Abstract

Abstract

The structural stability of Ti₃AlC₂ in Cu and the microstructure evolution of Cu–Ti₃AlC₂ composites prepared at different temperatures were investigated by high-resolution transmission electron microscopy and X-ray diffraction. A mild reaction between Ti₃AlC₂ and Cu occurred at 850–950^oC, and strong reactions occurred above 950^oC. The reaction was identified as diffusion of Al from Ti₃AlC₂ into Cu to form Cu(Al) solid solution. Ti₃AlC₂ retained its structure under the partial loss of Al. Further depletion of Al resulted in highly defective Ti₃AlC₂ accompanied by the inner diffusion of Cu into Ti₃AlC₂ along the passway left by the Al vacancies. When Al was removed, Ti₃AlC₂ decomposed and transformed into cubic TiC_x. In addition, TiC twins formed by the aggregation of C vacancies at twin boundaries. With the help of first-principles calculation and image simulation, an ordered hexagonal TiC_x was identified as a transition phase linking Ti₃AlC₂ and c-TiC_x. The effect of the reaction and phase transformation on the microstructure and properties of Cu–Ti₃AlC₂ composites was also discussed.

Abstract

In this paper, the mechanical properties of bulk single-phase γ-Y₂Si₂O₇ ceramic are reported. γ-Y₂Si₂O₇ exhibits low shear modulus, excellent damage tolerance, and thus has a good machinability ready for metal working tools. To understand the underlying mechanism of machinability, drilling test, Hertzian contact test, and density functional theory (DFT) calculation are employed. Hertzian contact test demonstrates that γ-Y₂Si₂O₇ is a ‘‘quasi-plastic’’ ceramic and the intrinsically weak interfaces contribute to its machinability. Crystal structure characteristics and DFT calculations of γ-Y₂Si₂O₇ suggest that some weakly bonded planes, which involve Y–O bonds that can be easily broken, are the sources of the low shear deformation resistance and good machinability

Abstract

Bulk Ta₄AlC₃ ceramic was prepared by an in situ reaction synthesis/ hot-pressing method using Ta, Al, and C powders as the starting materials. The lattice parameter and a new set of X-ray diffraction data were obtained. The physical and mechanical properties of Ta₄AlC₃ ceramic were investigated. Ta₄AlC₃ is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa .m1/2, respectively.  Typically, plate-like layered grains contribute to the damage tolerance of Ta₄AlC₃. After indentation up to a 200 N load, no obvious degradation of the residual flexural strength of Ta₄AlC₃ was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200^oC in air, Ta₄AlC₃ still retains a high strength and shows excellent thermal shock resistance, which renders it a promising high-temperature structural material.

Abstract

Nb₄AlC₃, a new compound belonging to the MAX phases, was discovered by annealing bulk Nb₂AlC at 1700^o C. The crystal structure of Nb₄AlC₃ was determined by combined X-ray diffraction, high-resolution transmission electron microscopy and ab initio calculations. It was reported that Nb₄AlC₃ follows the Ti₄AlN₃-type crystal structure. The lattice constants are a = 0.31296 nm, c = 2.41208 nm and the atomic positions are Nb1 at 4f (1/3, 2/3, 0.0553), Nb2 at 4e (0, 0, 0.1574), Al at 2c (1/3, 2/3, 1/4), C1 at 2a (0, 0, 0) and C2 at 4f (2/3, 1/3, 0.1086).

Abstract

In Cr_1-xAl_xN (0＜x＜1) coatings were fabricated by a reactive magnetron sputtering method on a K38G alloy. The composition and microstructure of the coatings were investigated. Phase segregation of cubic AlN was considered in Cr_0.65Al_0.35N using X-ray diffraction analyses. This segregation of cubic AlN from CrAlN matrix might be induced by the high micro-stress. The critical failure load determined by scratch tests of the coating with c-AlN segregation was highest among all the coatings studied in the present work, which indicated that the coating has the best adhesion.

Abstract

Cr_1-xAl_xN coatings have been deposited on a Ti₃Al based alloy by reactive sputtering method. The results of the isothermal oxidation test at 800–900 ^oC showed that Cr1-xAlxN coatings could remarkably reduce the oxidation rate of the alloy owing to the formation of Al₂O₃+Cr₂O₃ mixture oxide scale on the surface of the coatings. No spallation of the coatings or oxide scales took place during the cyclic oxidation at 800^oC. Ti was observed to diffuse into the coatings, the diffusion distance of which was very short, and the diffusion ability of it was proportional to the Al content in the coatings. Compared to Ti, Nb can diffuse much more easily through the whole coatings and oxide scales.

Abstract

Based on the structure characteristic of Ti₃SiC₂ and the easy formation of Ti₃Si_1-xAl_xC₂ solid solution, a transient liquid phase (TLP) bonding method was used for bonding layered ternary Ti₃SiC₂ ceramic via Al interlayer. Joining was performed at 1100–1500 ^oC for 120 min under a 5MPa load in Ar atmosphere. SEM and XRD analyses revealed that Ti3Si(Al)C2 solid solution rather than intermetallic compounds formed at the interface. The mechanism of bonding is attributed to aluminum diffusing into the Ti₃SiC₂. The strength of joints was evaluated by three point bending test. The maximum flexural strength reaches a value of 263±16MPa, which is about 65% of that of Ti₃SiC₂; for the sample prepared under the joining condition of 1500 ^oC for 120 min under 5MPa. This flexural strength of the joint is sustained up to 1000 ^oC

Abstract

This article presents the results of combined experimental and theoretical studies of elastic and thermal properties of Zr₂Al₃C₄ carbide. The full set of second order elastic constants, bulk modulus, shear modulus, and Young’s modulus of Zr₂Al₃C₄ were calculated and compared with those of Zr₃Al₃C₅ and ZrC. The experimentally measured Young’s modulus and shear modulus are in good agreement with theoretical ones. The calculated Debye temperature from elastic constants of Zr₂Al₃C₄ is 830 K, which is slightly higher than that of Zr₃Al₃C₅, and exhibits pronounced enhancement in comparison with that of ZrC. The highest Debye temperature of Zr₂Al₃C₄ is related with its highest specific stiffness, i.e., the stiffness-to-weight ratio. The heat capacity and thermal conductivity of Zr₂Al₃C₄ were measured by means of the flash method. The thermal conductivity of Zr₂Al₃C₄ decreases with increasing temperature, for instance the values at room temperature and 1600 K are 15.5 and 10.1 W/m K, respectively. The investigations provide information on elastic and thermal properties of Zr₂Al₃C₄ with promising high temperature applications.

Abstract

A Cr_0.50Al_0.50N coating has been prepared by a reactive-magnetronsputtering method on alloy K38G. The coating possesses mainly the B1 type with a small amount of B4-type crystal structure phase. Isothermal oxidation tests were performed at 900–1100^oC for 20 h by thermogravimetric analysis (TGA) in air.  The results reveal that the coated samples have much lower mass gain than that of the bare alloy. The parabolic rate constants of the coated samples decrease by 2 orders of magnitude compared with the bare alloy at 1000 and 1100^oC. During the oxidation of the coated samples below 1,000^oC, the main oxide is Cr₂O₃, but above 1000^o C, the scale changes to a-Al₂O₃. The observed oxidation behaviors demonstrate that the Cr_0.50Al_0.50N coating can provide good protection against corrosion over a wide temperature range.

Abstract

The theoretical mechanical properties and atomistic shear deformation mechanisms of γ-Y₂Si₂O₇, one of the most refractory silicates and potentially useful as a high-temperature structural ceramic, were investigated using first-principles calculations. The material shows low shear moduli to bulk modulus ratios, as well as a low ideal shear strength to tensile strength ratio. The unusual low shear deformation resistance of γ-Y₂Si₂O₇ originates from the inhomogeneous strength of its chemical bonds. The Y–O bond is weaker and readily stretches and shrinks; and Si–O bond is stronger and more rigid. The relative softer YO6 octahedron positively accommodates shear deformation by structural distortion, while the Si₂O₇ pyrosilicate unit is more resistant to deformation. The reported shear-load-bearing mechanism is quite similar to those found in the ‘‘quasi-ductile’’ LaPO₄ monazite and ternary layered carbides (the so-called MAX phases), and can endow γ-Y₂Si₂O₇ with quasi-ductility and damage tolerance.