The Al-B-Nb-Ti system

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb 2 SiO 5 top coat on SiC substrates. Solidified droplets in as-deposited Yb 2 SiO 5 and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb 2 SiO 5  + Yb 2 O 3 two-phase top coat and 2:1 mullite (2Al 2 O 3 *SiO 2 ) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb 2 O 3 in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al 2 O 3 . Mud-cracks were found in the Yb 2 SiO 5 layer and in precipitated Al 2 O 3 due to the thermal expansion mismatch between these coating phases and the substrate.

Plasma spray deposition of tri-layer environmental barrier coatings

ZrC and ZrN ceramics are of interest in the application of materials in extreme high temperature environments, particularly for nuclear applications in generation IV reactors. These materials demonstrate desirable characteristics such as high thermal and electrical conductivities along with high hardness and melting temperatures. Data reported in the literature often suffer from scatter due to differences in processing techniques and difficulty determining stoichiometry, which will significantly affect thermophysical properties. This article reviews the current available data for the properties of ZrC, ZrN and mixed carbonitrides phases and identifies causes of scatter in the literature and areas requiring further research.

https://www.tandfonline.com/doi/pdf/10.1179/1743676115Y.0000000061

Processing and properties of ZrC, ZrN and ZrCN ceramics: a review

In the present study, friction stir welding was applied to commercial purity titanium using a polycrystalline cubic boron nitride tool, and microstructure and hardness in the weld were examined. Additionally, the microstructural evolution during friction stir welding was also discussed. The stir zone consisted of fine equiaxed α grains surrounded by serrate grain boundaries, which were produced through the β → α allotropic transformation during the cooling cycle of friction stir welding. The fine α grains caused higher hardness than that in the base material. A lath-shaped α grain structure containing Ti borides and tool debris was observed in the surface region of the stir zone, whose hardness was the highest in the weld.

Stir zone microstructure of commercial purity titanium friction stir welded using pcBN tool

Anisotropic elastic and thermal properties of titanium borides by first-principles calculations

Thermodynamic assessment of the Ti-B system

Motivated by the thermodynamic assessment of the Ga–N and In–N binaries, the thermodynamic parameters of the Si–N system are optimized in a similar way. The intermetallic compound Si 3 N 4 is treated as stoichiometric and the terminal solution phase diamond_Si as the pure diamond_Si. The liquid solution phase is assumed to be a substitutional solution with Redlich–Kister formula for the expression of its excess Gibbs energy. The fugacity of nitrogen is considered when describing the behavior of the gas phase, especially with high pressure. The complete T – x phase diagram for the Si–N binary system is given. In this optimization a consistent thermodynamic data set for the Si–N system is obtained by means of CALPHAD (CALculation of PHAse Diagrams) method. The calculation results agree well with experiments.

Thermodynamic assessment of the Si-N system

Thermodynamic assessment of the Zr–N system

The isothermal sections of the Ti–B–N system at different temperatures 1273 K, 1673 K and 1893 K were constructed by extending the related binaries, Ti–B, B–N and Ti–N. The calculated phase diagrams reproduce the latest experimental results well. On the basis of the phase equilibrium calculation, the interfacial reactions and the phase sequences between the two pairs of the diffusion couples, Ti/BN, were analyzed theoretically under the reported experimental conditions of two different couples, the BN plates immersed into a loose Ti powder and the BN particles embedded into a dense Ti matrix at 1273 K. Different phase formation sequences and apparent spatial phase sequences for those two kinds of Ti/BN joints were obtained. It was proved that the different phase sequences of both the phase formation sequences and the apparent spatial phase sequences were dominated by the relative amounts and distribution of the two pairs in the diffusion couples.

Xiaoyan Ma

Papers

Thermodynamic assessment of the Ti-B system

Thermodynamic assessment of the Si-N system

Thermodynamic assessment of the Zr–N system

Study on the phase diagram of the Ti-B-N system and the interfacial reaction of the Ti/BN joints

The thermodynamic assessment of the Ti–Si–N system and the interfacial reaction analysis