A MICROMECHANICAL MODEL FOR γ-TiAl BASE PST CRYSTALS

An analytical micromechanical method is proposed to examine the dependence of plastic deformation on the microstructure for a PST crystal. The sub-domain microstructure of the γ phase and the effect of the α2 phase are taken into account by a proper micromechanical formulation, the dislocation slip and twinning deformation mechanisms are considered in the context of crystal plasticity. The model can well predict the dependence of stress-strain relations on loading angle with respect to the microstructure. The influence of the twinning and lamellar spacing on the deformation behavior and biaxial yield surfaces for PST crystals are also examined.     

MICROMECHANICAL STUDY ON YIELD STRESS AND THE EFFECTS OF TWINNING FOR γ-TiAl-BASED PST CRYSTALS

基于γ-TiAl基PST晶体的微结构及滑移系和形变孪晶的空间晶体学位向分布的变形机制,提出了分析PST晶体 屈服应力对外载轴和片层界面夹角θ间的依赖关系的细观力学解析模型,重点考察了PST晶体的微结构和普通位错与形变孪晶 的临界分切应力(CRSS)之闻的差别对晶体屈服的影响．结果表明: PST晶体屈服应力σy和外载轴与片层界面之间的夹角θ 存在着强烈依赖关系．在软取向,变形由普通位错和超位错控制,变形平行于片层界面;而在硬取向,变形由形变孪晶控制,PST 晶体的变形为穿过片层界面形式．在加载角θ分别为45°,0°和90°时,屈服应力间的关系为: σy(45°)<σy(0°)<σy(90°)． 同时,分析了滑移系和形变孪晶的具体开动情况．     

CREEP BEHAVIOR OF γ-TiAl POLYSYNTHETICALLY TWINNED CRYSTALS

ＣＲＥＥＰＢＥＨＡＶＩＯＲＯＦγ－ＴｉＡｌＰＯＬＹＳＹＮＴＨＥＴＩＣＡＬＬＹＴＷＩＮＮＥＤＣＲＹＳＴＡＬＳＹ．Ｇ．Ｚｈａｎｇ;Ｊ．Ｇ．Ｌｉｎ;Ｙ．Ｊ．ＺｈｏｕａｎｄＣ．Ｑ．Ｃｈｅｎ（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃｉｅｎｃｅａｎｄＥｎｇ...     

DEFORMATION BEHAVIOR OF γ-TiAl SINGLE CRYSTALS AT DIFFERENT TEMPERATURES

在－１９６－１１００℃温度范围内,利用压缩试验研究了γ－ＴｉＡｌ单晶（Ａｌ的原子分数为５６％）在「００１」,「１５２」,「０２１」,「２３３」,「１９１」,「２５１」,「１１０」七个晶体取向上的塑性变形行为,它们的屈服强度温度关系曲线都可分成三个部分：－１９６－６００℃屈服强度随温度的升高先是快速下降,达到室温附近以后基本保持不变;６００℃以上,屈服强度随温度的升高而反常升高,随晶体取向的不同在７     

A NEW MODEL FOR CAVITY NUCLEATION

ＡＮＥＷＭＯＤＥＬＦＯＲＣＡＶＩＴＹＮＵＣＬＥＡＴＩＯＮ①ＬｉｕＺｈｉｙｉＭｅｃｈａｎｉｃａｌＤｅｐａｒｔｍｅｎｔ，ＰｅｔｒｏｌｅｕｍＵｎｉｖｅｒｓｉｔｙ，Ｄｏｎｇｙｉｎｇ２５７０６２ＡＢＳＴＲＡＣＴＢｙｃｏｒｅｃｔｅｖａｌｕａｔｉｏｎｏｆｔｈｅａｃ...     

A MECHANICS MODEL FOR CIRCULAR FORM TOOL CUTTING

ＡＭＥＣＨＡＮＩＣＳＭＯＤＥＬＦＯＲＣＩＲＣＵＬＡＲＦＯＲＭＴＯＯＬＣＵＴＴＩＮＧＸｉａＷｅｉ；ＬｉＹｕａｎｙｕａｎ；ＺｈｏｕＺｅｈｕａ（ＤｅｐａｒｔｍｅｎｔｏｆＭｅｃｈａｎｉｃａｌＥｎｇｉｎｅｅｒｉｎｇ，ＳｏｕｔｈＣｈｉｎａＵｎｉｖｅｒｓｉｔｙｏｆ...     

A MODEL FOR NUMERICAL CALCULATION OF UNSTEADYTEMPERATURE FIELD

1.IntroductionItisessentialtocalculatetemperaturefieldinmaterialscienceandtechnology.UPtonowsagreatnumberofstudies[1--3]havebeendone.However,therearesomeproblemsneedtobeclarifiedfurther.Thefirstofthemishowtodescribetheequationofheatconductionwhenthermalcoefficientisdependentoftemperature.Usually,inonedimension,itcanbeexpressedbybut,itisverydifficulttosolve.Anotherproblemishowtodealwiththelatentheatofphasetransformation.Thethirdandgreatestofthemishowtoimprovethecalculatingaccuracytthatis,howt…     

THE BEHMAVIORS OF (1/2)〈110] DISLOCATIONS IN γ-TiAl SINGLE CRYSTALS

[021]取向的γ—TiAl单晶（Ti—56％Al,原子分数）在－196－1000℃温度范围内压缩变形时主要形变方式为｛111｝（110］普通位错的滑移低温时,（1／2）（110］普通位错大多沿螺型及60°交角两种方向排列,井倾向于成组出现;而在反常屈服温度区域内,（1／2）（110］普通位错大多平行于其螺型方向,且形成许多位错拐点,其数目随温度的升高而增加;在反常屈服峰温以上,普通位错不具有特殊的方向性,且倾向于通过攀移向其它滑移面运动     

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

ＣＲＥＥＰＯＦＰＯＬＹＣＲＹＳＴＡＬＬＩＮＥＮＥＡＲγ－ＴｉＡｌＷＩＴＨＡＬＡＭＥＬＬＡＲＭＩＣＲＯＳＴＲＵＣＴＵＲＥＷＲ．Ｃｈｅｎ；Ｊ．Ｔｒｉａｎｔａｆｉｌｌｏｕ；Ｊ．ＢｅｄｄｏｅｓａｎｄＬ．Ｚｈａｏ（Ｄｅｐｔ．ｏｆＭｅｃｈ．＆Ａｅｒｏ．Ｅｎｇ．，...     

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.     

FRACTURE MECHANISM AND MICROMECHANICAL ANALYSIS OF POLYSYNTHETICALLY TWINNED CRYSTALS OF γTiAl ALLOYS

１　ＩＮＴＲＯＤＵＣＴＩＯＮＧａｍｍａｔｉｔａｎｉｕｍａｌｕｍｉｎｉｄｅｈａｓｒｅｃｅｉｖｅｄｉｎｃｒｅａｓｉｎｇａｔｔｅｎｔｉｏｎｂｅｃａｕｓｅｏｆｉｔｓｈｉｇｈｓｐｅｃｉｆｉｃｓｔｒｅｎｇｔｈａｎｄｓｔｉｆｆｎｅｓｓａｔｒｏｏｍａｎｄｅｌｅｖａｔｅｄｔｅｍｐｅｒａｔｕｒｅ,ｃｏｍｂｉｎｅｄｗｉｔｈｓａｔｉｓｆａｃｔｏｒｙｃｒｅｅｐｂｅｈａｖｉｏｒａｎｄａｄｅｑｕａｔｅｏｘｉｄａｔｉｏｎｒｅｓｉｓｔａｎｃｅ［１］．ＴｉｒｉｃｈγＴｉＡｌｂａｓｅｄａｌｌｏｙｓｈａｖｅｇｏｏｄｐｒｏｓｐ…     

Internal friction measurements of γ-TiAl

Single-phase γ-TiAl polycrystals were prepared by extrusion. The compound possesses an Al content of 54.1 at.% and interstitial impurities typical for technical-purity materials. Owing to dislocation pinning by theses impurities and precipitates, internal friction measurements show no relaxation between room temperature and 950 K. At 1100 K a relaxation maximum is observed with an activation enthalpy of H_eff=(2.97 ± 0.1) eV and a pre-exponential factor of τ₀=2.5×10^−16±1 s, which can be attributed to the reorientation of structural defects. Above 1200 K the damping shows a frequency-dependent strong increase with temperature, which is characteristic for diffusion-controlled climb of dislocations.     

Diffusion bonding of γ-TiAl sheets

Using advanced γ-TiAl based alloys and various ingot and powder metallurgical routes, several aerospace engine components are now being developed. Further application of this material in the aerospace industry can only be implemented provided that successful joining and cost effective fabrication methods for this material are developed. In this context, diffusion bonding and superplastic forming of this material is considered to be of particular interest. The possibility of producing sound bonds in γ-TiAl sheets by industrially available diffusion bonding equipment designed for conventional Ti-alloys was investigated. Defect-free bonds were achieved at 1000°C with pressure levels of 5, 10 and 20 MPa, the holding time varying between 5 and 8 h. The bond qualities were assessed by shear testing at room temperature. Reasonable shear strength levels were obtained by bonding at 1000°C. The bonds were also post bond heat treated at 1430°C for 30 min, which improved the bond quality in all cases.     

Oxidation-Resistant Aluminide Coatings on γ-TiAl

The long-term application of TiAl alloys based on the -phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl₃, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of -TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl₃ coating and the -TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl₂ phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.     

Microstructural Study of Oxidized γ-TiAl

The microstructural development of oxidized-TiAl is presented with a focus on oxidation inair. The investigations were carried out usingconventional, analytical, and, especially,energy-filtered transmission electron microscopy (EFTEM). Threeimportant points were studied in detail: (1) thenitrogen effect, (2) thesurface-finish effect, and (3) thesubsurface zone. Nitrogen leads to the formation of TiN andTi₂AlN at the metal-scale interfaceinterrupting alumina and thereby preventing thedevelopment of a continuous alumina layer. TheAl-depletion layer formed during the oxidation process develops from a single-phaselayer, consisting of a cubic phase, to a two-phaselayer, consisting of the cubic phase and₂-Ti₃Al. The cubic phase isnot known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surfacepreparation of the sample with rapid oxidation kineticsfor fine polishing and slow kinetics for a 600-gritSiC-paper finish. The rougher surface finish leads to the development of a recrystallization zonenear the surface and supports the formation of acontinuous alumina layer in the early stages ofoxidation. As for the oxidation in air, the cubic phaseis formed first underneath the oxide scale,followed by ₂-Ti₃Alformation.     

DIRECTIONAL COARSENING OF γ'PRECIPITATES IN A SINGLE CRYSTAL NICKEL BASE SUPERALLOY

研究了［００１］取向单晶高温合金ＣＭＳＸ—４在１２５３Ｋ，３５０ＭＰａ拉伸蠕变中γ＇沉淀沿垂直于外应力轴方向的粗化过程及其机理。发现在定向粗化期间立方形γ＇沉淀的边缘部分优先生长，γ—γ＇界面呈内凹形方式推进，进而封闭了与之相邻的垂直基体通道（平行于［００１］）；相互连接的γ＇沉淀逐渐消耗并最终耗尽被封闭的残余垂直通道而发展成完整的筏形组织。定向粗化导致了γ＇沉淀沿［００１］方向的尺寸逐渐减小，水平基体通道（垂直于［００１］）的宽度大幅度增加。对γ＇沉淀的特殊生长方式和两共格相形貌的演变特征进行了分析与讨论。     

DEFORMATION AND FRACTURE OF TiAl PST CRYSTALS UNDER MONOTONIC AND CYCLIC LOADING

ＤＥＦＯＲＭＡＴＩＯＮＡＮＤＦＲＡＣＴＵＲＥＯＦＴｉＡｌＰＳＴＣＲＹＳＴＡＬＳＵＮＤＥＲＭＯＮＯＴＯＮＩＣＡＮＤＣＹＣＬＩＣＬＯＡＤＩＮＧＹ．Ｕｍａｋｏｓｈｉ；Ｔ．ＮａｋａｎｏａｎｄＨ．Ｙ．Ｙａｓｕｄａ（ＤｅｐａｒｔｍｅｎｔｏｆＭａｔｅｒｉａｌｓＳｃ...     

Isothermal forging of γ-TiAl based alloys

1　INTRODUCTIONBecauseoftheirattractive properties ,γ TiAlbasedalloysareconsideredforhightemperatureap plicationsinaerospaceandautomotiveindustries .Thesepropertiesincludelowdensity ,highspecificyieldstrength ,highspecificstiffness ,goodoxidationresistance ,andgoodcreeppropertiesuptohightem peratures.Researchanddevelopmentonγ TiAlbasedalloyshaveprogressedsignificantlywithinthelastdecade .Thisresearchhasledtoabetterunder standingofthefundamentalcorrelationsamongalloycompositionandmicros…     

A positron investigation of electron-irradiation induced defects in γ-TiAl intermetallic compounds

The isochronal recovery of irradiation induced defects was investigated in γ-TiAl intermetallic compounds (with 50 and 54 at% Al) by positron lifetime measurements after 2.5 MeV electron irradiation at 21 K. In the as-irradiated condition, the analysis of the results and the comparison with published data led to a value τ_d=230±5 ps for the lifetime of vacancy-trapped positrons. The lifetime variations observed during isochronal anneals at increasing temperatures are consistent with vacancy migration around 450 K. The observation of a progressive decrease in the lifetime of trapped positron, during the migration and elimination of vacancies, suggests that they do not form unrelaxed three-dimensional clusters, and that another type of positron traps is simultaneously present.     

Interaction between γ-TiAl alloy and zirconia

A study on the interaction between TiAI alloy and zirconia was carried out in argon atmosphere. The rnicrographic observations of the cross-section perpendicular to the interface were obtained using a scanning electron microscope with a dedicated energy dispersive spectrometer (SEM/EDS). The results showed that firstly TiAI alloy spreads on the ceramic surface, and then infiltrates into the pores between ceramic particles accompanied by a chemical reaction. The whole ceramic mold is broken into tiny blocks. A multiple fission reaction mechanism was developed to explain the interaction between TiAI alloy and zirconia.