Shanyi Du

Rapid prototyping and combustion synthesis of TiC/Ni functionally gradient materials

Abstract TiC–Ni functionally gradient materials (FGM) parts were fabricated by laminated object manufacturing (LOM), one of rapid prototyping (RP) techniques and combustion synthesis technique. The microstructure and phases of TiC–Ni FGM were analyzed with SEM and XRD. TiC–Ni FGM had anisotropic mechanical properties. It was stronger in the direction parallel to the thickness than in the direction perpendicular to the thickness. The maximum strength was 950 MPa in the TiC–20wt.%Ni region. The hardness of TiC–Ni FGM was larger than HRA 84 and the density was larger than 5.2 g cm −3 . When the content of Ni raised the density increased, then there was the largest relative density in the TiC–20wt.%Ni region.

Microstructure and mechanical properties of TiC-Ni functionally graded materials by simultaneous combustion synthesis and compaction

The simultaneous combustion synthesis and hot compaction of Ti, C and Ni powders under a hydrostatic pressure was undertaken to fabricate fully dense TiC-Ni functionally graded materials (FGM) in a single processing operation. The composition gradient was optimized by finite element analysis and obtained by stacking different powder mixtures of desired compositions. X-ray diffraction, scanning electron microscopy and microprobe analysis were employed to investigate the crystalline phase, microstructure and Ni distribution. Experimental results demonstrate that the combustion reaction was complete and the final products contained the phases TiC and Ni only, the microstructure varies coherently throughout the specimen with no distinct interface. The physical and mechanical properties were measured as a function of composition. It was found that the properties of the FGMs were dependent on the Ni content and approached the maximum values for the relative density, hardness and flexural strength at room temperature when the Ni content was increased to 20 wt%. The maximum in fracture toughness value was found in the TiC-30 wt% Ni material.

Effect of additives on self-propagating high-temperature synthesis of AlN

Abstract Effect of additives on the oxygen content and the growth mechanism of self-propagating high-temperature synthesis (SHS) of AlN were investigated in detail. Although the addition of NH 4 Cl made the product loosen, it also increased the water vapor concentration of the system at the same time, which brought a great increase of the oxygen content in the product. Carbon decreased the oxygen content of AlN by two kinds of mechanisms: carbothermal reduction of Al 2 O 3 , and decrease of the water vapor concentration that served as the catalyst of AlN oxidation at elevated temperature. The latter was found to be the dominant mechanism. Partly carbonized sucrose decreased the oxygen content of AlN more significantly than black carbon due to its ever-increasing surface area, arising from its carbonization during combustion. At the lower oxygen content, AlN grew from the vapor in the form of “platelet growth”. With increasing oxygen content, the growing steps were blocked up by the impurities containing oxygen. The “platelet growth” was gradually inhibited, and AlN inclined to grow into whiskers by VLS mechanism.

Effect of nitrogen pressure and oxygen-containing impurities on self-propagating high temperature synthesis of Si3N4

The effects of nitrogen pressure and oxygen-containing impurities on self-propagating high-temperature synthesis (SHS) of Si3N4 were studied. The growth mechanism of β-Si3N4 columnar crystal was investigated in detail using a gas-releasing method. The growth of α-Si3N4 occurred by a vapor-phase reaction, and the columnar β-Si3N4 grew by the vapor–liquid–solid (VLS) mechanism. Increase of the nitrogen pressure promoted volatilization of Si, and formation of α-Si3N4 during combustion, but did not favor the transition of the initial α-Si3N4 to β-Si3N4 during cooling. Therefore, the α/β ratio in the product increased with increasing nitrogen. Moisture in the nitrogen was helpful to the conversion of Si to α-Si3N4. Liquid formation, required by the VLS mechanism, depended upon the impurities containing oxygen in the reactant, rather than that in the nitrogen gas. The aspect ratio of columnar β-Si3N4 depended on the oxygen content in the reactant and the distribution coefficient of oxygen in the solid and liquid. Short β-Si3N4 columnar crystal was generally achieved because oxygen cannot get a supply from nitrogen gas. Increase of the oxygen content in the reactant resulted in a great improvement of the ratio of length and diameter of β-Si3N4.

Combustion synthesis of hexagonal boron–nitride-based ceramics

Abstract Pure hBN (hexagonal boron nitride) and hBN-based ceramic parts were fabricated by the combustion synthesis technique, i.e. self-propagating high-temperature synthesis (SHS). The components were manufactured by the combustion reaction of 80xa0MPa nitrogen and the compact made by cool isostatic pressing. In the hBN-based ceramic parts, hBN powders were used as diluent and SiO2 powders as the reinforcement phase. The relative density of pure hBN and hBN-based ceramic parts was 58 and 78%, respectively. The phases and microstructure of the ceramic parts were analyzed using XRD and SEM.

Structure Redesign of the Integrated Thermal Protection System and Fuzzy Performance Evaluation

Integrated thermal protection systems with both thermal protection and load-bearing capabilities are regarded as one of the most promising thermal protection concepts for future hypersonic vehicles. Yet improper design could cause the structure to be overweight and even fail prematurely during service. In this paper, the thermal–mechanical analysis model of integrated thermal protection systems is first established, and the thermal short and mismatch effects of typical corrugated web concept are studied. Three improved structural concepts are proposed. Thermal insulation capability and analyses of structural strength, stability, and stiffness are carried out on both the original and improved concepts. The results show that the web is a key component and that structural performance can be enhanced through web redesign. Then, a performance evaluation method is presented using the fuzzy decision-making model. Besides the aforementioned performance factors, two more factors with respect to structural complexi...

Kinetics and numerical simulation of self-propagating high-temperature synthesis in Ti–Cr–Al–C systems

In this paper, Ti–Cr–Al–C materials were investigated by self-propagating high-temperature synthesis (SHS) according to the experimental study and numerical simulation results. The highest adiabatic combustion temperature Tad of 2,467.45xa0K indicates that the 2Ti–0Cr–Al–C is the highest exothermic reaction system in the Ti–Cr–Al–C system. The adiabatic combustion temperature decreases with the increase of the Cr content. And a higher exothermal reaction would result in higher porosity which is induced by the high temperature and pressure of C reducing atmosphere and Al vapor. Combustion characterization of the products shows that the geometrical alternating layers result in the high exothermal reaction and flame-front propagating velocity. The higher the Tad is, the thinner the layer is. To demonstrate the process of the microscopic characterization and show the detailed combustion process closed to the experimental observations, the flame-front propagating velocity and temperature distribution were simulated numerically.

Synthesis, Microstructure and Properties of High-Strength Porous Ceramics

Porous ceramics containing tailored porosity exhibit special properties and features that usually cannot be achieved by their conventional dense counterparts. Thus, porous ceramics find nowadays many applications as final products and in several technological processes. Porous ceramics are of significant interest due to their wide applications in high-temperature filters, thermal gas separation, lightweight structural components and thermal structural materials (Peng, H.X et al, 2000; Corbin, S. F and Apte, P. S 1999; Fukasawa, T et al; 2001).

New Mn+1AXn compounds by thermal explosion in the Al-Cr-Si-C system

Ternary Cr2AlC and Cr3SiC2 compounds—belonging to the family of Mn+1AXn phases (n=1, 2, 3; M is a transition metal, A is an A group element, and X = C or N)—were prepared by thermal explosion in the Cr-Al-Si-C system in air. After mixing, drying, and compaction, the green powders taken in stoichiometric ratios of Cr: Si: C=3: 1: 2 plus 20 wt % Al were thermally exploded at 1100°C in an SHS reactor. The combustion products, Cr2AlC and Cr5-xSi3-zCx+z, were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Chromium-aluminum intermetallics and Al4C3 were identified as intermediate products in the synthesis of Cr2AlC. A Cr5-xSi3-zCx+z compound in the system under study was found to exist as Cr3SiC2.

Formation of Mn+1AXn phases in Ti–Cr–Al–C systems by self-propagating high-temperature synthesis

In this paper, phase composition of the Mn+1AXn phases by self-propagating high-temperature synthesis (SHS) was determined using Ti, Cr, Al, and carbon black as raw materials. And, phase composition and microstructures of the Mn+1AXn phases-contained bulk by SHS with the pseudo-hot isostatic pressing (SHS/PHIP) were investigated in Ti–Cr–Al–C systems raw materials. Rietveld XRD refinement was introduced to study the lattice parameters and phase composition of the resultant phases from the SHSed and SHS/PHIPed samples. Ti2AlCx, Ti3AlC2x, and Cr2AlCx by SHS were detected in the Ti–Cr–Al–C systems, as well as the binary carbide of TiC and intermetallics. The mechanical properties of the synthesized bulk samples were determined, exhibiting a high strength and toughness compared with the typical monolithic Mn+1AXn phase ceramics. It is indicated that the samples prepared by SHS/PHIP are identified to be a strategy for improving the mechanical properties of monolithic Mn+1AXn phase.