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1.
Nam-Kyun Kim Doh-Yeon Kim A. Kranzmann E. Bischoff Suk-Joong L. Kang 《Journal of Materials Science》1993,28(16):4355-4358
The aluminium concentration in -sialon grains of the 74Si3N4 ()-6Al2O3-20Nd2O3 (wt%) specimen has been determined. As the - transformation proceeded, the average aluminium concentration in -sialon grains decreased. For individual grains, the aluminium concentration at the centre was higher than that at the edge. The observed variation of aluminium concentration has been explained in terms of the compositional change of the liquid matrix during sintering. 相似文献
2.
The reaction hot-pressing at 1700 and 1750° C of a number of compositions in the system Al-Si-N-O corresponding to points spanning the-sialon phase line in the region of z=0.8 has been studied. Measurements have been made of densification rate and of the rate of conversion of-Si3N4 to-sialon. The densification process for these compositions may be described in terms of a liquid-phase assisted fast particle rearrangement process succeeded by Coble creep. The rates of these processes are sensitive to the volume of grain-boundary liquid phase present, which in turn is determined by the position of the compositional point in relation to the-sialon line. For systems containing very little grain-boundary phase, the vapour-phase transport of material may become important. 相似文献
3.
4.
Ya-Wen Li Pei-Ling Wang Wei-Wu Chen Yi-Bing Cheng Dong-Sheng Yan 《Journal of Materials Science》2001,36(4):807-810
Two R--sialon (R0.6Si9.3Al2.7O0.9N15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the phase transformation. Elongated -sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R--sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R--sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain. 相似文献
5.
The microstructure of a /- directionally solidified (DS) eutectic alloy with a nominal composition of Ni-30.26Mo-6.08Al-1.43V (wt%) was investigated by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. The -fibres exhibited a typical morphology with a rectangular cross-section and they displayed the Bain orientation relationship (OR) with the / matrix; that is, [001][001] and (110)(010). Misfit dislocations and lattice strain fields existed at the / interface for different habit planes; that is, (110)(010) and (100)(110) were analysed. EDAX (Energy dispersive X-ray) analysis showed that the composition of the -phase was approximately Ni4(Mo, Al, V); it contained 90° rotational domains of Ni3(Mo, Al, V) with a DO22 structure and Ni2(Mo, Al, V) with a Pt2Mo structure. 相似文献
6.
A potential heat-resistant ductile eutectic composite,/-, in the Ni-Al-Mo ternary system has been thermally cycled in the temperature range 200 to 1000° C for up to 1000 cycles in an attempt to examine dimensional as well as microstructural stability of the composite under thermal fatigue conditions. The composite examined has two types of initial microstructure; in one, blocky -Ni3Al encircles individual-Mo fibres (as-grown condition) whereas in the other, is in the form of fine cuboidal particles uniformly distributed in an Ni-rich fcc matrix (heat-treated condition). Dilatometric measurements upon temperature cycling show that the composite is stable against thermal ratchetting irrespective of initial microstructural conditions. However, the cycling induces microstructural change, which is characterized by segmentation of-Mo fibres or formation of a detrimental brittle phase identified as an intermetallic-NiMo that consumes-Mo fibres whether the fibres are encircled by or not. Post-cycling tensile tests at room temperature show that the fibre damage in the former has no fatal effect on tensile strength and ductility. A beneficial effect of the-encircling configuration is discussed on the basis of the recognition of a peritecto-eutectoid reaction:++ that has been disregarded. 相似文献
7.
-sialon whiskers and co-products of synthesis, such as -sialon powders and O-sialon powders, were annealed at 1623 K for 8 h in a closed graphite reaction tube under 1 atm nitrogen. Phase stabilities, Si/Al ratios, and crystallographic features were investigated. The O-sialon phase, which formed in the early stage of synthesis when oxygen partial pressure was relatively high, became less stable in the present annealing condition and decomposed. The majority of released aluminium and possibly oxygen from the decomposed O-powder was incorporated into -sialon whiskers with little change in its lattice parameters, when the -sialon whiskers were included in annealing. The aluminium contents were always lower in the -whiskers than in the powders even after increasing its aluminium content during 8 h annealing. The lattice parameters of both -whiskers and powders increased with increasing aluminium content and became closer after annealing. The lattice parameters of -whiskers remained the same before and after annealing despite the increased aluminium content, while the lattice parameters of -powders decreased despite its aluminium content remaining unchanged. The lattice parameters of O-sialon increased with increasing aluminium content, and the increase in thea direction is the largest when compared with other parameters. 相似文献
8.
Wang Hong Min Chen Quande Wu Yigui Zhang Yuangeng 《Journal of Materials Science》1992,27(5):1212-1216
The decomposition of the supersaturated solid solution of ZA27 alloy at room temperature has been investigated by X-ray diffraction, TEM and mechanical properties testing. Based on the results obtained, both continuous precipitation and cellular reaction occur during the decomposition process. The continuous precipitation follows the sequence:
1
+ spherical GP zones
2
+ elliptical GP zones
3
+ R + . The cellular reaction can be written: + + . The properties of the alloy depend on the microstructure. After 1 month of ageing, a series of changes of microstructure have taken place. The properties of the alloy are: b=500 MPa, =13%,H
v=148. 相似文献
9.
Self-reinforcement of Li--sialon ceramics by in situ growth of elongated and -sialon grains has been explored and analysed. Properties of Li--sialon ceramics are mainly determined by the overall starting composition and the crystalline modification of the starting Si3N4 powder which in turn determine the final microstructure of the materials. Both the morphology and crystalline phase of the elongated sialon grains have strong effects on the toughening mechanism. The results indicate that -sialons reinforced by elongated -sialon grains have advantages over similar materials reinforced by elongated -sialon grains because of the type of crack deflection toughening mechanism involved. 相似文献
10.
Powders of Si3N4, Al2O3 and SiO2 were mixed with Y2O3 as sintering aid and hot-pressed to form o- sialon. During sintering, the o phase preferentially precipitated in the temperature region of 1550 to 1600 °C, and the phase precipitated at temperatures of 1700 °C and above. The resultant microstructure consisted of dual phases of o and . The mechanical properties of o- sialon were improved with increasing amount of the phase. When SiO2 required for the formation of the o phase was all included in the grain boundary phase, o- sialon exhibited a three-point bending strength as much as 1400 MPa. The oxidation resistance of o- sialon is expected to improve due to the presence of the o phase, but actually was not so good due to the presence of the grain-boundary glassy phase and impurities. 相似文献
11.
An investigation has been made of the various phase transformations which occur when dilute Ti-Mo (0.5 and 1.0 wt% Mo) alloys are cooled from the phase field and are subsequently tempered in the + phase field. The morphology of the decomposition products varied with cooling rate and can be correlated with the competition between the formation of by martensitic and by diffusional processes. On isothermal transformation below theM
s, or upon tempering, phase precipitates grow heterogeneously from the supersaturated plates formed by shear. The precipitate habit plane is irrational and corresponds to the {334} plane.Formerly at Imperial College. 相似文献
12.
Metastable lath martensite (
L
) phase wires with high strengths have been produced in the Fe-Ni-Cr-Al-C alloy system by melt spinning in rotating water. These wires have a circular cross section and a white lustre and the wire diameter is in the range of 100 to 140m. The width and length of each lath in the
L
phase are as small as about 0.3 and 2m, respectively. The
y,
f and
p are about 900 and 1650 MPa and 2.0% for the
L
wires. The subsequent annealing causes an increase in p as well as y and f and the attained values are about 1000 and 1700 MPa and 3.0% for Fe-10Ni-10Cr-6.5 Al-1.0C wire annealed at 773 K for 1 h owing to the precipitation strengthening of a very fine unidentified carbide and to a high density of dislocations and lath boundaries in the
L
phase. Further annealing causes a significant decrease in p through decomposition of
L
to+M7C3+M23C6. Therefore, the high strength combined with relatively good ductility for the
L
wires is interpreted as due to the suppression of the phase transformation of
L
to a mixed structure of+M7C3+M23C6 by melt quenching. 相似文献
13.
An investigation has been made of the reversion of martensite () to austenite () in two stainless steels (i) Fe-16 wt% Cr-12 wt% Ni (of low interstitial content) (ii) Fe-15 wt% Cr-8 1/2 wt% Ni-2 wt% Mo-0.09 wt% C. The alloys were refrigerated to produce 12 to 15% martensite () and then heated for short times at various temperatures ranging from below A
s to above A
f. With rapid heating the reversion of to occurs largely by a shear mechanism. In the Fe-16Cr-12Ni alloy individual grains of transform to grains of reversed of similar size and shape. In the carbon-containing alloy there is evidence of break-up of the grains on reversion. An increase in the strength results from reversion and this is attributable mainly to the high dislocation density of the reversed . 相似文献
14.
Both lath-shaped and martensites are induced by tensile deformation within of a two-phase ( + ) Fe-Cr-Ni stainless steel, forms from the through the at an intersection of two crystals. These are observed both when is surrounded by and when borders . The amount of strain at which both and nucleate, increases with test temperatures in the range –196 to 50° C. Adjacent laths are either twin-related, or 5, 9, 15 or 19° off the twin relationship, as found by analysing electron diffraction patterns. 相似文献
15.
M. Cole K. P. J. O'Reilly M. Redington S. Hampshire 《Journal of Materials Science》1991,26(19):5143-5148
The general composition for an-sialon isM
x(Si, Al)12(O, N)16, (the value ofx lies between 0 and 2), where M is a modifying cation such as Li+, Ca2+, Y3+ or a number of the lanthanide cations (Ln3+). This paper reports extended X-ray absorption fine structure (EXAFS) spectroscopy measurements of a hot-pressed erbium containing-sialon, using the synchrotron radiation source at Daresbury, UK. This technique provides short-range structural information within this engineering ceramic. The results suggest that the erbium modifying cation shows a preference for being located in one particular site within the ceramic network. 相似文献
16.
Complex microstructural changes in as-cast eutectoid Zn-Al alloy 总被引:4,自引:0,他引:4
Phase transformations and microstructural changes of an as-cast eutectoid Zn-Al alloy (ZnAl22Cu2) were investigated during isothermal holding. The typical dendritic structures consisted of s phase as a core with the edge of decomposed s phase and decomposed s in the interdendritic regions. A series of complex phase transformations was observed. Both decompositions of s and s were determined at an early stage of ageing and a four-phase transformation, f+T+, was observed at the boundaries of f phase and the phase, instead of clearly observed at the boundaries of phase, in a solution-treated Zn-Al alloy during prolonged ageing. 相似文献
17.
Hardness-temperature relationships in metals 总被引:1,自引:0,他引:1
H. D. Merchant G. S. Murty S. N. Bahadur L. T. Dwivedi Y. Mehrotra 《Journal of Materials Science》1973,8(3):437-442
The high-temperature hardness data for thirty-seven metals have been collected from the published literature. Seven of these metals were further tested to confirm and improve the hardness-temperature data. All data were analysed to calculate the softening parameter B and the apparent activation energy for indentation B. The values of B are compared with the activation energies for lattice and dislocation pipe diffusion to obtain an estimate of the stress coefficient, , for creep. The B and values have been improved by considering the effect of elastic modulus. 相似文献
18.
Y. H. Zhu H. C. Man H. J. Dorantes-Rosales W. B. Lee 《Journal of Materials Science》2003,38(13):2925-2934
Phase transformations and microstructural changes of a furnace cooled eutectoid Zn-Al based alloy were studied during ageing at 100 and 170°C using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Three phase transformations occurred in the furnace cooled eutectoid Zn-Al alloy. The metastable FC phase decomposed during isothermal ageing. The four-phase transformation, + T + followed the discontinuous decomposition of the FC phase. Typical morphologies of the decomposition of the FC and phases were observed in scanning electron microscopy. Decomposition of Al-rich phase was observed during the prolonged ageing by transmission electron microscopy. The different types of decomposition of the different metastable phases dominated at different stages of ageing. 相似文献
19.
The dielectric properties of chemically vapour-deposited (CVD) amorphous and crystalline Si3N4 were measured in the temperature range from room temperature to 800° C. The a.c. conductivity (
a.c.) of the amorphous CVD-Si3N4 was found to be less than that of the crystalline CVD-Si3N4 below 500° C, but became greater than that of the crystalline CVD-Si3N4 over 500° C due to the contribution of d.c. conductivity (
d.c.). The measured loss factor () and dielectric constant () of the amorphous CVD-Si3N4 are smaller than those of the crystalline CVD-Si3N4 in all of the temperature and frequency ranges examined. The relationships of
n-1, (-
)
n-1 and/(-
) = cot (n/2) (were observed for the amorphous and crystalline specimens, where is angular frequency andn is a constant. The values ofn of amorphous and crystalline CVD-Si3N4 were 0.8 to 0.9 and 0.6 to 0.8, respectively. These results may indicate that the a.c. conduction observed for both of the above specimens is caused by hopping carriers. The values of loss tangent (tan) increased with increasing temperature. The relationship of log (tan) T was observed. The value of tan for the amorphous CVD-Si3N4 was smaller than that of the crystalline CVD-Si3N4. 相似文献
20.
Melora Larson Norbert Mulders Ravi Mehrotra Lori S. Goldner Guenter Ahlers 《Journal of Low Temperature Physics》1992,89(1-2):79-89
We measured the superfluid fraction s/ and the isobaric thermal expansion coefficient P of4He confined in an aerogel. Data were obtained near the -line along several isobars. Powerlaw fits to the results for s/ as a function of tT/Tc–1 (Tc is the transition temperature) give a pressure—independent exponent =0.755 when a confluent singular term is included. Fits to the P data of power laws yield the specific-heat exponents –0.6 and –1.0 above and below Tc respectively. When an analytic background term a×t is included in the fit, the pressure-independent value =–0.59 is permitted, but the amplitude ratio A/A is found to be near zero and the coefficient of the analytic term is large. The measured values for and or are inconsistent with hyperscaling in three dimensions. 相似文献