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.     

Quantitative analysis of the effect of heat treatment on microstructural evolution and microhardness of an isothermally forged Ti–22Al–25Nb (at.%) orthorhombic alloy

The microstructural features of the 980 °C isothermally forged Ti–22Al–25Nb (at.%) orthorhombic alloy during heat treatment were quantitatively investigated. The volume fraction of the O phase precipitates, the width and length of the lath O phase, and the diameter of equiaxed grains at different heat treatment temperatures were measured using an image analysis software. Quantitative relationships among heat treatment temperature, microstructure parameters, and microhardness were established. The relationship between microstructure parameters and microhardness was analyzed with a multiple regression analysis technique. The results indicate that the microstructure of this alloy is mainly depended on the heat treatment schedule. Only equiaxed O/α₂ grains and B2 matrix existed when the samples were solution-treated above 980 °C, while equiaxed α₂ grains, rim O around α₂, and equiaxed/lath O could be obtained after the samples were solution treated below 980 °C. The width of lath and acicular O phases, and volume fraction of total precipitates could be controlled in the range of 0.37–0.88 μm, 0.09–0.48 μm and 10.91–60.18%, respectively. Experimental and statistical analysis showed a linear relationship between the microstructure parameters and microhardness.     

Microstructural evolution of a PM TiAl alloy during heat treatment in α+γ phase field

In this study,the effect of temperatures and cooling rates of heat treatment on the microstructure of a powder metallurgy (PM) Ti-46Al-2Cr-2Nb-(B,W) (at.％) alloy was studied.Depending on the cooling rate and temperature,the different structures were obtained from the initial near-γ (NG) microstructures by heat treatment in the α+γ field.The results show that the microstructures of samples after furnace cooling (FC) consist primarily of equiaxed γ and α2 grains,with a few grains containing lamellae.Duplex microstructures consist mainly of γ grains and lamellar colonies were obtained in the quenching into another furnace at 900℃ (QFC) samples.However,further increasing of the cooling rate to air cooling (AC) induces the transformation of α→α2 and results in a microstructure with equiaxed γ and α2 grains,and no lamellar colonies are found.     

Fracture and R-curve behavior of an intermetallic β-stabilized TiAl alloy with different nearly lamellar microstructures

Four different microstructures of an engineering multi-phase Ti–43.5Al–4Nb–1Mo–0.1B alloy (in at.%) were fracture mechanically tested under monotonic loading conditions from room temperature up to 700 °C. Monotonic loading crack resistance curves were measured using the potential drop technique and the calculated crack lengths were compared to optically measured crack lengths during in-situ experiments under the light-optical microscope. By examining the fracture surfaces and the polished side faces of the samples in the scanning electron microscope the fracture mechanisms were evaluated. It was found that the fracture toughness mainly depends on structure and phase arrangement at the boundaries and can be enhanced up to 30% by having thick boundary areas with a few micrometers in width, consisting of unconnected β_O and γ grains. In contrast, a connected β_O-phase at the colony boundaries as well as a texture which provide lamellae parallel to the subsequent fracture surface reduce the fracture toughness.     

Thermomechanical fatigue behaviours of a third generation γ-TiAl based alloy

In order to clarify the behaviours of thermomechanical fatigue (TMF) of a third generation γ-TiAl based alloy, the influence of related microstructural instability during TMF process on stress–strain response, fatigue life and fracture way under in-phase (IP) and out-of-phase (OP) loading mode was investigated. Cyclic softening at high temperature (>700 °C) arises from the dissolution of α₂ lamellae and recrystallization of γ phase. Cyclic hardening at low temperature (<550 °C) is caused by strong interaction between dislocations. As temperature increases, the mean stress and remained plastic strain range increase, leading to severe TMF damage. Owing to the formation of superfine γ grains in IP condition, a superimposed effect of creep and fatigue damage contributes to the TMF failure. OP loading mode brings about the coarsening of primary equiaxed γ grains. Fatigue damage displays the intergranular fracture and transgranular cleavage fracture ways of coarse γ grains.     

High toughness and strength of Mo-12Si-8.5B-ZrB2 alloy resulting from a bimodal α-Mo grain structure

Designing a bimodal α-Mo grain structure in Mo-12Si-8.5B-ZrB₂ alloy was achieved via mechanical alloying followed by hot pressing. This bimodal structure consisted of a major of nano-scaled α-Mo grains and partial micron-scaled α-Mo grains. The effective cooperation of these fine and coarse α-Mo in the Mo-12Si-8.5B-ZrB₂ alloy was very beneficial for improving its toughness as well as strength. The high toughness was mainly attributed to the in-situ toughening and crack trapping effects originating from the coarse α-Mo. While the high strength was owing much to the grain boundary strengthening of the fine α-Mo. When the volume fraction ratio of the fine and coarse α-Mo grains was approximately 6:1, the fracture toughness value reached 13.1 MPa·m^1/2 in the bimodal alloy, showing a 18% increase compared to an ultrafine-grained alloy. Simultaneously, a high compressive strength with 2998 MPa was also maintained.     

The formation mechanism of the O phase in a Ti3Al–Nb alloy

It is reported that there are several different transformation mechanisms of the O phase in different heat treatment conditions in the Ti₃Al based alloys. However, very little work has been carried out on the α₂→O phase transformation in the Ti₃Al–Nb alloys of Nb amounts exceeding 12 at%. In this paper, the formation mechanism of the O phase in the Ti–24Al–14Nb–3V–0.5Mo (at%) alloy has been carried out by means of TEM and HRTEM. The results show that the O phase is directly derived from the primary equiaxed α₂ grains with a fine streak contrast, and exists in multivariant forms owing to its different orientations after the alloy is solution treated at 1000°C for 1 h followed by water quenching (WQ) and aged at 650°C for 24 h. The O plates in the primary equiaxed α₂ grains exist not only in the form of a single variant, but also in the form of fine α₂+O mixtures. The analysis indicates that the formation of the O phase is the result of a phase decomposition, that is the introduction of niobium as the preferred β stabilizer makes the supersaturation of niobium in the primary α₂ grains, and the α₂ phase containing Niobium separates into Niobium lean and Niobium rich regions through the Niobium diffusion: α₂→α₂(Nb-lean)+O(Nb-rich). Niobium rich regions transform to the ordered orthorhombic phase (O phase) with a lattice distortion and only a very small composition change. It appears, therefore, that the transformation involves nucleation, growth and coarsening of the O phase by a diffusion mechanism.     

Dynamic stress–strain properties of Ti–Al–V titanium alloys with various element contents

A series of Ti–Al–V titanium alloy bars with nominal composition Ti–7Al–5V ELI,Ti–5Al–3V ELI,commercial Ti–6Al–4V ELI and commercial Ti–6Al–4V were prepared.These alloys were then heat treated to obtain bimodal or equiaxed microstructures with various contents of primary a phase.Dynamic compression properties of the alloys above were studied by split Hopkinson pressure bar system at strain rates from 2,000 to 4,000 s-1.The results show that Ti–6Al–4V alloy with equiaxed primary a(ap)volume fraction of 45 vol%or 67 vol%exhibits good dynamic properties with high dynamic strength and absorbed energy,as well as an acceptable dynamic plasticity.However,all the Ti53ELI specimens and Ti64ELI specimens with ap of 65 vol%were not fractured at a strain rate of4,000 s-1.It appears that the undamaged specimens still have load-bearing capability.Dynamic strength of Ti–Al–V alloy can be improved as the contents of elements Al,V,Fe,and O increase,while dynamic strain is not sensitive to the composition in the appropriate range.The effects of primary alpha volume fraction on the dynamic properties are dependent on the compositions of Ti–Al–V alloys.     

The effects of long-term air exposure on the stability of lamellar TiAl alloys

The thermal stability of fine-grained and coarse-grained lamellar TiAl alloys was studied at 700°C in air. Both α₂ and γ lamellae were altered markedly after 1000- and 3000-h exposure. Three types of decomposition were found: (a) parallel decomposition of coarse α₂ into bunches of very fine α₂+γ lamellae, (b) perpendicular decomposition of fine γ (or γ+α₂) lamellae into short-ranged γ domains or (γ+α₂) grains and (c) perpendicular decomposition of fine α₂ lamellae into short-ranged α₂/γ grains. All three types are still at an intermediate stage after 3000-h exposure and neither break-up of lamellae nor globularisation of segments was observed. The thermal stability of microstructural constituents in increasing order was found to be: thick α₂, thin α₂, thin γ, and thick γ lamellae. Tensile properties were affected by these changes and the extent was found to vary from alloy to alloy.     

The melting behaviour of extruded Mg–8%Al–2%Zn alloy

Microscopical techniques were used to provide the microstructural details and identify mechanisms governing phase and morphological transformations during the heating of Mg–8%Al–2%Zn pellets in solid and semisolid states. It was found that an as-extruded matrix of equiaxed α-Mg grains with a twinning substructure, was thermally unstable and experienced complete recrystallization after reheating to 200 °C. The precipitates of the Mg₁₇Al₁₂ phase and augmented concentrations of alloying elements within migrating grain boundaries and triple junctions played a key role in transformation of the equiaxed grains into thixotropic structures during partial melting. A direct link exists between sizes of equiaxed grains in the solid state and unmelted particles in the semisolid slurry. Although the morphology of primary solid particles did not change during the melting progress, the rate of particles’ coarsening at high temperatures and their internal microstructure in subsequently solidified alloy were influenced by the solid–liquid ratio. The importance of these findings for semi-solid injection molding practice is emphasized.     

Enhancement of AZ80 joints using ultrasonic vibration-assisted welding process

Ultrasonic vibration-assisted welding process was developed to improve the microstructures and mechanical properties of AZ80 Mg alloy joints. Results show that the ultrasonic vibration-assisted welding process has a significant effect on the morphology and size of both β-Mg₁₇Al₁₂ precipitated phases and α-Mg grains. With ultrasonic vibration treatment, the primary α-Mg grains are transformed from coarse dendrites to nearly fine equiaxed grains, and the β-Mg₁₇Al₁₂ phases at α-Mg boundaries are refined and become discontinuous, owing to the cavitation effects and acoustic streaming effects induced by ultrasonic vibration. The mechanical properties of AZ80 Mg alloy joints under the ultrasonic vibration-assisted process are obviously improved. And the influence mechanisms of the ultrasonic vibration-assisted welding process on joints are discussed in detail.     

Microstructures and shape memory characteristics of dual-phase Co–Ni–Ga high-temperature shape memory alloys

The influence of microstructure on mechanical properties and shape memory characteristics of Co–Ni–Ga high-temperature shape memory alloys were investigated in this study. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to detect the microstructures. We found that these alloys were composed of dual phases, a non-modulated tetragonal L1₀ martensite and a face-centered cubic (fcc) γ phase. The martensite was twinned and well self-accommodated. The γ phase was a Co-based solid solution with 30% lower hardness than martensite. Although the fracture mode was intergranular, the strength and plasticity of the alloys increased markedly with the increasing volume fraction of the γ phase. The presence of the γ phase in grain boundaries rather than in the martensite is favorable to shape memory recovery. This was revealed by the maximum shape recovery strain over 5.0% that was obtained in the Co₄₆Ni₂₅Ga₂₉ alloy, with the γ phase formed mainly in grain boundaries.     

Thermal stability of an intermediate strength fully lamellar Ti–45Al–2Mn–2Nb-0.8 vol.% TiB2 alloy

A fully lamellar γ-TiAl alloy Ti–45Al–2Mn–2Nb + 0.8 vol.% TiB₂ was exposed at 700 °C for 10,000 h in air. No perpendicular decomposition of α₂ into α₂ + β/α₂ + γ sections and no recrystallisation of β equiaxed grains on decomposed α₂ + γ lamellae were observed all through the prolonged exposure. The dominant change in microstructure was that the α₂ lamellae thinned gradually and became increasingly discontinuous with increasing exposure time. The average thickness of α₂ lamellae reduced to a half and the volume fraction reduced by 1/4 after 10,000 h exposure. Correspondingly, the tensile properties were hardly changed, while fatigue strengthening was increased by 30%. The reasons for the relatively high degree of stable microstructure and mechanical properties are discussed based on production processing and alloy composition. The “exposure-induced fatigue strengthening” observed after 10,000 h exposure is assumed to be caused by structure relaxation and stress dissipation throughout the specimen.     

Microstructural evolution and mechanical properties of Al–40 wt.%Zn alloy processed by equal-channel angular extrusion

The Al-based Al–40 wt.%Zn alloy was subjected to multi-pass equal-channel angular extrusion (ECAE) via route-A and route-B_C. Before and after ECAE processing, microstructural evolution, the tensile properties, impact toughness and fracture behavior of the alloy were investigated.ECAE processing caused to elimination of the as-cast dendritic microstructure and formed a structure consisting of elongated, ribbon shaped α-phase via route-A and mostly equiaxed α-phase via route-B_C. ECAE processing also caused plastic instability as necking at early onset of deformation. As a result of more effective mechanical mixing via route-B_C, softening and necking occurred more apparently. The tensile and yield strength of the alloy increased just after first pass and then slightly decreased with increasing number of passes. On the other hand, its elongation to failure and impact toughness increased with increasing number of passes in both routes. The increase obtained via route-A is more pronounced in both properties. Fracture behavior of the as-cast alloy changed from brittle to ductile mode after multi-pass ECAE. Elimination of dendritic as-cast structure with reduction of porosities and deformation-induced homogenization by the effect of ECAE processing increased the ductility and impact toughness of the alloy and caused formation of a fracture surface consisting of micro-voids and dimples which indicates of ductile fracture. Attained experimental results indicate that multi-pass ECAE processing is very effective in improving the tensile elongation and impact toughness of binary Al–40 wt.%Zn alloy.     

热处理对Ti90钛合金显微组织及性能的影响

对比研究了退火温度、冷却速度及多重退火对一种新型近α钛合金Ti90显微组织、室温拉伸性能和腐蚀行为的影响。结果表明:在两相区退火时,随退火温度升高,变形组织逐渐球化,初生α相(α_p)体积分数降低,次生α相(α_s)增多并发生粗化,合金强度逐渐降低,塑性提高;β单相区退火后空冷,组织中原始β晶粒粗大,且有晶界α相析出(αGB),合金塑性急剧下降;β单相区退火后水冷,β晶粒内部析出细针状α’马氏体相,显著提高了合金强度,同时保持了较好的塑性;多重退火后αp和βt (β转变组织)尺寸增加,α_s粗化,导致合金强度和塑性同时降低;极化曲线测试结果显示,具有4种不同αp含量显微组织的Ti90合金在3.5%NaCl溶液中均表现出钝化行为,且钝化电流密度小,耐蚀性较好,耐蚀能力由强到弱依次为双态组织>等轴组织>片层组织。     

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.     

Microstructure,creep, and tensile behavior of a Ti–21Al–29Nb(at.%) orthorhombic+B2 alloy

The creep and tensile deformation behavior of a Ti–21Al–29Nb (at.%) alloy were studied. Monolithic sheet materials were produced through conventional thermomechanical processing techniques. Heat treatments at all temperatures above 1050 °C, followed by water quenching, resulted in fully-B2 microstructures. Below 1050 °C, either equiaxed or Widmanstätten O-phase precipitated within the B2 grains. RT elongation-to-failure values of less than 2% were recorded for aged microstructures containing 72–78 volume percent O phase. Tensile-creep experiments were conducted in the temperature range 650–710 °C and stress range 48–250 MPa. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress and microstructure. Microstructural effects on the tensile properties and creep behavior are discussed and the data was compared to that for other Ti₂AlNb-based alloys.     

不同原始组织TC6钛合金高温变形微结构演化及其力学性能

采用gleeble-1500热模拟试验机和Hopkinson压杆,对具有4种典型组织的TC6钛合金分别进行了高温准静态压缩和室温动态压缩试验,结合TEM观察,研究了不同原始组织的TC6钛合金高温变形微结构演化及其力学性能。结果表明:具有4种典型组织的TC6钛合金高温变形时随温度升高微结构的演化可分为等轴型组织演化和网篮型组织演化,前者演化过程为:等轴α相的拉长变形—动态再结晶—动态再结晶晶粒长大—α/β相变;后者演化过程为:板条状α相弯曲变形—板条状α相断裂—动态再结晶—动态再结晶晶粒长大—α/β相变,板条状α相变成短棒状。位错活动及动态再结晶是控制4种组织的TC6合金在高温变形过程中组织演化和力学性能的重要因素;网篮组织晶界众多,位错运动障碍较多,在高温下具有较其余3种组织更高的流变应力;等轴组织α相晶粒较大,位错运动障碍较少,其流变应力在4种组织中最低;双态组织、固溶时效组织的流变应力介于等轴组织与网篮组织之间。4种组织的TC6钛合金的室温动态力学性能均对应变率较敏感。4种组织的TC6钛合金在室温及应变率为2500～4000s-1动态压缩条件下,塑性由大到小依次为:等轴组织、双态组织、固溶时效组织和网篮组织,流变应力由大到小依次为:固溶时效组织、双态组织、网篮组织和等轴组织。     

Irradiation behaviour of α2 and γ phases in He ion implanted titanium aluminide alloy

A Ti–45Al–2Nb–2Mn + 0.8 vol.% TiB₂ (at.%) alloy with fully lamellar microstructure consisting of hexagonal-close-packed (hcp) α₂ and face-centred-tetragonal (fct) γ phases was irradiated by implanting helium ions to different fluences. Microstructural examination showed that helium cavities are formed in both the α₂ and γ phases after He-ion irradiation. However, the helium cavities and their size change with fluence are much larger in the α₂ phase than those in the γ phase, indicating that the γ phase exhibits better tolerance to the He-ion irradiation than the α₂ phase. Since α₂ and γ phases have different crystal structures, they possess differences in helium solubility and interstitial migration. These differences are responsible for the variation in radiation damage behaviour between the two phases.     

The Initial Oxide Scale Development on a Model FeNiCrAl Alloy at 900 °C in Dry and Humid Atmosphere: A Detailed Investigation

The investigated alumina forming FeNiCrAl model alloy shows protective oxidation behavior in dry and humid environment at 900 °C. Hence, this type of alloy may replace conventional chromia forming austenitic alloys in aggressive oxidizing/reducing environments. A detailed investigation of the oxide scale development reveals a complex initial scale development. Firstly, at alloy grain boundaries, a thin Al rich oxide forms which is replaced by transient alumina platelets in dry and equiaxed α-Al₂O₃ crystallites in humid atmosphere. The scale at alloy grain centers develops via a layered scale of external chromia:Fe/Ni metal inclusions:internal alumina to a layered external spinel:internal alumina scale in dry atmosphere. In humid condition an additional oxide feature appears on the center of large alloy grains i.e. thick oxide protrusions. Despite the initially different phase compositions a continuous protective α-Al₂O₃ scale forms both atmospheres.