Effect of microstructure on tensile properties and fracture behavior of intermetallic Ti2AlNb alloys

The tensile properties and fracture behaviors of Ti-22Al-27Nb and Ti-22Al-20Nb-7Ta alloys were investigated in the temperature range of 25-800℃ Three typical microstructures were obtained by ifferent thermomechanical processing techniques.The results indicate that the duplex microstructure has an optimum combination of tensile yield strength and ductility both at room and elevated temperatures.Adding Ta to Ti2AlNb alloy can improve the yield strength,especially at high temperature while retain a good ductility.The study on crack initiation and propagation in dedformed microstructure of Ti2AlNb alloys indicates that microstructure has ikmportant effect on the tensile fracture mechanism of the alloys.The cracks initiate within primary O/α2 grains along O/B2 boundaries or O phase laths in B2 matrix,and propagate along primary B2 grain boundaries for the duplex microstructure.The fracture mode is transgranular with ductile dimples for the duplex and the equiaxed microstructures,but intergranular for the lath microstructure.     

Strength and fracture of two hot-rolled Ti–Al–Si–Nb dual phase alloys based on Ti3Al

Ti–Al–Si–Nb dual phase alloys are mainly composed of α₂-Ti₃Al matrix and Ti₅Si₃ silicide phases. In this paper, two alloys (402 and 405) whose Si contents are 2 and 5 at% respectively were arc melted and hot-rolled into sheets with different amounts of deformation. The silicide phase (Ti,Nb)₅(Si,Al)₃ was broken up into small pieces and redistributed in the α₂ matrix during the hot-rolling. Improved strength and ductility of the two alloys were observed after hot-rolling, which can be attributed to both the finely distributed reinforcement silicide phase and refinement of the matrix grain size. The mechanical properties of the two alloys are dependent on their volume fractions of the silicide phase: the strength of alloy 405 is higher than that of alloy 402, while alloy 402 is more ductile than alloy 405. The brittle–ductile transition temperature of the two dual phase alloys is between 600 and 800°C. The surface slip on the dual phase alloys was also observed. Obvious separation between the (Ti,Nb)₅(Si,Al)₃ particles and the α₂ matrix is found on the fracture surfaces obtained at high temperature, showing dimple-like morphology.     

Composition dependence of the microstructure and mechanical behavior of Ti–Zr–Cu–Pd–Sn–Nb bulk metallic glass composites

Ti-based bulk metallic glass composite alloys Ti₅₆Zr₆Cu_19.8Pd_8.4Sn_1.8Nb₈, Ti₆₄Zr₄Cu_13.2Pd_5.6Sn_1.2Nb₁₂ and Ti₆₈Cu_13.2Pd_5.6Sn_1.2Nb₁₂ were designed to obtain the microstructure composing of β-Ti dendrites and glassy matrix. The compressive and three-point bending properties were investigated. It was found that the actual microstructure of the Nb-added alloys consisted of primarily precipitated β-Ti dendrites, network-like glassy matrix, and extra island-like Ti₂Cu intermetallic phase with different volume fractions. Under compressive loading, all the Nb-added alloys presented higher yield strength combined with remarkably increased plasticity. Under bending condition, however, the alloys Ti₅₆Zr₆Cu_19.8Pd_8.4Sn_1.8Nb₈ and Ti₆₄Zr₄Cu_13.2Pd_5.6Sn_1.2Nb₁₂ with higher Ti₂Cu volume fractions became completely brittle. The alloy Ti₆₈Cu_13.2Pd_5.6Sn_1.2Nb₁₂ could keep its plastic deformability due to the decreased Ti₂Cu volume fraction. Compressive deforming behavior of the Nb-added alloys was determined by the ductile β-Ti phase and glassy matrix, nevertheless, bending deforming behavior of the alloys was determined by the volume fraction and distribution of the brittle intermetallics.     

Molecular-dynamics simulation of the synthesis of intermetallic Ti–Al

In the present paper, results of a molecular-dynamics simulation study of the synthesis of intermetallic Ti–Al in the regime of thermal explosion at constant volume are reported. The structure of the polycrystalline intermetallic compound formed upon cooling was studied, and the dependence of crystallite sizes on the cooling rate was examined.     

Crack-resistance curve of a Zr–Ti–Cu–Al bulk metallic glass with extraordinary fracture toughness

For the emerging bulk metallic glasses (BMGs), damage tolerance is a key mechanical property needed for their practical applications. To reach a fracture toughness on a par with, or even better than, conventional engineering alloys, the only route reported so far is to compositionally base the BMG on high-cost palladium (Pd), which has a very high Poisson’s ratio (～0.42). Here we report the discovery of a Zr₆₁Ti₂Cu₂₅Al₁₂ (ZT1) BMG that has a toughness as high as the Pd-based BMG, but at the same time consists of common engineering metals and has robust glass-forming ability. The new BMG, while having an unimpressive Poisson’s ratio of 0.367, derives its high toughness from its high propensity for crack deflection and local loading-mode change at the crack tip due to extensive shear band interactions. The crack-resistance curve (R-curve) of this BMG has been obtained from fatigue pre-crack samples, employing standard “single-specimen” and “multiple-specimen” techniques.     

Glass forming ability and crystallization behavior of Al–Ni–RE metallic glasses

Atomic size may change due to the interaction between different elements when pure metals are synthesized into alloys. In present paper, the effective atomic radii instead of the nominal radii were introduced in the cluster line method to predict the optimum glass forming compositions in Al-based Al–Ni–RE (RE = La, Y, Ce, Gd, Dy) systems. Wedge-shaped samples of the alloys were suction cast under well-controlled condition to experimentally determine the dependence of the glass forming ability (GFA) on the composition. It is found that such a modification to the method makes the prediction very close to the experimental results. When the effective atomic radii are used to calculate the topological instability parameter originally proposed by Egami and Waseda, λ′, the λ′ corresponding to the best GFA in each Al–Ni–RE system linearly changes with the radius of the RE element. Meantime, the onset temperature of crystallization and mixing enthalpy linearly increase with the λ′.     

Influence of Al content on cast microstructures of Ti–Al intermetallic compounds

Cast microstructures of Ti–(35–58) at% Al alloys have been studied using optical microscope, scanning electron microscope and transmission electron microscope. The critical compositions of transition from primary β to α phase and from α to γ phase were determined to be about 49.5 at% Al and 55.5 at% Al respectively. The peritectic compositions of α_p and γ_p were measured to be about 47 and 54 at% Al respectively. A minimum dendrite cell size was observed at 46.5 at% Al, which is believed to arise primarily from the variation behavior of the partition coefficient and liquidus slope with Al content. The mode of solid state transformation β→α during solidification at a given cooling rate varied from the massive transformation to the precipitation of Widmanstätten α plates and of α grains with Al content. The mode of transformation α→γ, on the other hand, varied from the precipitation to the massive transformation with Al content. This variation of transformation modes at a given cooling rate was explained in terms of the variation of the driving force and mobility with Al content.     

Analysis of fracture behavior and stress–strain distribution of martensite/austenite multilayered metallic sheet

The properties of a multilayered metallic sheet tend to be different from those of a sheet made of monolithic material because of the layer interaction. Because of the change in the ductility and strength of each layer, the stress gradient changes; a brittle layer is deformed over the fracture limit of the monolithic material. Further, a new stress–strain distribution is observed in the form of a new fracture within materials. In the case that the observed strain is more than the fracture strain of a monolithic brittle material, micro-voids and cracks are generated and a tunnel crack is formed. As the layer interaction is changed by the suppression stress in the direction of necking, the stress components of two layers are changed. The suppression stress changes the features of the plastic zone, such as the defects range of dislocation, slip band, and void; consequently, the fracture strain is changed. The thinner the brittle layer, the greater is the increase in the fracture strain. This study suggests a method for predicting the fracture strain from the relation of the thickness change and the volume fraction in accordance with the lamination numbers.     

Microstructure and mechanical properties of Fe–Si–Ti–(Cu,Al) heterostructured ultrafine composites

The influence of partial substitution of Fe by Cu or Al in Fe_75?xSi₁₅Ti₁₀(Cu, Al)_x (x = 0 and 4) ultrafine composites on the microstructure and mechanical properties has been investigated. The Fe₇₁Si₁₅Ti₁₀Cu₄ ultrafine composite exhibits a favorable microstructural evolution and improved mechanical properties, i.e., large plastic strain of ～5% and pronounced work hardening characteristics. The mechanical properties of the ultrafine eutectic composite are strongly linked to the length scale heterogeneity and the distribution of the constituent phases.     

Correlation between microstructure and tensile behavior of metal–intermetallic laminate compound with different initial Ni foil thickness

Ni-base metal–intermetallic laminate composites were obtained from in situ reaction synthesis between Ni and Al foils by utilizing plasma activated sintering. The effects of Ni foil thickness on the microstructure and tensile properties of the composites were investigated. The results show that the phases forming during reaction synthesis are independent of the starting thickness of the Ni foils. However, thicker reacted layers are obtained in the samples fabricated from 100 lm Ni foils(Ni100) than those obtained in the samples from 50 lm Ni foils(Ni50)when treated at the same process. The tensile strength of Ni100 samples increases with the temperature increasing at the expense of ductility. Dissimilarly, Ni50 composites treated at higher temperatures exhibit enhanced strength and ductility. Both Ni50 and Ni100 laminate fracture in a similar mechanism. Cracking first occurs in the brittle intermetallic layers. These original cracks result in shear bands in Ni layers emitted from the crack tips, and thus producing local stress concentration, which initiates new cracks in adjacent intermetallic layers. The multiplication of cracks and shear bands leads to the failure of the laminates.     

Fabrication and wear behavior of CNT/Al composites

Aluminum matrix composites reinforced with carbon nanotube were fabricated by a powder metallurgy method. The effects of carbon nanotube content on the relative density, the hardness, and the friction and wear behavior of the composites under dry sliding condition were investigated using the ball （pin）-on-block tester. By scanning electron microscopy （SEM）, the worn surfaces and worn chips were observed, and the wear mechanism of composites was analyzed and discussed. The results indicate that the addition to the aluminum matrix of 2.0%（mass fraction） carbon nanotube causes the increase in the Vickers hardness of about 80%. Within the range of carbon nanotubes content from 1.0% to 2.0%, both the friction coefficient and wear rate of composites decrease with the increase of carbon nanotube content. The delamination wear is the main wear mechanism for the composites.     

Hot corrosion of Ti–46Al–8Ta (at.%) intermetallic alloy

Hot corrosion behaviour of a fully lamellar Ti–46Al–8Ta (at.%) alloy was studied in air under thermal cycling conditions (20-h cycles) at 700 and 800 °C. The samples were purposely contaminated with salt deposits consisting of NaCl or Na₂SO₄ or a mixture of these. The progress of degradation was followed by mass change measurements and visual inspection. Post-exposure examination involved scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The composition of salt deposits clearly influenced the rate and type of corrosion. Sodium chloride appeared especially harmful because of the formation of volatile chloride species.     

Hot deformation behavior and microstructure evolution of TiC–Al_2O_3/Al composites

Hot compression behavior of TiC–Al2O3/Al composites was studied using the Gleeble-1500 system at a temperature range of 300–550 °C and at strain rate range of 0.01–10.00 s-1. The associated structural changes were studied by TEM observations. The results show that stress level decreases with deformation temperature increasing and strain rate decreasing, which can be represented by a Zener–Hollomon parameter in an exponent-type equation with hot deformation activation energy Q of 172.56 kJ·mol-1.Dynamic recovery occurs easily when strain rates are less than 10.00 s-1. Dynamic recrystallization can occur at strain rate of 10.00 s-1.     

Ti–Cu–Ni shape memory bulk metallic glass composites

New Ti–Cu–Ni shape memory bulk metallic glass composites were obtained by carefully controlling the cooling rate upon quenching. This allows for the formation of a metastable microstructure consisting mainly of ductile, spherical martensitic Ti(Ni,Cu) precipitates embedded in an amorphous matrix also containing a small volume fraction of TiCu and Ti₂Cu precipitates. These composites exhibit large ductility and high strength combined with a strong work-hardening behaviour. A deformation mechanism is proposed with the help of experimental observations and finite-element simulation. The simulation results demonstrate that stress concentrations occur around the precipitates, which promotes a heterogeneous stress distribution and the formation of multiple shear bands. Additionally, different transformation temperatures were observed for martensitic precipitates depending on whether they are completely or partially embedded in the amorphous matrix.     

Machining of a Zr–Ti–Al–Cu–Ni metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass machining chips were characterized using SEM, X-ray diffraction and nano-indentation. Above a threshold cutting speed, oxidation of the Zr produces high flash temperatures and causes crystallization. The chip morphology was unique and showed the presence of shear bands, void formation and viscous flow.     

Microstructure and formation mechanism of SHS joining between C_f/Al composites and TiAl intermetallic with Al–Ni–CuO interlayer

In this study, it was reported a novel approach for joining Cf/Al composites and TiAl intermetallic by selfpropagating high-temperature synthesis(SHS). Mixed powders of 14Al–2Ni–3CuO were used as the SHS interlayer, and differential thermal analysis test of Al–Ni–CuO interlayer was conducted to analyze the exothermic characteristic. Sound joint was got by SHS joining under the conditions of 600 °C, 30 min, and 5 MPa. The joint was characterized by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS), and X-ray diffraction(XRD). TiAl3 and NiAl3 are, respectively, formed in the TiAl/interlayer and Cf/Al/interlayer interfaces. Reaction products of Ni2Al3, NiAl3, Al2O3, and Cu were observed in the interlayer. And the formation mechanism of SHS joining was investigated.     

Kinetics and fracture behavior under cycle loading of an Al–Cu–Mg–Ag alloy

The behavior of aluminum alloy AA2139 subjected to T6 treatment, including solution treatment and artificial aging, has been studied using cyclic loading with a constant total strain amplitude. Upon low-cyclic fatigue in the range of total strain amplitudes ε_ac of 0.4–1.0%, the cyclic behavior of the AA2139-T6 alloy is determined by the processes that occur under the conditions of predominance of the elastic deformation over plastic deformation. The AA2139 alloy exhibits stability to cyclic loading without significant softening. The stress-strained state of the alloy upon cyclic loading can be described by the Hollomon equation with the cyclic strength coefficient K' and the cyclic strain-hardening exponent n' equal to 641 MPa and 0.066, respectively. The dependence of the number of cycles to fracture on the loading amplitude and its components (amplitudes of the plastic and elastic deformation) is described by a Basquin–Manson–Coffin equation with the parameters σ′/E = 0.014, b =–0.123, ε′_f= 178.65, and c =–1.677.     

Effects of test orientation on fracture and fatigue crack growth behavior of third generation as-cast Ti–48Al–2Nb–2Cr

The effects of changes in test orientation and load ratio on the room temperature fracture and fatigue crack growth behavior of as-cast Ti–48Al–2Nb–2Cr titanium aluminide was investigated to determine the presence of any anisotropy in mechanical properties. As-cast samples were tested in the longitudinal and transverse directions to the casting direction at room temperature in air. Load ratios ranging from R = 0.1 to R = 0.9 were used in the fatigue tests in order to determine its effects on the threshold for fatigue cracking, the Paris law slope, and fatigue crack instability toughness, K_c, in addition to determining both notched and fatigue-precracked values for toughness. Optical metallography and SEM fractography were used to document the effects of orientation on the fracture path and morphology. Significant effects of changes in load ratio were obtained on the fatigue threshold and Paris law slope, while its effects on K_c and the effects of sample orientation were found to be minimal. These are rationalized by considering microstructural effects on the properties measured and are compared to similar materials processed via different techniques.     

Microstructure and microhardness of Ti3Al matrix composites produced by XD^TM synthesis

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｌｏｗｄｅｎｓｉｔｙ ,ｈｉｇｈｅｌｅｖａｔｅｄｔｅｍｐｅｒａｔｕｒｅｓｔｒｅｎｇｔｈ,ｈｉｇｈｓｐｅｃｉｆｉｃｓｔｒｅｎｇｔｈａｎｄｅｘｃｅｌｌｅｎｔｃｒｅｅｐｒｅ ｓｉｓｔａｎｃｅｏｆｔｉｔａｎｉｕｍａｌｕｍｉｎｉｄｅｓｍ     

Basic principles of baroelectric synthesis of Ti–Al<Subscript>3</Subscript>Ti metal–intermetallide laminate from Al–Ti foil package

A procedure for calculation of basic parameters of the baroelectric treatment of an Al–Al₃Ti–Ti multilayer package in the synthesis of the Ti–Al₃Ti monolithic metal–intermetallide laminate from a set of Al–Ti foils is proposed. Loading conditions at which the treatment time substantially decreases in comparison with hot isostatic compaction are shown.