Materials Science Forum Vols. 546-549

Abstract: A series of NiTiAl based alloys with different amount of Nb addition were prepared and the effects of Nb on both the mechanical properties and microstructure evolution were investigated. The addition of Nb can remarkably enhance the high temperature yield strength of these alloys. The highest yield strength of the alloys with 10 at% Nb reaches 1237MPa at 873K and 875MPa at 973K, respectively. The Ti2Ni(Al,Nb) precipitates and the solution strengthening effect might be responsible for the high yield stress achieved at high temperature.

Abstract: The effect of Mo on the oxidation behavior of TiNiAl at 1073K has been investigated. It is found that 1at.% Mo addition can increase the diffusion of Al in the alloys and promote the formation of dense and continuous Al-rich oxide layer. Therefore the oxygen diffusion can be effectively impeded and the oxidation behavior of TiNiAl is improved. The observation of the cross-sectional oxidation layer showed that beneath the top oxide scale a Mo-rich oxide layer formed. Because the oxide of Mo is volatile at high temperature, voids formed in the oxide scales during the oxidation process. 3at.% Mo addition could cause cracks between the oxide scale and the substrate, resulting in poor adhesion of the oxide scale to the substrate.

Abstract: The effects of Cr and Al on high temperature oxidation resistance of Nb-Si system intermetallics have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and weight gain method. The results showed that the high temperature oxidation resistance can be substantially improved by proper Cr or Al addition. The further analysis revealed that Cr promotes the formation of CrNbO4 in scale and improve the adherence between the oxide scale and the substrate. It also found that Al improves the surface morphology of oxide scale and changes oxidation products by promoting the AlNbO4 formation.

Abstract: Nb-16Si-24Ti-6Cr-6Al-2Hf-xB(x=0, 0.5, 1, 2, 4, 6) in situ composites were prepared by arc-melting. Microstructure and the effect of boron on 1250
C oxidation resistance of the composites were investigated by scanning electron microscopy(SEM) and X-ray energy disperse spectrum(EDS) as well as X-ray diffraction(XRD). The experimental results showed that the high temperature oxidation resistance of the alloy was remarkably improved by adding proper amount of boron. This may be resulted from several beneficial roles of boron, i.e., boron improves the resistance of Nb5Si3 by solid solution strengthening, inhibits the diffusion of oxygen in the matrix, improves the adherence between the oxide scale and the substrate and increases the cracking resistance of the oxide scale.

Abstract: A Nb-Si system in-situ composite Nb-16Si-24Ti-6Cr-6Al-2Hf(at%) was fabricated using vacuum arc melting method, and then was metallurgically reacted with four kinds of ceramics ZrO2, Y2O3 stabilized ZrO2, Y2O3, SiC+Si3N4 at elevated temperature. The microstructures in the interface reaction zones were investigated by SEM and EDS. The results showed that different extent chemical reactions between the composite and these four kinds of ceramics took place. It has been found that HfO2 riched layer formed near the interfaces between the Nb-Si in-situ composite and three ceramics of ZrO2, Y2O3 stabilized ZrO2 and Y2O3, and thickness of the reaction zones were about 100μm, 10μm, 1μm respectively. C and N elements in SiC+Si3N4 ceramic diffused to the composite at elevated temperature, and a new phase that rich of Ti, C and N elements appeared nearby the interface and accumulated inhomogenously. It may be concluded that the interface reactions of both ZrO2 and SiC+Si3N4 with Nb-Si composite were very violent, therefore they are not suitable as face-coat materials of shell molds for investment casting; the interface reaction of Y2O3 stabilized ZrO2 and Nb-Si composite was slighter than ZrO2 and SiC+Si3N4, and it can be used as face-coat material of shell molds, the interface reaction of Y2O3 and Nb-Si composite was very slight, and is recommended as an ideal face-coat materials of shell molds.

Abstract: Porous C/C composite with certain porosity prepared by Chemical vapor infiltration (CVI) was chosen as the preforms to develop the C/C-SiC composites through precursor infiltration and pyrolysis(PIP), using PCS (polycarbosilane) as the precursor and divinylbenzene as solvent and cross-linking reagent for PCS. The effect of the infiltration solution with different PCS/DVB ratio on the final density, microstructure, and mechanical properties of composites was investigated and the proper PCS/DVB ratio to prepare the C/C-SiC composites was suggested. The experimental results showed that the final densities and the mechanical properties of the composites were close related to the PCS/DVB ratio. Higher PCS/DVB ratio resulted in higher final density and better mechanical properties, but not the highest PCS/DVB ratio could get the best mechanical properties. The main reason is that too high PCS/DVB ratio will make the infiltration process become difficult and lead to the formation of lots of pores in the final composite, at last lowers the mechanical properties. It is believed that the 50% PCS content is proper to prepare the C/C-SiC composites. The composite from 50% PCS infiltration solution could get the final density of1.696g/cm3, the flexural strength of 171Mpa, and shearing strength of 21.6Mpa, which are the best mechanical results among the obtained materials.

Abstract: The fabrication of AlN-SiC-TiB2 ceramics with powder mixtures of Al, 6H-SiC and TiB2 was investigated by self-propagating high temperature synthesis (SHS) and hot isostatic pressing (HIP). The powder mixtures were shaped by isostatic cool pressing method and the combustion reaction was carried at the pressure of 100-200 MPa N2 by an ignitor. The compositions and morphologies of the combustion product were studied by XRD and SEM. The determined bending strength and the fracture toughness of the ceramics were 350 MPa and 3.5 MPa⋅m1/2 respectively.

Abstract: Unidirectional carbon fiber reinforced fused silica (uni-Cf/SiO2) composite was prepared by slurry infiltration and hot-pressing. The room and elevated temperatures flexural properties were investigated and the fracture features of the composite were observed. This composite exhibited non-catastrophic failure at room and elevated temperatures. The oxidation of carbon fiber at elevated temperatures was the main reason for the degradation of flexural strength and elastic modulus. The flexural strength tested at 1200
was 376MPa and exhibited anomalously higher than that at 1000
(277MPa), which was attributed to the viscous flow of fused silica matrix and therefore the occurrence of microcracking in the matrix was deferred. And it was inferred that the brittle to plastic transition temperature (Tb-p) of uni-Cf/SiO2 composite corresponded to a certain temperature around 1200°C.

Abstract: The thermotropic liquid crystalline copolyamide (TLCPa) was synthesized and the in situ composites for TLCPa/Polyamides 66 (PA66) were prepared by melting extrusion. As revealed by differential scanning calorimeter (DSC), depression of the melting point and the crystallinity of PA66 indicated that the miscibility was enhanced via intermolecular H-bonds. Characteristic absorption shifts of C=O groups of TLCPa/PA66 in Fourier transform infra-red spectra (FTIR) confirmed the existence of H-bonds. Scanning electron microscope (SEM) observation showed that the shape of TLCPa phase change in matrix with increasing TLCPa content. Mechanical properties of blends were significantly improved by good interface adhesion and TLCPa fibrils generation.

Abstract: The influence of needling parameters on mechanical properties of carbon-carbon (C-C)composites reinforced by needled carbon cloth was studied in the present investigation. The results showed that needling density (ND)and needle-punching depth (PD) both had a larger effect on interlaminar shear strength (ILSS) than on compression strength and flexible strength of Needling C-C(NC-C). The mechanical properties of NC-C increased with increasing punching density in the range of 20-45 punch/cm2 and also with increasing punching depth range from10 to16mm. The NC-C with the highest ND value of 55punch/cm2 had highest ILSS value of 27 MPa.The optimized ND and PD parameters was 30 punch/cm2 and 12mm respectively, which resulted in a high value of tensile strength(119MPa), bending strength (220MPa) and compression strength (235 MPa) in the X-Y direction .