Microstructure and fracture toughness of a β phase containing TiAl alloy

Microstructures and fracture toughness of Ti-45Al-2Nb-1.5V-1Mo-0.3Y alloy have been investigated. The alloy exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with mixtures of γ and residual β phases along lamellar colony boundaries. Precipitation of both β and γ phases from α₂ lamellae was found after aging at 950 °C for 48 h. Phase transformations involving β phase both in α₂ laths and along colony boundaries are discussed. This TiAl alloy possesses a higher K_IC value up to 23.5 MPam^1/2 at room temperature, compared with fully lamellar Ti-45Al-5Nb-0.3Y alloy. The toughening mechanism for current alloy is concluded as trans-lamellar fracture, ligament bridges and crack deflection, together with precipitation of β and γ phases. The precipitation of fine β and γ particles is considered as intrinsic toughening mechanism, because α₂/β and α₂/γ interfaces generating due to precipitation can restrict dislocation motion effectively.     

INFLUENCE OF ALLOYING ELEMENTS（Nb，Mo，V） ON MICROSTRUCTURE OF Ti_3Al BASE ALLOYS

ＩＮＦＬＵＥＮＣＥＯＦＡＬＬＯＹＩＮＧＥＬＥＭＥＮＴＳ（Ｎｂ,Ｍｏ,Ｖ）ＯＮＭＩＣＲＯＳＴＲＵＣＴＵＲＥＯＦＴｉ_３ＡｌＢＡＳＥＡＬＬＯＹＳ￥ＳＯＮＧＤａｎ;ＤＩＮＧＪｉｎｊｕｎ;ＷＡＮＧＹａｎｄｏｎｇ（ＡｎａｌｙｓｉｓａｎｄＴｅｓｔｉｎｇＣｅｎｔｅ?..     

Microstructure and mechanical properties of the intermetallic alloy Ti-45Al-6(Nb, Mo)-0.2B

The microstructure and tensile properties of the intermetallic γ-TiAl + α₂-Ti₃Al alloy Ti-45Al-6(Nb, Mo)-0.2B are studied after various heat and thermomechanical treatments. Its cast state is found to be characterized by a homogeneous, relatively fine-grained structure in the cases of both a model 30-g ingot and a bulk laboratory ?120 × 180-mm ingot. The cast material subjected to heat treatment with furnace cooling and aging at T = 1100°C exhibits strength properties at T = 20–900°C that are relatively high for cast γ + α₂ alloys. Due to the rational choice of the alloy, its thermomechanical treatment is facilitated, a fine-grained structure can easily be formed upon hot deformation, and a superplastic state with elongations in the test-temperature range T = 900–1000°C that are very large for γ + α₂ alloys is reached. However, the high contents of niobium and molybdenum in the Ti-45Al-6(Nb, Mo)-0.2B alloy hinder the formation of an equilibrium lamellar structure upon heat treatment, and an increase in the aging temperature to T = 1100°C leads to the development of the α₂ → ß(B2) phase transformation, which makes it impossible to reach a high level of the mechanical properties in the temperature range of the potential application of γ + α₂ alloys. Our study has revealed that the compositions of the γ + α₂ alloys need further optimization of the way of refining the niobium and molybdenum contents.     

Effect of carbon on microstructures of Ti-45Al-3Fe-2Mo-xC alloy

The effect of carbon addition on the microstructures of TiAl-based alloy (Ti-45Al-3Fe-2Mo) was studied. The proportion of β/B2 phase reduces with the content of carbon increasing, while the colony size increases. With increasing the carbon content, the lamellar spacing first decreases from 267 nm (Ti-45Al-3Fe-2Mo) to 237 nm (Ti-45Al-3Fe-2Mo-0.3C) and 155 nm (Ti-45Al-3Fe-2Mo-0.5C), but then increases to 230 nm (Ti-45Al-3Fe-2Mo-1.0C) with further increase in C level, which is affected by the inhibition of carbon atom and precipitation of carbides at the lamellar interface. Precipitation of carbides shows a response to aging time at 800 °C. P-type carbides grow up at the boundaries and near the dislocation areas with the prolonging of aging time. And these carbides are projected different morphology in different beam directions (BD). The effects of these microstructural modifications were examined and the observations were discussed.     

Effects of minor yttrium addition on hot deformability of lamellar Ti-45Al-SNb alloy

The effects of 0.3%（molar fraction, the same below） yttrium addition on hot deformability of lamellar Ti-45Al-5Nb alloy were investigated by simulated isothermal forging tests. The ingots with the nominal compositions of Ti-45Al-5Nb and Ti-45Al-5Nb-0.3Y were prepared by induction skull melting. Simulated isothermal forging tests were conducted on Gleeble 1500D thermo-simulation machine using a 6 mm in diameter and 10 mm in length compressive specimen at the deformation temperatures of 1 100, 1 150, 1 200 ℃ and strain rates of 1.0, 0.1, 0.01 s^-1. The results show that yttrium addition remarkably improves hot deformability of Ti-45Al-5Nb alloy. An appropriate hot deformation processing parameter of Ti-45Al-5Nb-0.3Y alloy is determined as 1 200 ℃, 0.01 s^-1. The flow stresses are decreased by yttrium addition under the same compressive conditions. The activation energies of deformation Q are calculated as 448.6 and 399.5 kJ/mol for Y-free and Y-containing alloys, respectively. The deformed microstructure observation under 1 200 ℃, 0.01 s^-1 condition indicates that Ti-45Al-5Nb-0.3Y alloy shows more dynamic recrystallization. The improvement of hot deformability of Ti-45Al-5Nb-0.3Y alloy induced by yttrium addition should be attributed to that the smaller the original lamellar colonies, the lower the deformation resistance and activation energy of deformation are, and the more the dynamic recrystallization is.     

Enhancement of creep properties and microstructural stability of intermetallic β-solidifying γ-TiAl based alloys

Carbon, Mo and Si represent promising alloying elements with respect to increase the operating temperature of intermetallic titanium aluminides. The influence of these elements on microstructural stability and creep properties is investigated on a β-solidifying γ-TiAl based alloy, named TNM, with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%) in two different microstructural conditions. The first condition after casting and hot isostatic pressing has a coarse “nearly lamellar γ” microstructure. The second condition is adjusted by a subsequent heat treatment and shows a “nearly lamellar β” microstructure with finely spaced α₂ and γ lamellae and areas of discontinuous precipitation (cellular reaction) in the α₂/γ-colonies. Creep tests were carried out at 815 °C and 150 MPa to examine the influence of microstructure and its change on and during creep. Alloying of C and Si or Mo to the TNM based material led to improved creep properties and microstructural stability by inhibiting the progress of discontinuous precipitation.     

Ta含量对Ti-6Al-3Nb-2Zr-1Mo-xTa合金力学性能和腐蚀性能的影响

系统研究了Ti-6Al-3Nb-2Zr-1Mo-x Ta(x=0,0.2,0.5,1.0,3.0,5.0)合金的微观组织、拉伸性能、夏比冲击韧性和耐海水腐蚀性。结果表明,经α+β两相区锻造后,Ti-6Al-3Nb-2Zr-1Mo-5Ta合金获得片层组织,Ti-6Al-3Nb-2Zr-1Mo-x Ta(x=0,0.2,0.5,1.0,3.0)均获得双态组织。XRD、TEM和选区电子衍射表明,在添加Ta元素后,Ti-6Al-3Nb-2Zr-1Mo-x Ta合金没有新相产生。对于双态组织Ti-6Al-3Nb-Zr-1M0-x Ta合金,随着Ta含量的增加,其Mo当量逐渐增加,导致其屈服强度、抗拉强度和显微硬度均有所提高。而Ta含量对冲击吸收功的影响规律与屈服强度和抗拉强度的影响规律相反,其大小与冲击断口剪切唇区面积一致。当Ta含量超过1.0%(质量分数)时,由于α和β相之间的标准平衡电位差逐渐增大,Ti-6Al-3Nb-2Zr-1Mo-x Ta合金的耐海水腐蚀逐渐降低。综合考虑强度、冲击韧性和耐海水腐蚀性能,Ti-6Al-3Nb-2Zr-1Mo-1Ta合金综合匹配性最好,具有良好的海洋工程应用潜力。     

Effect of V and Nb additions on microstructure,properties,and deformability of Ti-45Al-9 (V,Nb,Y) alloy

Ti-45Al-9(V, Nb, Y) alloys with four different x=V/Nb (atomic ratio x = 1, 1.5, 2 and 3.5) have been prepared, and the microstructures, properties and hot deformation behaviors were investigated. SEM, XRD and TEM results showed that Ti-45Al-9(V, Nb, Y) alloys were mainly composed of γ, α 2 , and β phase, and the volume fraction of β phase increased with the increase of the atomic ratio of V/Nb. The alloys were featured with lamellar microstructure with β and γ phases locating at the colony boundaries, and some β precipitates appearing at γ/γ interfaces. It was found that the colony size decreased with the increase of x. The alloys exhibited moderate mechanical properties at room temperature, with a yield strength of over 600 MPa, and fractures showed mainly translamellar character. The alloy with x=3.5 exhibited the best deformability at elevated temperature and that with x=1 had superior oxidation resistance at 800 ℃.     

全片层高Nb含量TiAl合金长期热循环后的显微组织不稳定性

对全片层Ti-45Al-8.5Nb-（W,B,Y）合金在900℃下进行长期热循环（500次和1000次）实验,采用扫描电镜（SEM）及透射电镜（TEM）研究该合金长期热循环后的显微组织不稳定性。结果表明：合金经热循环后主要产生两种类型的组织不稳定性：1）长期热循环特别是1000次热循环后,在Al偏析处易产生因晶界迁移引起的不连续粗化,随着循环次数的增加,元素扩散致使Al偏析逐渐减少;2）1000次热循环后,α2片层变细且发生断裂,这是由α2→γ相变导致的α2片层溶解所致。同时,γ晶粒在α2片层或（α2＋γ）片层内部以任意方向形核。     

Sintering Behavior and Microstructure Formation of Titanium Aluminide Alloys Processed by Metal Injection Molding

The sintering behavior of metal injection molded titanium aluminide alloys, their microstructure formation and resulting mechanical properties were investigated. As reference material, the alloy Ti-45Al-5Nb-0.2B-0.2C at.% (TNB-V5) was selected. Additionally, two other variations with Mo and Mo + Si additions were prepared: Ti-45Al-3Nb-1Mo-0.2B-0.2C at.% and Ti-45Al-3Nb-1Mo-1Si-0.2B-0.2C at.%. The results indicate that the optimum sintering temperature was slightly above the solidus line. With proper sintering parameters, very low porosities (<0.5%) and fine microstructures with a colony size <85 µm could be achieved. Considering the sintering temperatures applied, the phase transformations upon cooling could be described as L + β → β → α + β → α → α + γ → α₂ + γ, which was in agreement with the microstructures observed. The effects of Mo and Si were opposite regarding the sintering behavior. Mo addition led to an increase in the optimum sintering temperature, whereas Si caused a significant decrease.     

Effect of carbon addition on solidification behavior,phase evolution and creep properties of an intermetallic β-stabilized γ-TiAl based alloy

Improving mechanical properties of advanced intermetallic multi-phase γ-TiAl based alloys, such as the Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is limited by compositional and microstructural adaptations. A common possibility to further improve strength and creep behavior of such β-solidifying TiAl alloys is e.g. alloying with β-stabilizing substitutional solid solution hardening elements Nb, Mo, Ta, W as well as the addition of interstitial hardening elements C and N which are also carbide and nitride forming elements. Carbon is known to be a strong α-stabilizer and, therefore, alloying with C is accompanied by a change of phase evolution. The preservation of the solidification pathway via the β-phase, which is needed to obtain grain refinement, minimum segregation and an almost texture-free solidification microstructure, in combination with an enhanced content of C, requires a certain amount of β-stabilizing elements, e.g. Mo. In the present study, the solidification pathway, C-solubility and phase evolution of C-containing TNM variants are investigated. Finally, the creep behavior of a refined TNM alloy with 1.5 at.% Mo and 0.5 at.% C is compared with that exhibiting a nominal Ti-43.5Al-4Nb-1Mo-0.1B alloy composition.     

烧结温度对快速凝固TiAl合金组织及力学性能的影响(英文)

将快速凝固Ti-46Al-2Cr-4Nb-0.3Y(摩尔分数,%)合金薄带破碎成粉末,然后通过等离子烧结(SPS)制备成块体。研究烧结温度对烧结块体的组织和力学性能的影响。在1200℃烧结时得到完全致密的块体。进一步升高烧结温度对合金密度的影响不大,但是对烧结块体的显微组织及相结构有显著影响。烧结块体主要由γ和α2相组成,随着烧结温度的升高,α2相的体积分数降低,块体合金由近γ组织演变为近层片组织,且组织逐渐粗化,但是长大不明显。1260℃烧结得到的块体组织细小、均匀,没有明显微偏析,具有良好的综合力学性能,室温压缩断裂强度和压缩率分别为2984MPa和41.5%,高温(800℃)拉伸断裂强度和伸长率分别为527.5MPa和5.9%。     

Precipitation behaviors in a quenched high Nb-containing TiAl alloy during annealing

The precipitation of γ phase and heterogeneous nucleation of ω_o phase within β_o phase areas are common phenomena in TiAl alloys. However, detailed explanation on the corresponding phase transformation mechanisms is still lacking. In this study, the precipitation behaviors of γ and ω_o phases in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy are investigated. The results show that large γ grains form after quenching whereas small γ particles can directly nucleate within the remaining β_o phase during annealing. Semi-coherent interfaces are observed between γ and β_o phases and the average distance between dislocations is evaluated. The heterogeneous nucleation of ω_o phase at the lamellar colony boundary is imaged by HRTEM. Edge-to-edge method is used to calculate the orientation relationship between γ and ω_o phases. The γ phase grows up faster than ω_o phase within the β_o phase areas during annealing at 800 °C.     

Microstructure and mechanical properties of large size Ti-43Al-9V-0.2Y alloy pancake produced by pack-forging

A pancake of Ti-43Al-9V-0.2Y (at.%) alloy with dimensions of ϕ480 mm × 46 mm was fabricated by pack-forging with a thick reduction of 80%. The as-forged Ti-43Al-9V-0.2Y alloy pancake has a duplex (DP) microstructure, which is composed of B2/α₂/γ lamellar colonies and massive B2 and γ phase regions distributed along the boundaries between the lamellar colonies. Different microstructures were obtained by heat treatment of samples cut from the as-forged Ti-43Al-9V-0.2Y alloy pancake. A fully lamellar (FL) structure consisting of B2/α₂/γ lamellar colonies was obtained after the heat treatment of 1350 °C/8 h. Tensile test results exhibited that the yield strength (YS) and ultimate tensile strength (UTS) of the alloy with DP microstructure were decreased from 680.7 MPa to 834.3 MPa at room temperature to 589.5 MPa and 693.1 MPa at 700 °C, respectively, and the elongation (δ) of the alloy with DP microstructure was increased from 1.99% at room temperature to 12.12% at 700 °C; the elongation (δ) of the alloy with FL microstructure was increased from 1.52% at room temperature to 85.84% at 800 °C.     

TiAl合金的热加工、组织和性能

采用水冷铜坩埚感应熔炼技术制备了高质量的Ti-43Al-9V-0．3Y合金铸锭,该合金铸态组织为近层片组织结构,层片团簇的体积分数为85％左右,大小约为80μm,块状卢和γ相位于层片团簇边界。层片结构中除了γ和α2相外,还存在少量的口相析出物。Ti-43Al-9V-0．3Y合金具有良好的热加工性能,通过包套锻造和包套轧制技术,成功制备了大尺寸TiAl合金锻饼和国内最大尺寸TiAl合金板材,其尺寸分别为犯60mm×24mm和500mm×300mm。经热变形后,Ti-43Al-9V-0．3Y合金的显微组织明显细化,力学性能得到了显著提高。     

Microstructural instability in surface layer of a high Nb-TiAl alloy processed by shot peening following high temperature exposure

Microstructural instability induced by shot peening was investigated in a Ti-45Al-8.5Nb-(W, B, Y) alloy following high temperature exposure. After shot peening and thermal exposure at 1000 °C for 300 h, fine grains are formed in the outermost (FG layer). Underneath, coarse grains (CG layer) are formed. The FG layer is composed of completely recrystallized γ grains. The CG layer is composed of incompletely recrystallized γ grains, where the critical strain for recrystallization is not reached. During long term thermal exposure, α₂ lamellae can undergo dissolution, precipitation and growth. After thermal exposure for 300 h, large α₂ grains precipitated at the γ/γ interface or inside the γ grains in both FG and CG layers. The precipitated α₂ particles almost have the same orientation with primary α₂ lamellae, indicating that nearly no recrystallization phenomena occur for α₂ phase. So the γ lamella is easier to recrystallize than that of α₂ lamella at the same temperature and residual strain.     

Processing,microstructure, and properties of β titanium alloys modified with boron

The development of next-generation βTi alloys is expected to involve very attractive combinations of strength-toughness-fatigue resistance at large cross sections, improved and affordable thermomechanical processing, and enhanced elevated temperature capability. This article describes the development of βTi alloys that are modified with small boron (B) additions to achieve these goals. Two important aerospace alloys, Ti-15Mo-2.6Nb-3Al-0.2Si and Ti-5Al-5V-5Mo-3Cr microalloyed (0.1%) with B were considered. Ingots that were 70 mm in diameter and 500 mm in length were cast using induction skull melting. A detailed microstructural characterization and tensile property evaluation were conducted. Microalloying with B refines the cast grain size to about 50 μm, which enhances strength and ductility. The effect of B additions on the microstructural stability and properties in the as-cast condition was established. The implications of B additions on the microstructural evolution and affordability of subsequent processing is also discussed. This paper was presented at the Beta Titanium Alloys of the 00’s Symposium sponsored by the Titanium Committee of TMS, held during the 2005 TMS Annual Meeting & Exhibition, February 13–16, 2005 in San Francisco, CA.     

Surface transformation of Ti-45Al-2Nb-2Mn-1B titanium aluminide by electron beam melting

Microstructure and phase evolution on the surface of Ti-45Al-2Nb-2Mn-1B (at.%) gamma based titanium aluminide was investigated by a series of electron beam melting with different beam energies and scanning speeds. X-Ray Diffraction (XRD), Glow Discharge Spectroscopy (GDS), Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were performed to characterize the phase modification and morphology after the EBM treatment.At beam energies of 250 W and scanning speed of 16 mm s⁻¹, the lamellar structure of Ti-45Al-2Nb-2Mn-1B transformed into a dendritic structure composed of initial α₂ (Ti₃Al) dendrites and an interdendritic phase of the γ (TiAl). While at higher energies of 350 W and lower beam speeds of 7 mm s⁻¹, mainly B2 and α₂ (Ti₃Al) phases with higher titanium formed on the surface.All Phase transformations increased the hardness of the surface to a maximum of 600 HV if compared to 330 HV for untreated material. Lower energies and higher speeds induced cracks in the surface layers, while higher energies and lower speeds produced hard surface layers without cracking.     

Microstructure and tensile properties of cast Ti-44Al-4Nb-4Hf-0.1Si-0.1B alloy with refined lamellar microstructures

The tensile properties of a cast Ti-44Al-4Nb-4Hf-0.1Si-0.1B alloy with fine lamellar structures have been measured at room temperature. The average lamellar colony size was in the range of 70–130 μm, depending on heat treatment condition. All the tensile ductilities measured at room temperature are below 0.45% with the majority between 0.3 and 0.4%. Microstructural examination and fractographic examination revealed that there are many lamellar colonies with their lamellar interfaces parallel to each other, in contrast to observations on an extruded sample, which showed 1% ductility. Cleavage along the lamellar interfaces in those parallel colonies in the cast samples forms clusters of small parallel cracks close to each other and their collective effect, together with that from the debonding between boride ribbons and the metal matrix, may contribute to the low ductility in the cast alloy. Analysis shows that those parallel lamellar colonies probably originate from the Burgers alpha grains (obeying Burgers OR with the beta phase) formed during the beta-to-alpha solid phase transformation.     

High-Temperature Oxidation Behavior of Ti-22Al-27(Nb, Zr) Alloys

研究了Ti2Al Nb基合金Ti-22Al-(27-x)Nb-x Zr(x=0,1,6,at%)在650~800℃的氧化行为。采用XRD和SEM等测试技术对此温度区间形成的氧化层特征进行了分析。结果表明,相比Ti-22Al-27Nb,含锆合金具有较好的抗氧化性能。Ti-22Al-(27-x)Nb-x Zr合金在650℃氧化100 h,主要氧化产物为Ti O2,而在800℃氧化100 h,Ti O2,Al2O3和Al Nb O4为主要产物,但是在Ti-22Al-21Nb-6Zr合金中还有Zr O2生成。Ti-22Al-26Nb-1Zr合金具有优异抗氧化性能,归因于氧化产物细化形成了致密的氧化层,而Ti-22Al-21Nb-6Zr合金,虽然在800℃也形成了较多Al2O3,但是氧化层中的Zr O2为氧的快速扩散提供通道,进而导致该合金氧化增重明显。