TiAl Specimens Research Articles

On real-time control of microstructure of TiAl specimens with varied cross-sections based on numerical calculation and machine learning

In our previous study, TiAl alloys with varied cross-sections were prepared via the electromagnetic cold crucible zone melting (ECCZM) method. However, due to the unstable temperature field, the equiaxed grain region appeared in the columnar grain, disrupting the microstructure continuity. Herein, the processing method was redesigned based on numerical calculation and machine learning (ML). Real-time adjusted processing parameters were established to stabilize the temperature field. Firstly, the equation of absorbed heating power about withdrawing distance was calculated based on the magnetic field distribution in the cold crucible, and the input power was adjusted in real-time accordingly. Subsequently, a combination of real-time adjusted input power and withdrawing rate was established based on the balance of absorbed heating power and heat loss in the mushy zone. The specimen with real-time adjusted input power and withdrawing rate was also prepared. In addition, the matched input power and withdrawing rate were optimized by ML, and the TiAl alloy without equiaxed grain was obtained via the new processing method. Microstructure and mechanical properties of the directionally solidified TiAl alloy with the conventional ECCZM method (CM) and the real-time adjusted (RA) method were compared. It was found that equiaxed grains would induce intergranular fracture. The ML-optimized RA method can eliminate equiaxed grain and obviate the occurrence of intergranular fracture during tensile deformation, thus preventing early fracture and improving the tensile strength.

Chemical, microstructure, and mechanical property of TiAl alloys produced by high-power direct laser deposition

• A gradient TiAl specimen whose chemical composition varied greatly along the building direction was fabricated by high-power direct laser deposition. • The microstructure transforms from basket-weave microstructure into acicular martensite microstructure as the height increases, and the mechanical properties also vary greatly when the height changes. • By characterizing the deformation behaviors under the indentations, it is concluded that the phase constitution is responsible for the variation of the mechanical properties. Additive manufactured metals sometimes exhibit extraordinary microstructures and mechanical properties due to the particular processes. In this paper, we focus on a novel gradient TiAl alloys fabricated by high-power direct laser deposition, whose chemical composition, microstructure, and mechanical property vary along the building direction. The results indicate that Al concentration dramatically decreases from 39.5 at.% to 30.1 at.% as the height increases from the bottom to the top. Meanwhile, microstructural characterization indicates that the specimen appears basket-weave microstructure at the bottom, then the α 2 and γ phase gradually decrease, and eventually it transforms into acicular martensite microstructure in the top region. The indentation analysis shows that the associated hardness increases as the height increases, while the plasticity reaches a minimum value in the middle region. The increasing amount of β o (ω) is considered to be responsible for the increasing hardness because of the strong precipitation strengthening effect. The high plasticity in the bottom and top regions results from the strong deformation behaviors of the γ and β o phases.

Size-Dependent Fracture Characteristics of Intermetallic Alloys

BackgroundLightweight alloys such as intermetallic titanium aluminide (TiAl) alloys are poised to be a potential candidate for replacing heavier nickel based super alloys in an aero engine. However, before an industry wide implementation is possible, it is indispensable to develop physically accurate computational material models which account for essential deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl components.ObjectiveThe objective of this work is to determine the effect of size of tested specimens on their fracture energy and provide a physically motivated scaling law.MethodsIn this work, the quasi-brittle behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup. Since the final abrupt failure of each specimen is preceded by plasticity, a theoretical and numerical framework which accounts for both elastic and plastic work densities is applied in simulations.ResultsThe fracture energy density for each tested size is calculated numerically which is found to be lower for larger volumes, thereby, confirming the size effect in intermetallic TiAl alloys. A novel size effect law is proposed which is based on two physically motivated coefficients.ConclusionsThe work concludes with the quantitative knowledge of the size-dependent fracture energy of intermetallic alloys and an empirical scaling law to predict the same. Excellent predictive capability of the proposed law is successfully established with data of various quasi-brittle materials from literature.

Volume dependent fracture energy and brittle to quasi-brittle transition in intermetallic alloys

Intermetallic alloys such as titanium aluminide (TiAl) are lightweight with superior specific strength and high temperature oxidation resistance. They have a potential to replace heavier nickel based super alloys in low pressure high temperature sections of an aero engine. To facilitate an industry wide implementation, it is however indispensable to develop physically accurate computational methods which account for key deformation and fracture mechanisms. This assists the virtual prototyping required for the new product development using TiAl alloys. In this work, the quasi-brittle and volume dependent behavior of TiAl alloys is experimentally and numerically investigated. A total number of 29 geometrically identical TiAl specimens of three different sizes are tested in a three-point bending setup to investigate the volume dependence of fracture properties. In addition, three compact tension tests with identical specimen geometries are performed to analyze brittle to quasi-brittle transition and a switch in failure mechanism. Since the material fracture is observed to be preceded by plasticity, a theoretical and numerical framework of phase field ductile fracture which accounts for both elastic and plastic work densities is applied. The averaged fracture energy density for each test is evaluated numerically which is found to be lower for larger volumes. The work concludes with the proposal of an empirical law relating the fracture energy density with the specimen volume. Data from literature for different classes of materials is used for validation of the proposed empirical law. The mechanisms of transition from brittle to quasi-brittle fracture are investigated on the basis of local stress triaxiality in three-point and compact tension simulations.

The joining of gamma titanium aluminides via the powder interlayer bonding method

Powder interlayer bonding (PIB) is a joining technique originally developed to enable high-integrity repairs of aerospace components. The technique has previously been employed for the joining of titanium and nickel alloys utilised in the aerospace industry. This study expands on the application of the novel joining technique known as powder interlayer bonding (PIB), to the bonding of γ titanium aluminide (TiAl) material. PIB has been used to facilitate high-integrity joints in gamma titanium aluminides (TiAl), where full densification of the joint was achieved. The PIB technique described here used a metallic powder interlayer between the two faying surfaces of γ TiAl specimens. Bonds were formed in an inert atmosphere under induction heating. The PIB technique proved capable of producing high-integrity bonds in terms of microstructural evaluation, with very limited porosity retained after the bonding cycle. A brittle Ti2Al phase can be produced with heavily oxidised powder which is susceptible to cracking and will negatively affect mechanical properties.

The crack and pore formation mechanism of Ti–47Al–2Cr–2Nb alloy fabricated by selective laser melting

In this study, a broad range of parameter combinations (laser power: 100–400 W; scanning speed: 10–90 mm/s) were used to fabricate Ti-47Al-2Cr-2Nb alloy at the layer thickness of 100 μm by selective laser melting (SLM). The preparation of the TiAl-single track by SLM was prone not only to balling and irregularity but also to cracking. Although the optimized process parameters were used to fabricate TiAl specimens, many pores and cracks still existed and a low density was achieved. To understand the mechanism for the crack and pore formation, the connections among the cracks, pores, and the process parameters were investigated in addition to the variation in the crack propagation with an increase in the number of deposition layers. The results indicated that the cracks originated in the third layer, because of the accumulation of residual stresses and the changes in the composition of Ti-47Al-2Cr-2Nb deposition layers. Additionally, the frequency of cracks constantly increased with an increase in the number of deposition layers. Preheating the substrate to 200 °C improved the degree of cracking to a certain extent, as the initiation layer for the cracks increased from the third layer to the fifth layer. Despite the achieved improvement, it was not possible to produce crack-free specimens on the SLM machine used for this study. Finally, there was a good metallurgical bond between the Ti-6Al-4 V substrate and the Ti-47Al-2Cr-2Nb deposition layers that was free of pore and crack defects. These findings suggest that using SLM to fabricate Ti-6Al-4 V/TiAl intermetallic laminate composites may potentially eliminate cracking and improve the properties of TiAl alloys.

Laser Surface Texturing of TiAl Multilayer Films—Effects of Microstructure and Topography on Friction and Wear

Laser surface texturing is an efficient way to control the friction and wear properties of materials. Although described in many papers, most previous work relates to a pure topographic view of laser-textured surfaces. As lasers are heat sources, their thermal impact during treatment can be high enough to modify the material’s microstructure or surface chemistry and affect tribological properties as well. This research took a closer look at the microstructure of laser-textured TiAl multilayers, besides topographic aspects. Direct laser interference patterning was used to create well-defined line-like surface textures in TiAl multilayers with differing lateral feature sizes in the micron range. High-resolution techniques such as TEM and XRD highlighted the effect of this method on microstructure, and in particular, the phase situation of the TiAl multilayer. Thermal simulations demonstrated that the maximum achievable temperatures were around 2000 K, thus being high enough to melt Ti and Al. Cooling rates on the order of 109 K/s depended on the lateral feature size, potentially leading to metastable microstructures. Finally, ball-on-disk tests on as-textured TiAl specimens showed a reduction in wear under dry conditions depending on the periodicity of the line-like textures used.

In situ micro-compression testing of He2+ ion irradiated titanium aluminide

A titanium aluminide (TiAl) alloy 45XD has been irradiated by a He ion beam with an energy of 5MeV on a tandem accelerator at the Australian Nuclear Science and Technology Organization (ANSTO). The total fluence of He ions was 5×1017ioncm−2. A 17μm uniform damage region from the material surface with a helium concentration of about 5000appm was achieved by using an energy degrading wheel in front of the TiAl target. The micro-size test specimens from the damage layer were fabricated using a focused ion beam & scanning electron microscope (FIB-SEM) system. The in situ SEM micromechanical compressive testing was carried out inside an SEM and the results indicated irradiation embrittlement in the helium affected region. Electron back scatter diffraction (EBSD) analysis has been applied to reveal the orientation of the lamellae in the TiAl specimens, and used to understand the deformation processes in the sample. The irradiation damage of gallium ion beam from FIB on the surface of TiAl sample was also investigated.

Porous Ti–Al Intermetallic Based Alloys Fabricated by Pressure-Sintering Elemental Powders with a Space Holder Powder

ABSTRACTIn the present study, we have attempted to produce the porous Ti–Al intermetallic based alloys by pressure-sintering of the elemental powders of Ti and Al through a combustion reaction, with a NaCl space holder to achieve the development of a highly porous structure. The powders of Ti (particle size: <45 μm) and Al (particle size: <45 μm) were mixed to various total compositions of Ti–0-40 Al (at.%), and NaCl (particle size: 300-400 μm) was mixed with the Ti/Al powder to give a highly porous structure. The mixed powder was sintered at 700 oC under a constant pressure of 30 MPa to form cylindrical specimens with a diameter of 20 mm. The soaking of the specimens in pure water allows for the removal of NaCl (with a volume fraction of 60 %) and the formation of the pores to achieve a controlled relative density of 0.4. The present process can produce porous Ti–Al alloys with various compositions. In as-sintered Ti–Al specimens, an inhomogeneous microstructure consisting of Ti particles surrounded by the a Ti–Al alloy layer (various Ti–Al intermetallic phases) was observed. The as-sintered specimens exhibited the brittle behavior under compression, which is independent of their Al content. However, in a Ti–20Al alloy heat-treated at 1300 °C, a nearly Ti3Al single-phase microstructure appeared in the porous specimen. The heat-treated porous Ti–20Al alloy shows both a high plateau-strength level of approximately 100 MPa and a high plateau-end strain over 50%, resulting in a high absorption energy. The present results demonstrate that controlling the microstructure for the formation of the Ti3Al single-phase would be an effective method to achieve high energy absorption capacity of the porous Ti–Al alloy.

Microstructures of as-Fabricated and Post Heat Treated Ti-47Al-2Nb-2Cr Alloy Produced by Selective Electron Beam Melting (SEBM)

The microstructures of as-fabricated and post heat treated Ti-47Al-2Nb-2Cr (at%) alloy produced by selective electron beam melting (SEBM) were examined. The solidification path and the phase transformation of Ti-47Al-2Nb-2Cr alloy during SEBM process were analyzed. Results show that the microstructure is fully lamellar (FL), consisting of fine γ -TiAl and α 2 -Ti 3 Al, which is formed in the as-fabricated TiAl specimen. Due to the rapid cooling and the cyclic heat treatment during the SEBM process, the lamellar colonies and the lamellae width become coarser and wider, respectively from the top to the bottom of the specimen, solidification takes place solely through the β phase, and the phase transformation path is L → L + β → β → β + α → α → α + γ . Different post heat treatments were carried out to get a homogeneous microstructure. Results indicate that a fine and homogeneous microstructure can be achieved after oil quenching from 1250 °C and holding at 1200 °C for 2 h.

Fracture characteristics of notched investment cast TiAl alloy through in situ SEM observation

TiAl alloys were produced by investment casting method combined with induction skull melting (ISM) technique. In situ scanning electron microscopy (SEM) was utilized to study the fracture characteristics and crack propagation of a notched investment cast TiAl specimens in tension under incremental loading conditions. The whole process of crack initiation, propagation and failure during tensile deformation was observed and characterized. The results show that the fracture mechanism was sensitive to not only the microcracks near the notched area but also lamellar orientation to loading axis. The high tensile stress leads to the new microcracks nucleate along lamellar interfaces of grains with favorable orientation when local stress intensity reaches the toughness threshold of the material. Thus, both plasticity and high tensile stress are required to cause notched TiAl failure.

Fracture Behavior of Directional TiAl Lamellar Structures

In order to assess the influences of macro- and microstructure as well as the cast flaw on the tensile properties, the fracture behavior of a number of ambient tensile TiAl specimens containing directional lamellar microstructure have been analyzed. It is found that all the fractures of tensile specimens are triggered by a critical Griffith crack. According to the desirable ambient tensile properties coincide with the initial cracking facets smaller than 600 mm, it is proposed that the detrimental effect of the random oriented lamellar colonies on the tensile properties can be eliminated through cast technology improvement.

Study on compression deformation, damage and fracture behavior of TiAl alloys: Part I. Deformation and damage behavior

The compression deformation, damage and fracture behaviors of TiAl alloys in the fully lamellar and duplex microstructures were systematically studied. The fracture behaviors in compression tests of TiAl specimens are detailed in Part II of this work. In this paper a series of compression preloading–unloading–reloading tests were carried out. Results indicated that the compression engineering stress–strain curves were unchanged and were not affected by the numbers of multiple preloading–unloading processes until the preloading stresses reached the ultimate compression stress. However the tensile strength was significantly reduced by the multiple compression preloading–unloading–reloading processes. At the microscopic scale, the development of the microcracks on the specimen surfaces increases up to ∼1000 cracks/mm 2 with the compression up to the ultimate fracture stress of ∼2200 MPa. The linking of microcracks to form macrocrack damage in the bulk is examined. A preloading–unloading–reloading process with step-by-step observation of the initiation, extension and formation of microcracks on the specimen surface with increasing compression load is described in detail. From these observations, mechanisms of crack linking and propagation are identified in duplex and fully lamellar microstructures.

Study on compression deformation, damage and fracture behavior of TiAl alloys: Part II. Fracture behavior

Compression deformation, damage and fracture behaviors of TiAl alloys are studied systematically. The deformation and damage processes in compression tests of TiAl specimens are detailed in Part I and the fracture behaviors are described and discussed here the Part II of this work. Results indicate that: the compression properties are superior to the tensile properties and specially show a much higher fracture strength and plasticity. The macroscopic features of the fracture surfaces indicate that the fracture mechanisms are different between the full lamellar (FL) alloy and the duplex (DP) alloy. In FL alloy, the shear cracking dominates the fracture, and the shear crack extends throughout the whole specimen. At both ends of the specimen, the shear cracks initiate and propagate in directions inclined to the compression axis with an angle near 45°. In the middle of the specimen, the shear cracks propagate parallel to the compression axis. While in the DP alloy after two short shear cracks initiate and extend at both ends, the longitudinal normal cracking is triggered and propagates parallel to the compression axis and dominates the fracture. In the microscopic features the regions of shear cracking show the smooth interlamellar shear fracture facets with some translamellar shear fracture steps. The longitudinal normal cracking in DP specimens shows mixed interlamellar and translamellar cleavage fracture facets. Effect of loading rate manifests itself in increasing the shear yield strength of FL and the fracture strength of DP alloys.

Effects of repeated loading on the fracture behavior of notched TiAl specimens

Notched specimens of a fully lamellar TiAl alloy and a duplex TiAl alloy were in situ tensile tested with repeated loading–unloading–reloading processes in a scanning electron microscope (SEM). The step-by-step processes of initiation and extension of the main crack were captured by SEM. The fracture surfaces were observed and one-sector-to-one-sector related to the crack extension. Effects of loading–unloading damage on the fracture behavior were evaluated by combining the pictures of propagating crack configurations, corresponding fracture surfaces and the load locus. The results revealed the following events: (1) at the elastic regime, the loading–unloading process had negligible effect on the fracture behavior produced by subsequent reloading; (2) at the plastic regime, even at a value much lower than that of the preload, the reload extended further the existing main crack; (3) after a heavy loading–unloading process, the main crack extended and resulted in final fracture at a value of the reload, which was lower than that of the preload and (4) microcracks produced in the loading–unloading process had minor effects on the fracture behavior.

Oxidation resistance of TiAl alloy treated by plasma Nb–C alloying

In this paper, the duplex treatment of plasma surface niobizing and carburization (Nb–C alloyed or duplex treated) has been performed on TiAl substrates. Cyclic-oxidation tests for untreated and duplex treated TiAl were carried out in static air at 900 °C for 103 h. The mass gain due to oxidation was measured using an electrobalance. The chemical compositions of the Nb–C alloyed layer as well as its microstructure were examined by glow discharge spectrum (GDS) and the optical microscopy (OM). The electronic diffraction spectrum (EDS) and scanning electronic microscopy (SEM) were used to analyze and observe the oxide layers. The phase structures of the oxide scale were identified by X-ray diffraction (XRD). The results show that the oxidation rate of the duplex treated TiAl specimens decreased significantly, compared with the untreated TiAl. The roles of duplex treatment for improving the oxidation resistance are discussed.

A Novel Canning Technology for Forging of Gamma-TiAl Alloys

A novel canning technology to forge gamma-TiAl alloys was developed at the BTU Cottbus. A TiAl specimen was encapsulated with multilayer stainless steel foil and glass. The steel foil layers prevented the heat loss through radiation and the glass layer reduced the temperature decrease through conduction. First, the effect of steel foil on the cooling rate was investigated. Cooling curves were recorded for TiAl specimens without steel foil layer, with 1, 2, 3 and 4 layers of steel foil, as well as with 3 coated steel foil layers, respectively. While the unprotected specimen cooled from 1200 °C to 1100 °C within 12 s, the specimen with 3 coated steel foil layers needed 52 s for the same temperature decrease. The efficiency of the glass layer was examined with forging of steel specimens. The cooling rate during forging of the specimen with a glass layer was only half of that without a glass layer. Based on the results, Ti-45Al-0.5Mo-0.5Cu-0.2Si specimens, canned with steel foil and glass, were successfully forged at strain rates of 0.1 s-1 and 0.04 s-1 with warm dies which were heated to 500 °C. Visual and metallographic examinations revealed no cracks, pores or micropores. The microstructures are fine-equiaxed grains.

Crack opening due to deformation twin shear at grain boundaries in near-γ TiAl

Microcrack nucleation has been observed at apparent deformation twin interactions with grain boundaries in a duplex near-gamma TiAl specimen deformed to surface tensile strain of about 1.4%. To prove that these microcracks are a result of twins and not dislocation slip bands, detailed characterization of the surface topography and crystallography associated with the microcracks was conducted and analyzed. Electron backscatter diffraction patterns were used in conjunction with selected area channeling patterns to determine the crystallography near the observed microcracks. Transmission electron microscopy of a selected twin, extracted using a focused ion beam, and atomic force microscopy were used to show that the observed microcracks could only have been caused by local strain heterogeneities caused by twin interactions with grain boundaries and not by dislocation slip bands.

A Micromechanics Study of Lamellar TiAl

Microdeformation patterns of lamellar TiAl specimens with various grain sizes under uniaxial tension are mapped using the micro/nano experimental mechanics technique called SIEM (Speckle Interferometry w ith Electron Microscopy). The stress–strain relationships were obtained from deformations within decreasing areas ranging from mm2 to μm2. We found that the stress–strain relationship of the material depends on the size of strain measuring area in relation to the grain size. The stiffness at a grain boundary can be as large as 7–10 times more than that of the grain itself. From the data obtained so far, it seems that the traditional way of using PST (polysynthetically twinned) single crystal to predict polycrystalline behavior may not be appropriate.

Improvement of resistance of TiAl alloy against high temperature oxidation by electroplating in AlCl 3–NaCl–KCl–CrCl 2 molten salt

For improvement of the oxidation resistance of a TiAl alloy under high temperature conditions, aluminum chromium alloys were electroplated on the TiAl alloy in an AlCl 3–NaCl–KCl molten salt containing CrCl 2 at 423 K. The deposit electroplated at −0.1 V vs. Al/Al 3+ consists of γ-Al 8Cr 5 single phase, including chromium content at 41 at.%. A mixture phase of γ-Al 8Cr 5 and Al is, however, formed at potential from −0.2 to −0.4 V. The oxidation resistance of the electrodeposit layer was evaluated by a high temperature oxidation test at 1173 K for 24 h. On the TiAl specimen covered with γ-Al 8Cr 5 which was deposited at −0.1 V, a uniform AlCr 2 layer was formed during the oxidation at 1173 K. The AlCr 2 layer was covered by a thin dense Al 2O 3. Both AlCr 2 and Al 2O 3 layers play a role of superior protection from the high temperature oxidation of the TiAl alloy; therefore, the plating of the γ-Al 8Cr 5 single layer deposited at −0.1 V reduced oxide thickness by 1/40 during the oxidation at 1173 K for 24 h from that on the TiAl without any plating.