooling（AC）). However, when the solution-treated specimens are aged at 600 ℃（600 ℃/2 h,AC）,α;phase precipitates in the primary α phase, and the size of α;phase increases with the aging temperature increasing to 750 ℃. Meanwhile, 50-100-nm S2-type silicide particles precipitate along lamellar phase boundaries of transformed β phase after aging at 750 ℃. BTi-6431 S alloy shows the best650 ℃ ultimate tensile strength（UTS） and yield strength（YS） when treated in solution treatment. However, aging treatment results in a decline in 650 ℃ ultimate tensile strength. This may be attributed to the loss of solution strengthening due to the depletion of Al, Si and Zr of the matrix caused by the precipitation of Ti;Al and（TiZr）;Si;.Silicide is a brittle phase; therefore, its precipitation causes a sharp decrease in the room-temperature ductility of BTi-6431 S alloy.

Experimental investigation of three-dimensional mixed-mode fracture of a titanium alloy at room and elevated temperatures

Damage tolerance of titanium alloy structures is very important for the safety of modern aircraft under complex loading and environmental conditions. However, there is no available systematic knowledge about the effect of alloy thickness under mixed-mode loading at elevated temperatures. In the present study, a newly developed fracture experimental technique based on high-temperature moiré interferometry was employed to investigate experimentally I-II mixed-mode fracture in titanium alloy TC11 of various thicknesses at room and elevated temperatures. Compact shear specimens with thickness ranging from 1.8 to 7.1 mm were tested. The effects of temperature, thickness, and loading angle on the load capacity and crack initiation angle were investigated systematically. The TC11 alloy was shown to possess varied fracture performance at elevated tem-perature, and an opposite thickness effect at room temperature. Increasing temperature would enhance the fracture load capacity of thick specimens but reduce the fracture load capacity of thin specimens. Crack initiation angles under I-II mixed-mode loading showed the thickness-temperature coupling effects. These complex effects call for new development in three-dimensional mixed-mode fracture theory and technologies for damage tolerance assessment.

Temperature-insensitive microwave absorption of TiB2-Al_(2)O_(3)/MgAl_(2)O_(4) ceramics based on controllable electrical conductivity

The design of the high-temperature microwave absorbing materials (MAMs) with temperature-insensitive and considerable microwave absorption (MA) capacity is a tremendous challenge.Ti B2-Al_(2)O_(3)/Mg Al_(2)O_(4) MAMs are prepared by the spark plasma sintering (SPS).Further,the influences of the Al_(2)O_(3) content on the flexural strength,thermal stability,high-temperature electrical conductivity,dielectric and MA properties are discussed.The results show that the Al_(2)O_(3) content is associated with flexural strength,high-temperature thermal stability,electrical conductivity,dielectric and MA properties.Meanwhile,the temperatureinsensitive and considerable MA properties with the minimum reflection loss (RLmin) ranging from -19.4 to -14.3 dB and the effective absorption bandwidth (EAB,RL<-5 dB) of 3.19–3.55 GHz are attained at 25°C–700°C in 8.2–12.4 GHz as the Al_(2)O_(3) content increases from 70.6 wt%to 80.6 wt%.This is ascribed to the compensating effect of the positive and negative temperature coefficient materials on the high-temperature electrical conductivity as well as the co-play of the dipolar and interfacial polarization at elevated temperatures.Consequently,Ti B2-Al_(2)O_(3)/Mg Al_(2)O_(4) ceramics exhibit the ideal prospect as the high-temperature MAMs.

Low-temperature superplasticity of β-stabilized Ti-43Al-9V-Y alloy sheet with bimodalγ-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100℃was systematically investigated in the temperature range of 750-900℃under an initial strain rate of 10^(-4)s^(-1).A bimodalγgraindistribution microstructure of Ti Al alloy sheet,with abundant nano-scale or sub-micronγlaths embedded insideβmatrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m=0.27 at 800℃,which is the lowest temperature of superplastic deformation for Ti Al alloys attained so far.The maximum elongation reaches~360%at 900℃with an initial strain rate of 2.0×10^(-4)s^(-1).To elucidate the softening mechanism of the disorderedβphase during superplastic deformation,the changes of phase composition were investigated up to 1000℃using in situ high-temperature X-ray diffraction(XRD)in this study.The results indicate thatβphase does not undergo the transformation from an ordered L2;structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of bothγandβdynamic recrystallization(DR)after tensile tests as characterized with electron backscatter diffraction(EBSD),the superplastic deformation mechanism can be explained by the combination of DR and grain boundary slipping(GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic deformation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic Ti Al alloy.