Wenru Sun

Characteristics of Ca-α-sialon : Phase formation, microstructure and mechanical properties

Abstract Ca-α-Sialon (Ca x Si 12 -( m+n ) Al m+n O n N 16-n ,) ceramics with extensive compositions on the line of Si 3 N 4 –CaO:3AlN (where m =2 n ) ranged from x =0·3 up to x =2·0 were fabricated by hot-pressing. An exploration for Ca-α-Sialon involving reaction sequences, phase compositions, cell dimensions, microstructure and mechanical properties was carried out in the present work.

Phase relationships in the system Y-A1-O-N

Abstract The sub-solidus phase relationships and the isothermal section at 1850°C in the system Y-Al-O-N were determined. No quaternary compound occurs in the system. In the sub-systems there exist Y 3 O 3 N, YAM (2Y 2 O 3 -Al 2 O 3 ), YAP (Y 2 O 3 · A1 2 O 3 ), YAG (3Y 2 O 3 · 5Al 2 O 3 ) and aluminium oxynitride spinel. No compound occurs in the YN-AlN sub-system.

Studies of the formation of α′ and α′-β′ sialon

Abstract Recently α′ and α′-β′ composite sialon ceramics have received much attention because of the hardness and the better thermal shock properties of α′-sialon materials. Phase relationships determined in the α′ plane show that α′-sialon expands from the pure nitride side towards the alumina side and there exists a fairly big region of α′-sialon in equilibrium with β′-sialon, thus providing the possibility to fabricate α′-β′ composite sialon ceramics. Because of the small difference in composition between α′ and β′ sialons, transformations between α′ and β′ occur very rapidly at firing temperatures in the presence of an appropriate grain-boundary liquid phase. Therefore, to control the transformation is of importance for fabricating both α′ and α′-β′ composite sialons and for this an understanding of reaction mechanisms is necessary. This paper describes the reaction sequences under different firing circumstances of α′ and α′-β′ sialon with Y as “modifying” cation, doped with or without La 2 O 3 as densifying additive. The effect of firing conditions on transformation between α′ and β′ and the role of La 2 O 3 on densification as well as the formation of α′-sialon are discussed.

Effect of Phosphorus on Mechanical Properties and Thermal Stability of Fine-grained GH761 Alloy

The effect of phosphorus on mechanical properties and thermal stability of fine-grained GH761 alloy has been investigated. The results show that, when the content of phosphorus is in the range of 0.0007%-0.040%, there are no significant effects on the tensile properties. Phosphorus strongly affects the stress rupture life. The content of 0.023% P results in the longest life in this experiment. Compared with that of the alloy with 0.0007% P. the life is increased more than 2 times in the alloy with 0.023% P. After ageing at 700 degrees C for 1000 h, the life reduced to some extent. However, phosphorus exhibits the similar effect on the life as before ageing. The tensile properties of the alloy are not influenced by long term ageing obviously. The present results suggested that the optimum content of phosphorus is 0.023%. The phosphorus with this content can optimize the precipitates at the grain boundaries and prevent the excessive growth of the precipitates. The combining mechanical properties of GH761 alloy can be largely improved by phosphorus addition combined with grain refinement.

Phase relationships in the system NdAlON

The phase relationships in the system NdAlON at both subsolidus temperatures and 1700°C were determined. A nitrogen-containin Nd-β-alumina, NdAl12O18N, was identified. The unit cell dimensions were determined tobe a = 5.557A andc=22.00 A. At 1700°C an extensive liquid region exists in the triangle Nd2O3Nd2AlO3NNdAlO3.

The effect of GPS parameters on mechanical properties of Y-α-SiAlON ceramics

A dense, oxygen-rich Y—α-sialon ceramic, with a small amount of YAG as a grain boundary phase, has been fabricated through GPS (gas pressure sintering). The effect of nitrogen pressure during GPS on densities and mechanical properties of the materials has been studied, which was observed to be relevant to the YAG content. The α-sialon with 2.5 wt% YAG, GPSed under 3 MPa and at 1800 °C, possesses a flexural strength of 500 MPa, hardness (Vickers) of 1816 kg/ mm2 and fracture toughness (K1c) of 3.4 MPa · m12.

Phase relationships in the system CaAlON

Abstract Phase relationships in the system CaAlON at both subsolidus temperatures and 1700°C were determined. In the region AlNCaOAl 2 O 3 there is no new phase and an extensive liquid phase region exists at 1700°C. The effect of CaO on the densificati of AlN reported by Komeya is discussed on the basis of phase equilibrium.

Chemical compatibility of RE0.6M0.4Mn0.8Co0.2O3-δ (RE = La, Pr, Nd, Sm and Gd; M = Sr and Ca) with yttria stabilized zirconia

Abstract The chemical compatibility of one new series, RE 0.6 M 0.4 Mn 0.8 Co 0.2 O 3-δ (RE=La, Pr, Nd, Sm and Gd; M=Sr and Ca), with yttria stabilized zirconia (YSZ) has been examined. Powder mixtures of perovskites and YSZ were annealed at 1200° for 24h in air. RE 0.6 M 0.4 Mn 0.8 Co 0.2 O 3-δ showed different compatibility towards YSZ for M=Sr and Ca, respectively. The formation of SrZrO 3 was identified by X-ray diffraction analysis as a reaction product for RE 0.6 Sr 0.4 Mn 0.8 Co 0.2 O 3-δ . In contrary, there is no reaction product for RE 0.6 Ca 0.4 Mn 0.8 Co 0.2 O 3-δ . EDX analysis revealed that there is a larger difference between the solubility of Sr and Ca in YSZ. The bond-valence model was used to discuss the different compatibility of Sr and Ca series perovskites with YSZ. The present studies also showed that the formation of SrZrO 3 was favored for larger disorder effect due to size difference between A-site RE 3+ and Sr 2+ in RE 0.6 Sr 0.4 Mn 0.8 Co 0.2 O 3-δ .

Influence of Fe on Microstructure and Mechanical Properties in P-doped Ni–Cr–Fe Alloys

The influence of Fe on the microstructure and mechanical properties of P-doped Ni–Cr–Fe alloys has been investigated. Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe. In addition, the precipitation of M23C6 was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.

Re-recognition of the Effects of Phosphorus and Boron on the γ″ and γ′ Phases in IN718 Alloy

The effects of P and B on the matrix strength and precipitations of γ′ and γ″ phases in the grain interior were re-recognized in this study. The combination addition of P and B markedly accelerated the precipitations of γ′ and γ″ phases and strengthened the matrix of IN718 alloy when air-cooled from high temperature, while made no difference when water-quenched from high temperature. The effect of single addition of P on the precipitations of γ″ and γ′ phases was the same with that of the combination addition of P and B, while the single addition of B had no effect on the precipitations of the two phases. Therefore, it was P rather than B which accelerated the precipitations of γ′ and γ″ phases. P could take part in the precipitations of γ′ and γ″ phases, which was revealed by electrochemical extraction and quantitative analysis of chemical composition. It also revealed that P atoms were dissolved in the γ matrix to a relatively high degree at the temperature that γ′ and γ″ phases began to precipitate, and consequently the precipitations of γ′ and γ″ phases were accelerated. The first-principle calculation indicated that P decreased the formation enthalpies of γ′ and γ″ phases when it occupied the Ni lattice sites in the two phases, which explained the effect of P on the γ′ and γ″ phases.