Microstructural evolution and mechanical properties of a β-solidifying γ-TiAl alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed Ti–44Al–6Nb–1Mo–0.2Y–0.1B (at.%) powder by spark plasma sintering (SPS). The powder was produced by a plasma rotating electrode process (PREP), and then SPS densified at temperatures between 1200 and 1320 °C. At SPS temperatures below 1240 °C, the α₂-dominated dendritic structure in the PREP powder particles disappeared and the fully dense microstructure mainly consisting of γ and B2 grains formed during SPS, but several original powder particle boundaries (OPBs) still remained. While sintered above 1240 °C, OPBs vanished entirely and an uniform duplex microstructure emerged. Furthermore, fully-lamellar (FL) microstructure with mean colony size smaller than 20 μm was produced via β-homogenization annealing. This FL microstructure renders a good tensile elongation of 1.25% and yield strength of 665 MPa at room temperature. However, instability of α₂/γ lamellar structures was induced by final stabilization annealing, resulting in sharp reduction of both room-temperature ductility and high-temperature strength.     

TiAl合金全片层组织的热稳定性研究

研究了Al含量、冷却速率和添加硼元素对TiAl合金全片层组织在1150℃的热稳定性的影响。研究表明：Al含量在46％～48％（原子分数,下同）范围的二元TiAl合金的Al含量越高,γ偏析程度越严重,铸造片层组织的热稳定性越差;Ti-48AI合金α单相区固溶处理后炉冷的粗片层组织的稳定性远远优于空冷的细片层组织,空冷细片层组织容易在晶界处发生不连续粗化转变,并且空冷片层晶粒内的魏氏片层（LW）与基体的界面往往与晶界一同成为片层组织发生分解的起始部位;Ti-48A1合金中添加0．8％B因晶界TiB2相的存在能有效抑制细片层组织的晶界不连续粗化,但γ相从TiB2／基体界面和晶界重新形核生长可使片层组织转变为均匀的细晶近γ组织。     

Microstructural stability of a cast Ti–45.2Al–2W–0.6Si–0.7B alloy at temperatures 973–1073 K

Microstructural stability of a cast intermetallic Ti–45.2Al–2W–0.6Si–0.7B (at.%) alloy after ageing in the temperature range from 973 to 1073 K up to 10,000 h was studied. The microstructure of the alloy consists of lamellar and TiAl-rich regions. Lamellar regions consist of fine lamellae of γ(TiAl) and α₂(Ti₃Al) phases, fine B2 and Ti₅Si₃ precipitates on the lamellar interfaces. Fine α₂ lamellae are partially decomposed before ageing. Small volume fraction of coarse B2 particles and few ribbon-like borides were found within lamellar regions. TiAl-rich regions contain coarse α₂ lamellae, rod-like B2 particles, small volume fraction of coarse Ti₅Si₃ precipitates and ribbon-like borides. The as-received microstructure of the alloy is unstable during long-term ageing at temperatures ranging from 973 to 1073 K. The α₂ phase in the lamellar and TiAl-rich regions transforms to the γ phase and fine needle-like B2 precipitates. The microstructural instabilities lead to softening of the alloy. The microhardness decrease is measured to be faster in the lamellar regions compared to that in the TiAl-rich regions.     

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.     

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.     

Microstructural observations of as-prepared and thermal cycled NiCoCrAlY bond coats

Microstructures of as-prepared and 1100 °C/100 h isothermally annealed NiCoCrAlY bond coat specimens as well as a bond coat obtained from an end of life turbine blade were characterized with TEM. In all specimens the γ grains were observed to consist of fine γ′ precipitates, which form during cooling and are unstable at the higher operating temperatures. The β grains present in the as-prepared specimens were observed to transform to L1₀ martensite in the 1100 °C/100 h isothermally annealed specimen. As a result of substrate-bond coat interdiffusion the M_s temperature increases during thermal cycling due to an increase in Ni and decrease in the Cr concentrations of the β-phase. The turbine blade bond coat was also found to contain Cr and Co-rich σ-phase precipitates.     

Microstructure and mechanical properties of a spark plasma sinteredTi–45Al–8.5Nb–0.2W–0.2B–0.1Y alloy

A high Nb containing TiAl alloy from pre-alloyed powder of Ti–45Al–8.5Nb–0.2B–0.2W–0.1Y was processed by spark plasma sintering (SPS). The effects of sintering temperature on the microstructure and mechanical properties were studied. The optimized conditions yield high densities and uniform microstructure. Specimens sintered at 1100 °C are characterized by fine duplex microstructure, leading to superior room temperature mechanical properties with a tensile strength of 1024 MPa and an elongation of 1.16%. Specimens sintered at 1200 °C are of fully lamellar microstructure with a tensile strength of 964 MPa and an elongation of 0.88%. The main fracture mode in the duplex microstructure was transgranular in the equiaxed γ grains and interlamellar in the lamellar colonies. For the fully lamellar structure, the fracture mode was dominated by interlamellar, translamellar and stepwise failure.     

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.     

Microstructural changes and mechanical behaviour of a near lamellar γ-TiAl alloy during long-term exposure at 700 °C

Ti-44Al-5Nb-1W-1B with a near lamellar microstructure was exposed at 700 °C for up to 10000 h in air. The changes in microstructure were investigated using scanning and transmission electron microscopy. It has been found that the combined addition of Nb and W can restrict parallel decomposition of α₂ lamellae into ultrafine γ lamellae, but causes prevalent precipitation of fine β(B2+ω) particles from α₂ lamellae and precipitation/growth of ω particles from β(B2) grains. However, although 3/4 of α₂ lamellae dissolved and majority of them transformed to β(B2+ω), tensile ductility is reduced only by 30% while the strengths remain essentially unchanged for the thermally exposed alloy. This is attributed to the widespread distribution of β(B2+ω) particles. On the other hand, fatigue limit was found to decrease during the first 5000-h exposure but finally increased by 11% after 10000-h exposure. The reasons for the decrease and increase of fatigue strength at different exposure stages are discussed by considering two contradictory effects on the exposed alloy: 1) exposure-induced embrittlement due to microstructural changes (harmful); 2) annealing of fatigue samples in a warm air environment for prolonged time (beneficial).     

Microstructure of aluminized stainless steel 310 after annealing

The aluminized coating on type 310 stainless steel prepared by high-activity Al pack cementation method has been annealed at 900 °C for 12 h to transform the brittle δ-Fe₂Al₅ phase into the more ductile β-FeAl phase. The microstructure is studied in detail with transmission electron microscopy. The thick outer layer has β-(Fe, Ni)Al as matrix with cube-like Cr₂Al precipitates. The interfacial layer has a thin layer of metastable FCC phase (layer I) and then mixed β-(Fe, Ni)Al grains and α-(Fe, Cr) grains (layers II and III). The Cr₂Al precipitates are present in the β-(Fe, Ni)Al grains in layer II but not in those in layer III, while β-FeAl precipitates are present in the α-(Fe, Cr) grains in both layers. The orientation relationships between various phases, the formation of the layers, and the precipitation of Cr₂Al in β-(Fe, Ni)Al are discussed.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

等离子电火花烧结温度对TiAl合金微观组织和力学性能的影响

以等离子旋转电极球形Ti-45Al-2Cr-2Nb-0.2W预合金粉末为原料,采用等离子电火花烧结工艺在1150到1250℃范围内制备了高致密度和显微组织均匀一致的细晶钛铝基合金。烧结温度为1150℃时可获得均匀组织的α2+γ双态组织,并呈现出烧结温度范围内最高的断裂强度(1026MPa)和室温延伸率(1.12%);烧结温度为1250℃时可获得全片层α2/γ组织,烧结体的断裂强度和室温延伸率分别为953MPa和0.92%。双态组织(DP)的断裂模式是等轴γ晶内的穿晶断裂和片层晶团内的晶间断裂;而DP组织则为穿片层断裂、片层间断裂和台阶撕裂3种模式的复合模式。     

Solidification structure in a cast γ alloy

Solidification microstructure of a cast Ti–50Al–6Mo alloy has been examined. The as-cast microstructure consists of β-dendrites, lamellar α₂+γ regions, eutectoid β+γ and a thin γ layer at the interface between β dendrite and lamellar structure. The microscopic basis of crystallographic texture present in the material as a consequence of the solidification path in the alloy Ti–50Al–6Mo, which solidifies through a L+β phase field has presented.     

Superplastic behaviour of two extruded gamma TiAl(Mo,Si) materials

The superplastic properties of two intermetallic Ti–46.8Al–1.2(Mo,Si) and Ti–46Al–1.5(Mo,Si) (at.%) materials produced by arc melting and processed by hot extrusion in the temperature range between 1200 and 1250 °C were studied. The materials exhibited an equiaxic near γ microstructure with γ grains finer than 1 μm and some band like region of γ grains with a size ranging from 5 to 20 μm. The finer grained zone contained a volume fraction of about 12 vol% in the 46.8Al material and about 25 vol% in the 46Al material of finely dispersed α₂-Ti₃Al particles. Mechanical tests performed on both materials at strain rates ranging from 4.6×10⁻⁴ to 10⁻² s⁻¹ in the temperature range of 975–1050 °C showed strain rate sensitivity exponents of about 0.5 at most strain rates. A maximum elongation to failure of about 300% was obtained for the 46.8Al material while about 900% was recorded for the 46Al material at 1050 °C at a relatively high strain rate of 8×10⁻³ s⁻¹. This difference is attributed to the larger volume fraction of α₂-phase particles in the 46Al material that leads to a decrease of the number and size of band like regions of coarse γ grains. The microstructure in the fine-grained areas of both materials remains essentially constant during deformation. The mechanical behavior at high temperature of these superplastic materials can be explained by considering grain boundary sliding as the dominant deformation mechanism.     

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.     

Ceramic nanocomposites obtained by sol–gel coating of submicron powders

MgAl₂O₄–ZrO₂ nanocomposites were fabricated by conventional sintering of composite powders obtained by sol–gel coating of a submicron spinel powder. In the composite powder the zirconia grains remain narrow sized and completely tetragonal even after being heat treated at temperatures where a free xerogel is completely monoclinic. The sintered material exhibits a dense, fine and highly homogeneous microstructure. The zirconia nanoparticles are located at both inter- and intragranular positions and exhibit heteroepitaxial relationships with the surrounding crystals. Tetragonal zirconia seems to be stabilised by an interface effect. Both the scale of the microstructure and the fraction of intragranular grains were controlled by adjusting the mean grain size of spinel grains before coating and sintering conditions.     

Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy

The present work characterizes the microstructure of a hot-extruded Ti–45Al–5Nb–0.2B–0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L1₀ lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep.     

Experimental and ab-initio study of the Zr- and Cr-enriched aluminide layer produced on an IN 713C Inconel substrate by CVD; investigations of the layer morphology,structural stability,mechanical properties,and corrosion resistance

The paper discusses the effect of zirconium and chromium on the microstructure and properties of the aluminide layers produced on an Inconel 713C nickel superalloy substrate. The aluminizing process was conducted using the chemical vapor deposition (CVD) method in AlCl₃ + ZrCl₃ vapors and a hydrogen atmosphere as the carrier gas. This low-activity aluminizing process yielded a diffusive multi-component aluminide layer composed of three main zones: the outer zone, about 3 μm thick, chiefly built of AlNi₂Zr, Ni₃Zr and Al₃Zr₄, the intermediate zone, about 6 μm thick, containing the β-NiAl phase, and the inner zone, with a thickness of about 7 μm, mostly composed of the Cr₂Al and β-NiAl grains. The substrate contained semi-coherent γ′-phases (Ni₃Al) separated from the γ-austenite matrix by a dislocation net. DFT calculations have shown that Cr added to β-NiAl markedly increases the elastic constant C11 and the isotropic shear modulus G, whereas the addition of Zr decreases the C44 component. Moreover, zirconium added to β-NiAl increases its plasticity thanks to the formation of wide-spread metallic ZrNi bonds. It has been found that the Zr + Cr-modified aluminide layer formed on the Inconel 713C nickel superalloy improves its corrosion resistance (as measured in a 0.1 M Na₂SO₄ solution).     

Allotropic transformation induced stacking faults and discontinuous coarsening in a γ-γ′ Co-base alloy

The stability of a Co-based alloy designed to possess a microstructure comprising of L1₂, γ′ Co₃Ti-type precipitates embedded in an A1, γ Co solid solution matrix has been investigated. The alloy showed acute microstructural instabilities upon ageing at 700 °C, resulting in the degeneration of the γ-γ′ aggregate into i) a faulted Co-based martensite and Co₃Ti and ii) a lamellar aggregate of A3-Co and Co₃Ti. The faulted Co-based phase was formed by isothermal diffusionless transformation of the metastable A1-phase, whilst the lamellar aggregate was a discontinuous reaction product.     

Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.