CREEP OF POLYCRYSTALLINE NEAR γ-TiAl WITH A LAMELLAR MICROSTRUCTURE

Creep of a polycrystalline near γ-TiAl alloy in two fully lamellar conditions is presented. A lamellar structure with fine interface spacing and planar grain boundaries provides improved creep resistance. The lamellar structure with wide interface spacing and interlocked grain boundaries has <1/2 the creep life, five times the minimum strain rate and greater tertiary strain.Creep strain is accommodated by dislocation motion in soft grains, but the strain rate is controlled by hard grains. The resistance to fracture is controlled by the grain boundary morphology, with planar boundaries causing intergranular fracture.To maximize the creep resistance of near γ-TiAl with a lamellar microstructure requires narrow lamellar interface spacing and interlocked lamellae along grain boundaries.     

CREEP OF A POLYCRYSTALLINE NEAR γ-TiAl Alloy WITH A LAMELLAR MICROSTRUCTURE

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TRANSFORMATION KINETICS AND COARSENING OF LAMELLAR STRUCTURE IN γ-TiAl ALLOY

研究了Ｔｉ－４７．８ａｔ．－％Ａｌ合金从α相区冷至室温再在双相区时效后，层状组织的转变机理及不连续粗化．层状组织转变过程与时效温度和形成过饱和α＿２单相时的冷却速度有关．建立了层状组织转变的动力学模型．层状组织的稳定性随片间距减小而降低，表现为时效时发生不连续粗化．     

FIRST-PRINCIPLES STUDY OF OXYGEN ATOM ADSORPTION ON γ-TiAl(111) SURFACE

运用第一性原理方法对氧原子在γ-TiAl（111）表面的吸附研究表明，氧原子倾向于吸附在近邻表面层多Ti的位置，随着覆盖率的增加，表面不同位置吸附能差别减小．电子结构分析发现，氧原子同表面金属原子形成以离子特性为主的化学吸附．氧化学势对TiAl表面稳定性影响的研究表明，γ-TiAl（111）清洁表面只能在氧化学势很低时可以稳定存在，氧化学势稍高，清洁表面就变得不稳定，氧原子开始吸附，并迅速达到高覆盖率的情况．     

PLASTIC DEFORMATION AND DISLOCATION STRUCTURE IN γ'-PHASE OF A DIRECTIONALLY SOLIDIFIED Ni-BASE SUPERALLOY

The dislocation structure of directionally solidified(DS)Rene 80 superalloy during tensileand creep deformation has been studied by TEM.In tensile deformation,γ'shearing occurs byslip of APB-coupled(1/2)<110>dislocation pairs on{111}planes at temperatures below760℃.and by slip of SISF-seperated(1/3)<112>superpartials which have dissociated on{111}planes into a(1/2)<110>screw superpartial and a(1/6)<112>edgesuperpartial with an APB in between at temperatures equal to or above 760℃.In creepdeformation,γ'shearing occurs again by slip of SISF-seperated(1/3)<112>superpartialswhich have not dissociated on{111}planes and by diffusive slip of APB-coupled(1/2)<110>dislocation pairs at 760℃,618 MPa.However,no γ'shearing occurs at 980℃,190MPa.During secondary creep at 760℃,618 MPa or at 980℃,190 MPa,2-dimensional net-works have formed at γ/γ'interfaces,and the strain is mainly the result of diffusion control-led climb of dislocation networks at γ/γ'interfaces.The dependence of secondary creep rate εupon size a and volume fraction v_■ of cubic γ' has been found to be ■∝α/v_f~(2/3).     

THE α_2/γ INTERFACIAL STRUCTURE CHANGES IN A DUAL-PHASE γ-TiAl ALLOY AFTER HOT DEFORMATION AND SUBSEQUENT SHORT-TIME ANNEALING

1.Introduction Thetwo--phaseac--Ti-Al--basedalloysarecurrentlyunderextensiveinvestigationdueto theirbettermechanicalproperties['].Afullylamellaroranearlylamellarmicrostructure consistingofalargenumberoflamellarcolonieswithafewequiaxedac--grainsisusually preferred.Theacplateinthelamellarstructureusuallykeepsaconventionalorientation relationshipwiththeadZplate,{III}.//{0001}.,and<110>.//<1120>...Thisisformed bythephasetransformatiollofeitheror~or ac~adZ acoror~adZ~adZ ac,de…     

Influence of silicide on fracture behavior of a fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar binary γ-TiAl alloys is extremely anisotropic with respect to the lamellar orientation.For the fully lamellar Ti-46Al-0.5W-0.5Si alloy,the existence of silicide ...     

Influence of columnar grain boundary on fracture behavior of as-cast fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar γ-TiAl alloys depends on the angle between the lamellar orientation and loading axis,but the role of the presentation of grain boundary cannot be ignored.To investigate the influence of the grain boundary on the initiation and propagation of cracks,the tensile test of the alloy was conducted at room temperature with loading axis parallel and perpendicular to the lamellar orientation,respectively.The cracks adjacent to the fracture zone of the tensile specimens have been investigated to analyze the fracture behavior.Results show that the grain boundary has dual influences on the fracture behavior.When the loading axis is parallel to the lamellar orientation,cracks are preferentially initiated at and propagate along the grain boundaries.When the loading axis is perpendicular to the lamellar orientation,the grain boundaries can prevent the propagation of cracks from running across.Additionally,serrated-shape grain boundaries have a better inhibiting effect on the propagation of cracks than planar boundaries.     

晶体取向对单晶γ-TiAl合金纳米切削的影响研究

为了研究晶体取向对单晶γ-TiAl合金纳米切削过程的影响,采用分子动力学数值方法对不同切削晶向下的切削力、切削温度、材料去除及晶格结构变化进行分析和探讨,揭示不同的晶体取向对单晶γ-TiAl合金纳米切削质量作用机制.结果表明:在纳米切削过程中,随着晶面和晶向的变化,切削力、切削温度、材料去除和晶格结构都会有不同程度的变...     

γ-TiAl合金表面辉光等离子Ni-Cr共渗层的组织与性能

采用双层辉光等离子表面冶金技术在γ-TiAl合金表面进行Ni-Cr共渗处理,形成均匀致密的Ni-Cr合金层,通过SEM、EDS、XRD等对其渗层显微组织、化学成分和相结构进行分析,并测试了合金层的显微硬度及耐磨性。结果表明,合金层有效渗层厚度为25μm,主要由Cr2 Ti、AlTi、Cr、NiTi2、Cr1.75 Ni0.25 Ti等相组成;合金层中合金元素Cr和Ni由表及里呈梯度分布,合金层与基体结合牢固;合金层的显微硬度、动态硬度及摩擦磨损性能显著高于基体。     

Characterization of Si-aluminide coating and oxide scale microstructure formed on γ-TiAl alloy during long-term oxidation at 950 °C

Recently γ-TiAl intermetallics have been successfully applied on low pressure turbine blades of modern jet engines mostly due to their low density, high specific strength and creep resistance, all of which make them excellent alternatives for Ni-based superalloys. However, due to insufficient high temperature oxidation resistance above 800 °C there is the necessity for development of protective coatings that will allow the formation of a thin and slow growing α-alumina oxide scale.The paper presents the results of investigations concerning the behavior and influence of silicon on the substructure of aluminized ɣ-TiAl alloy in the as-deposited state as well as after long-term isothermal high temperature oxidation. The Si-modified aluminide coating was produced by pack cementation method and oxidized isothermally at 950 °C for 3000 h. The detailed microstructural examination was performed using analytical SEM and STEM techniques along with Focused Ion Beam (FIB) technique which was utilized for the preparation of a sample from the metal-scale interface after long-term oxidation test. The addition of silicon to the aluminide coating resulted in the formation of nanometric Ti₅Si₃ precipitates that were found to bind both Ti and Nb from the alloy. It was demonstrated that the investigated ɣ-TiAl with Si-modified aluminide coating is capable of forming a continuous and uniform α-alumina oxide scale at 950 °C that remains adherent for 3000 h. The oxide scale was found to obey the growth mechanism typical for Ni-based superalloys and consisted of outer equiaxed and inner columnar grains.     

磁性Fe3O4/Ag复合纳米粒子制备与抗菌性能

采用箔-纤维-箔法和1150℃/150 MPa/30 min的真空热压工艺成功制备了SiCf/Ti-43Al-9V复合材料,并使用金相显微镜、X射线衍射仪、扫描电镜及能谱仪对该复合材料微观组织的形成进行了研究.结果表明,制备过程中SiC纤维与TiAl基体合金发生反应,并形成一定厚度的反应层;基体组织为等轴晶,粒径约为8 μm,与原始合金组织相比明显细化;从反应层到远处的TiAl基体合金,基体合金的组织由全γ相转变为α2/γ片层组织、γ晶粒和晶间B2相的混合组织.其中全γ相区域的厚度为2～4 μm,并围绕纤维分布.根据Ti-Al-V相图、C原子和V原子的扩散,分析了这两个基体区域的形成机理,并结合热压成形过程中的塑性变形和变形储存能,解释了基体合金晶粒大小的变化.     

Oxidation protection of gamma-titanium aluminide using glass-ceramic coatings

γ-TiAl intermetallic alloys are presently considered an efficient structural material for advanced turbine blades and aero-engine components due to their various advantages compared to the traditionally used superalloys. However, their poor oxidation resistance at temperatures > 750 °C severely limits their wider application. The present study dealt with the improvement of oxidation resistance of this alloy by applying impervious glass-ceramic coatings by vitreous enameling technique. Results showed that MgO-SiO₂-TiO₂ glass-ceramic coating could offer excellent oxidation resistance to γ-TiAl at 800 °C even up to 100 h with negligible weight gain (~ 0.10 mg/cm²) compared to that of the bare alloy (~ 1.3 mg/cm²). The coatings those were belonging from BaO-MgO-SiO₂, ZnO-Al₂O₃-SiO₂ and BaO-SiO₂ systems also extend appreciable improvement in the oxidation resistance of the alloy at 800 °C up to 100 h. At further higher temperature such as at 1000 °C, the ABK-13 and ABK-103 glass-ceramic coatings offered significant protection to the alloy up to 25 h of exposure in air with minimum weight gain (~ 0.34 mg/cm²). However, after that the coated layers started to peel off from the alloy surface.     

Hydrogen permeation resistant glass-ceramic coatings for gamma-titanium aluminide

Hydrogen embrittlement is no doubt a limiting factor in the high performance applications of many intermetallics. This is mainly because of the inward diffusion of hydrogen atoms and formation of brittle hydride phases during hydrogen exposure of the intermetallic components that resulted in lowering the component's ductility and fracture toughness. γ-TiAl intermetallic is not an exception in this regard. The present study dealt with the method of providing oxide based glass-ceramic coatings on the surface of γ-TiAl by vitreous enameling technique to protect the substrate from the detrimental effect of hydrogen at high temperatures like 800 °C at 0.1 MPa gas pressure for up to 75 h. Results showed that although the uncoated γ-TiAl alloy was severely affected by the hydrogen exposure test, the coated samples were remained mostly unaffected after the test with minimum changes in their microstructure. While the gain in weight of the uncoated alloy after 75 h of H₂ permeation test was ~ 1.05 mg/cm², the weight gain of the two coated samples were only ~ 0.12 mg/cm² for BaO-SiO₂-MgO and ~ 0.15 mg/cm² for MgO-SiO₂-TiO₂ glass-ceramic coated substrates. As revealed from the XRD phase analysis, after the high temperature exposure of the coated samples in flowing H₂ for up to 75 h, the coated layers were only enriched with their major crystalline phases with little or no trace of the detrimental hydride phases, whereas, in the uncoated alloy, presence of aluminum hydride and titanium hydrides were observed.     

载荷对钛合金Al2O3Y2O3/AlY复合涂层摩擦学性能的影响

为了提高耐摩擦磨损性能,采用磁控溅射技术在γTiAl钛合金表面制备了Al2O3Y2O3/AlY复合涂层。检测了涂层的厚度、表面硬度、微观组织和成分变化。通过在130g、230g和330g载荷下采用GCr15钢球作摩擦副进行的摩擦磨损试验,研究了有和无复合涂层的γTiAl合金的耐摩擦磨损性能。结果表明:Al2O3Y2O3/AlY复合涂层的厚度约为33μm,由Al2O3Y2O3层、AlY层和扩散层组成,平均表面硬度为433.4HV0.1。带复合涂层的γTiAl合金摩擦因数和比磨损率均比无涂层γTiAl合金的小。在不同的摩擦磨损试验载荷下,无涂层γTiAl合金的磨损机制为磨粒磨损和黏着磨损,而有Al2O3Y2O3/AlY复合涂层的γTiAl合金则主要是磨粒磨损和氧化磨损。