Full Lamellar Microstructure Research Articles

Optimization of microstructure and high-temperature mechanical properties of Ti4822/Ti2AlC composites through multiple solution-aging treatments

In this paper, a Ti-48Al-2Cr-2Nb (Ti4822, at%) based composite with uniformly distributed Ti2AlC nano-micro precipitates was prepared by powder metallurgy followed by a multiple solution-aging treatment. Multiple solution treatments eliminated the equiaxial γ grains in the sintered composites and refined the coarse-grained primary Ti2AlC phases, resulting in a fully lamellar microstructure with fine Ti2AlC precipitates embedded between the α2/γ lamellae. The high-temperature ultimate tensile strength of the heat-treated Ti4822/Ti2AlC composites reached 543 MPa at 800 ℃, and the elongation was 7.7 %, increased by 9.7 % and 0.8 % correspondingly compared to that of the sintered composite. The enhancement of the properties of Ti4822/Ti2AlC composite in the heat-treated state was due to the elimination of γ grains in the microstructure to transform the full lamellar microstructure on the one hand, and the refinement and dispersion of agglomerated Ti2AlC precipitates on the other hand. This study provided a valuable reference for the refinement and dispersion of the second phase in TiAl composites.

Multiple ceramic particles help to improve oxidation resistance of TiAl alloy

A novel microstructural regulation strategy was utilized to improve the oxidation resistance of TiAl alloy by the controllable multiple ceramic particles. TiAl composites were prepared using spark plasma sintering (SPS) by introducing SiC and graphene oxide (GO). TiAl composites showed nearly full lamellar microstructure, and the Ti5Si3 particles in-situ precipitated at the lamellar colony boundaries and Ti2AlC in-situ precipitated at the α2/γ interfaces. Grain refinement of TiAl composite was obvious, displaying a decrease of lamellar colony size from 259 to 81 μm after addition 0.3 wt% SiC and 0.5 wt% GO. The isothermal oxidation results showed that the mass gain of TiAl composite was decreased by 56% compared to TiAl alloy after oxidation at 950 ℃ for 100 h. This was mainly attributed to the in-situ precipitated Ti2AlC and Ti5Si3 ceramic particles inhabiting the interior diffusion of O atoms and the outward diffusion of Ti and Al atoms, then the activity of O and Ti was decreased. Furthermore, the Ti2AlC and Ti5Si3 ceramic particles contributed to release the internal stress between the oxide layer and matrix, strengthening the bondability between the oxide layer and matrix.

Fretting wear behaviors and mechanism of a high Nb–TiAl alloy with full lamellar microstructure at ambient temperature

Fretting wear behaviors and mechanism of a high Nb–TiAl alloy with full lamellar microstructure at ambient temperature

Microstructure inducement of different tensile fracture mechanisms at 800 ℃ of directional solidified Ti-45Al-5Nb alloys produced by electromagnetic confinement

Directional solidified TiAl alloys have low density and good comprehensive high temperature performance, which is an ideal substitute material for high temperature structural parts. In this paper, Φ 16.5 mm directional solidified Ti-45Al-5Nb sample with full-lamellar microstructure was prepared successfully by electromagnetic confinement technique at 10 µm/s and relative tensile tests at 800 ℃ were also performed. The tensile strength of Ti-45Al-5Nb alloy at 800 ℃ is 765 MPa and the elongation is 16%. Moreover, results of tensile process characteristics and fracture analysis show that this material has two distinct fracture mechanisms at 800 ℃, which are cleavage fracture mechanism and microporous aggregation fracture mechanism. Two fracture mechanisms do not exist at the same time, and for a particular specimen, the fracture process is dominated by only one of them. Different fracture mechanisms are determined by the lamellar orientation difference and element segregation. When the orientation of adjacent grains is similar and the segregation is light, the fracture process is dominated by the cleavage fracture mechanism. Otherwise, proportion and scale of the γ phase at the grain boundary increase and the B2 phase is generated. In this case, the microporous aggregation fracture mechanism will dominate the fracture process.

Three-dimensional lamellar orientation and stress condition induced sudden fracture of directional solidified γ-TiAl alloys produced by electromagnetic confinement

Ф 16.5 mm directional solidified Ti-45Al-5Nb alloys with full-lamellar microstructure were successfully obtained through electromagnetic confinement and seed crystal methods. Tensile experiments at ambient temperature were performed to evaluate the industrial processability of the alloy. Although the angle between the full-lamellar structure and the applied stress is approximately 0°, the elongation is still extremely low. The fracture morphology and internal microstructure were observed from multiple directions. Combined with external mechanical stress condition, the reason for sudden fracture was discovered and the lamella orientation control plan is proposed based on different specimen conditions. The cognition of the relationship between the lamellar orientation and mechanical performance was supplemented and expanded on the three-dimensional scale.

Control of kink-band formation in mille-feuille structured Al/Al2Cu eutectic alloys

Kink bands have recently received significant attention owing to their ability to increase the strength and ductility of some Mg alloys. In this study, we first demonstrated that it is also expected even in Al alloys, by controlling the morphology of the introduced kink bands. Several directionally solidified Al–Cu alloys, wherein an Al/Al2Cu eutectic lamellar microstructure developed, were focused, and the variations in deformation behavior with microstructure were examined. The alloys can be deformed at room temperature, and the yield stress exhibits strong anisotropy. A high yield stress appears when stress is applied parallel to the lamellar interface, accompanied by kink-band formation. The microstructure was found to play an important role in controlling kink-band formation and the resultant mechanical properties. Only a few large kink bands form in a eutectic alloy specimen with a full lamellar microstructure, which is accompanied by a lower yield stress. Thus, the kink band cannot be used positively in it. In hypoeutectic alloys in which primary Al grains coexist with the lamellar microstructure, however, significantly high yield stresses appear, which are accompanied by the homogeneous formation of small kink bands. The results demonstrate that microstructural control can vary the role of the deformation kink band from the fracture (buckling) mode to the deformation mode via the change in its morphology. This enables a large increase in yield stress while maintaining the ductility of the Al/Al2Cu eutectic alloy. The high yield stress is maintained at temperatures up to ~300 °C owing to the high thermal stability of the lamellar microstructure. These findings provide new ways to develop novel high-temperature high-strength Al alloys.

Effect of heat treatment and thermomechanical processing on microstructure and tensile property of Ti-44Al-8Nb-0.2W-0.2B-0.5Y alloy

High Nb-TiAl (Ti-44Al-8Nb-0.2W-0.2B-0.5Y, at.%) ingot was fabricated by vacuum arc remelting (VAR). The as-cast ingot was hot-isostatic pressed (HIP) and homogenizing annealing processed. The influence of heat treatment temperature and thermomechanical processing on the microstructure and tensile property of the alloy was investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and tensile tests. It was found that the high Nb-TiAl alloy after HIP and annealing was mainly composed of coarse α2/γ lamellae, β/B2 phase and γ phase and the solidification path of this alloy was: L→L+β→β→α+β→α→α+β+γ→α2+β+γ. The water quenching results showed that the alloy was in α single phase region at 1,340 °C. After heating at 1,340 °C for 30 min followed by furnace cooling, the alloy showed a full lamellar microstructure and its ultimate tensile strength was about 538 MPa, with an elongation of 0.3% at room temperature. Free-crack forged pancakes with fine-grained fully lamellar structure (FFLS) were obtained with an initial deformation temperature of 1,340 °C and the ultimate tensile strength of forged alloy was about 820 MPa, with an elongation of 0.9% at room temperature, which was much higher than that of alloy after HIP and annealing because of microstructural refinement.

Effects of multi-step heat treatment on the microstructure, segregation and property of a directional solidified Ti-45.5Al–3Nb-0.11C-0.3Si alloy produced by electromagnetic confinement

Multi-step heat treatments and corresponding room temperature tensile tests of Ti-45.5Al–3Nb-0.11C-0.3Si alloys with directional full-lamellar microstructure are performed. Effects of the temperature and the number of experimental cycles on the microstructure, segregation and property of the alloy are studied. In full-lamellar microstructure, the thickness and the proportion of the α lamellae are reduced and the lamellar decomposition phenomenon increases as the temperature rises. When the temperature exceeds 1070 °C, particle phases begin to precipitate along the α/γ interface. Results show that there are three different stages in the content and segregation changes of each element in different processes, and changes in Ti and Al are linear and uniquely related to the initial composition. With different processes, the tensile properties at room temperature increase first, then decrease and slowly climb at last. The elongation increases up to 2.69 times and the tensile strength increases to 2.45 times after appropriate heat treatment.

Microstructural evolution, tensile property and dynamic compressive property of FSWed Ti–6Al–4V alloy

Friction stir welding was applied to Ti–6Al–4V plates with 5 mm in thickness. The microstructure and mechanical properties were investigated. A full lamellar microstructure was developed near the top surface, and the size of prior β grain gradually decreases as the distance from the top surface increases. The microstructure of the bottom is fine equiaxed α grains, and the mean size is 2 μm. A mixture microstructure consisting of primary α, lamellar α + β and fine equiaxed α is discovered in thermomechanically affected zone (TMAZ). Results of transverse tensile test show that the tensile strength of the joint reaches 98% that of the base material (BM). Quasi-static compression test shows that the joint exhibits larger compressive strength and failure strain than the BM. Dynamic compressive strength of the joint is close to that of the BM; furthermore, the strain at the peak stress and energy absorption of the joint are larger than those of the BM.

Analysis of shear stress promoting void evolution behavior in an α/β Ti alloy with fully lamellar microstructure

With the help of X-ray synchrotron tomography, in situ tensile tests have been performed on Ti-6Al-4V titanium alloy with full lamellar microstructure. The rendered 3D images have shown that void could nucleate as soon as the material yields, although local stress triaxiality (T = 0.472) seems not to be high and even tends to maintain a constant value in the following damage steps(T = 0.474–0.510). It can be inferred that local stress triaxiality could not be the dominating factor during damage development in this current work. By combining this result with the result of post mortem analysis by SEM and EBSD, it can be found that voids preferentially nucleate and propagate along either the α/βinterface, or along shear bands within one grain/colony, or at grains/colonies boundary. This could be interpreted by the role of shear stress during this processing. Based on the location of voids, shear stress can be taken into consideration at α/β interface, shear band within one grain and grains/colonies boundaries, respectively. The origins of shear stress at different scales seems correlated to local microstructure inhomogeneity and subsequent heterogeneous plastic deformation at different scales. Void nucleation and propagation could depend on this local shear stress instead of stress triaxiality.

Relationship between microstructures, facet morphologies at the high-cycle fatigue (HCF) crack initiation site, and HCF strength in Ti-6242S

The effect of microstructures on the high-cycle fatigue (HCF) properties of Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si) was investigated. Five different microstructural conditions were generated through combinations of hot-deformation and heat treatments, and each was fatigue-tested using smooth round bar specimens at an R ratio of 0.1. The HCF behavior was found to be highly dependent on microstructures. The bimodal microstructure showed the highest HCF strength in the whole cycle range with 750 MPa at 1.0E+07 cycles. The fatigue ratio S of the HCF strength at 1.0E+07 cycles to tensile strength showed its highest value of 0.65 for the acicular α-microstructure whereas it showed its lowest value of 0.49 for the full lamellar microstructure. The flat facets were observed at the fatigue crack initiation sites irrespective of the microstructures, and were always located in the subsurface regions. Concurrent observation of the fatigue initiation facet and the underlying microstructure was made using the precision sectioning method. Based on these experimental results, the main factor contributing to the improvement of HCF strength was analyzed, along with the correlation between facet morphologies and fatigue life-controlling microstructural units, the fatigue initiation mechanisms and the dependence of HCF strength on the facet size.

Microstructural Investigation of Routes for Gamma Titanium Aluminides Production by Powder Metallurgy

The alloy design and efficient routes of TiAl processing are important technological challenges for the development of new aerospace systems. Gamma-TiAl alloys are potential replacements for nickel and conventional titanium alloys in hot sections of turbine engines, as well as in sub-structures of orbital platform vehicles. Powder metallurgy (P/M) of Ti-based alloys may lead to the obtainment of components having weak-to-absent textures, uniform grain structure and higher homogeneity compared with conventional wrought products. This paper aims to investigate the microstructural evolution and densification aspects involved in the obtainment of Ti-48Al-2Cr-2Nb (at.%) alloy by three P/M-processing routes. Samples were prepared from elemental and pre-alloyed powders mixed for 2 h, followed by cold uniaxial and isostatic pressing followed by sintering and hot pressing stages between 1100°C up to 1400°C, for 1 h. After metallographic preparation, sintered samples were characterized by means of scanning electron microscopy (SEM) in the backscattered mode (BSE), X-ray diffraction (XRD), and density measurements. The results showed the potential of TiAl pre-alloyed powders to prevent Kirkendall porosity. A full lamellar microstructure was obtained by the pressureless route while a duplex microstructure was observed in samples produced by the hot pressing route.

Recrystallization Behavior at Diffusion Bonding Interface of High Nb Containing TiAl Alloy

A series of diffusion bonding tests are conducted on a high Nb containing TiAl alloy with full lamellar (FL) microstructure and the effect of interfacial recrystallization behavior on the shear strength is discussed. Microstructural observations reveal that bonding above 1 100 °C and 30 MPa results in recrystallization at the bonding interface flanked by FL microstructure, thus promoting the interface migration and leading to a sound joint. Based on the present results, the nucleation mechanism of recrystallization at the bonding interface is studied. Shear testing results show that recrystallization at the bonded zone improves the bonding quality of the joints by changing the failure mode. The shear strength of the joints attains 400 MPa when bonding at 1 150 °C/30 MPa/45 min and 1 100 °C/40 MPa/45 min.

Additive manufacturing of a high niobium-containing titanium aluminide alloy by selective electron beam melting

Additive manufacturing (AM) offers a radical net-shape manufacturing approach for titanium aluminide alloys but significant challenges still remain. A study has been made of the AM of a high niobium-containing titanium aluminide alloy (Ti–45Al–7Nb–0.3W, in at% throughout the paper) using selective electron beam melting (SEBM). The formation of various types of microstructural defects, including banded structures caused by the vaporization of aluminum, was investigated with respect to different processing parameters. To avoid both micro- and macro-cracks, the use of higher preheating temperatures and an intermediate reheating process (to reheat each solidified layer during SEBM) was assessed in detail. These measures enabled effective release of the thermal stress that developed during SEBM and therefore the avoidance of cracks. In addition, the processing conditions for the production of a fine full lamellar microstructure were identified. As a result, the Ti–45Al–7Nb–0.3W alloy fabricated showed outstanding properties (compression strength: 2750MPa; strain-to-fracture: 37%). SEBM can be used to fabricate high performance titanium aluminide alloys with appropriate processing parameters and pathways.

Reaction-Diffusion Bonding of High Nb Containing TiAl Alloy

Abstract In this study, high Nb containing TiAl alloy (TAN) was welded by reaction-diffusion bonding using Al foil as interlayer at 1200 °C for 2 h. The desired full lamellar microstructure ( γ +α 2 ) was obtained across the bonding seam when the joint was heat-treated according to the heat treatment process of TAN alloy. The present paper also investigated microstructure and bonding mechanism in details. The results show that liquid Al can react with TAN to form TiAl 3 intermetallic layer, and then the TiAl 3 layer will transform to γ -TiAl by solid-state diffusion at high temperature, finally a full lamellar structure is formed after heat treatment. Furthermore, sound joints with full lamellar structure can be also obtained when TAN alloy is directly bonded in accordance with the heat treatment cycle of TAN alloy.

The Influence of Initial Microstructures on the Diffusion Bonding Interface of High Nb Containing TiAl Alloy

The solid-state diffusion bonding experiments of high Nb containing TiAl alloy were successfully carried out at 950°C under a uniaxial pressure of 30MPa for 45min, and the influence of different initial microstructures, such as initial forged microstructure (named duplex microstructure) with different grain sizes, near lamellar microstructure and full lamellar microstructure, on the interface of the bonding joints were investigated. And the microstructure characterization of interfaces was taken by OM, SEM, EDS and micro-hardness tester. The results indicated that the grain size and strain energy are of great importance to improve the quality of interfacial bonding. Besides, the interfacial microstructure was found different from matrix and changed during the diffusion bonding process. Meanwhile, micro-hardness tests of the three kinds of joints showed that the micro-hardness in the interface was slightly higher than matrix in all the joints, resulted from the working hardening of the interface under the uniaxial pressure.

Transgranular shearing introduced brittlement of Ti–5Al–5V–5Mo–3Cr alloy with full lamellar structure at room temperature

β titanium alloys with full lamellar structure often exhibit the low elongation, and the brittle fracture mechanism of Ti–5Al–5V–5Mo–3Cr β titanium alloy with full lamellar microstructure is investigated in this paper. After tensile test, the fracture surface characterization and the cross sectional observation show that the transgranular dimple fracture, the intergranular fracture and the transgranular shearing fracture are the fracture mode of the alloy. It is discovered that the cracks form by the coalescence of the microvoid at the α/β interfaces, and the shearing is triggered by the gradual increment of the crack dimension inside the prior β grain at a high stress level. The first two fracture progresses contribute to the plastic deformation of the alloy, and the transgranular shearing occurs in the large scale β grain and is the main brittle fracture mechanism of Ti-5553 alloy.

Production of Aerospace Tial Intermetallics for High Temperature Applications by Powder Metallurgy

During the recent years, alloys based on the intermetallic compound TiAl have attracted a considerable interest as potential competitors to steels and superalloys. Gamma-TiAl alloys are potential replacements for nickel and conventional titanium alloys in hot sections of turbine engines, as well as in orbital platform vehicles. The alloy design and efficient routes of TiAl processing are important technological challenges. Powder metallurgy is a near net shape process that allows the parts production with complex geometry at low costs. In this work, samples of Ti-48Al-2Cr-2Nb (at.%) were prepared from elemental and pre-alloyed powders mixed for 2 h, followed by cold uniaxial and isostatic pressing and sintered between 800 up to 1400°C, for 1 h, under vacuum. After metallographic preparation, sintered samples were characterized by SEM (Scanning Electron Microscopy), density analyses and Vickers microhardness measurements. The results indicated the viability of the pre-alloyed route and the tendency of a full lamellar microstructure of alternating gamma and α2 phases in high sintering temperatures.

Effect of heat treatment on microstructures and mechanical properties in a full lamellar PM TiAl alloy

The effect of heat treatment on the microstructures and tensile properties of a powder metallurgical (PM) TiAl based alloy has been investigated in this paper. The near gamma (NG) microstructure is transformed to a full lamellar (FL) microstructure with an average grain size of 100 μm by heat treatments. The lamellar spacing of FL structure decreases with the increase of cooling rate. For cooling rates of 5, 10 and 50 °C/min, the lamellar spacing is 1.9, 1.0 and 0.8 μm respectively. The room temperature tensile properties exhibit an increasing trend with decrease of lamellar spacing.

EFFECTS OF MELT MIXING WITH HIGH AND LOW TEMPERATURE MELTS ON SOLIDIFACATION MICROSTRUCTURES OF Au-20Sn EUTECTIC ALLOY

The effects of melt mixing with high-temperature and low-temperature melts on Au-20Sn(mass fraction,%)eutectic alloy solidification microstructures have been studied.The solidification microstructure evolutions of Au-20Sn eutectic alloy by the temperatures of high-temperature melt were investigated.Melt mixing with high-temperature and low-temperature melts can effectively improve the solidification microstructure of Au-20Sn alloy.Adopting an appropriate melt mixing condition,which is high-temperature melt with 350℃and low-temperature melt with 283℃,the precipitation of primary phaseζ'-Au_5Sn will be inhibited during solidification process,and the full lamellar eutectic microstructure was obtained.When the temperature of high-temperature melt is high (360℃)or low(340℃),the primary phaseζ'-Au_5Sn will also exist in the solidification microstructure. Melt mixing with high-temperature and low-temperature melts can effectively decrease the Au atom segregation and modify the precipitation behavior of primary phaseζ'-Au_5Sn.The compressive behavior at 220℃exhibits a low yielding stress and a low stress platform for the alloy with full lamellar eutectic microstructure prepared by melt mixing,which indicates that the hot-workability of Au-20Sn alloy can be improved by melt mixing.