Creep Behaviour of Si3N4 Ceramics Sintered with RE2O3

2006 ◽  
Vol 514-516 ◽  
pp. 759-763
Author(s):  
Claudinei dos Santos ◽  
Kurt Strecker ◽  
Francisco Piorino Neto ◽  
Cosme Roberto Moreira Silva ◽  
Flávia A. Almeida ◽  
...  
Keyword(s):  
Solid Solution ◽  
Relative Density ◽  
Creep Behaviour ◽  
Rare Earth Oxide ◽  
Grain Boundary Sliding ◽  
Creep Tests ◽  
Sintering Additive ◽  
Compressive Stresses ◽  
Oxide Solid Solution ◽  
Boundary Sliding

The objective of this work was to evaluate the creep behaviour of Si3N4 based ceramics obtained by uniaxial hot-pressing. As sintering additive, an yttrium-rare earth oxide solid solution, designed RE2O3, that shows similar characteristics to pure Y2O3, was used. Samples were sintered using high-purity α-Si3N4 powder, with additive mixtures based on RE2O3/Al2O3 or RE2O3/AlN, at 5 and 20 vol.%, respectively. The sintered samples were characterized by X-ray diffractometry, scanning electron microscopy and density. Specimens of 3x3x6 mm3 were submitted to creep tests, under compressive stresses between 100 and 350 MPa at temperatures ranging from 1250 to 13750C in air. Samples with RE2O3/Al2O3 showed β-Si3N4 as crystalline phase, with grains of high aspect ratio, and a relative density around 99% of the theoretical density. The Si3N4/RE2O3/AlN samples presented α-Si3N4 solid solution, designed α-SiAlON, with a more equiaxed microstructure and slightly lower relative density (96-98%). The results of creep tests indicated that these ceramics containing α-SiAlON are the more creep resistant, with steady-state creep rates around 10-4 h-1, with stress exponents (n) in the range 0.67-2.53, indicating grain boundary sliding as the main creep mechanism.

Microstructures and Creep Behaviour of Mg-4Al Alloy Containing Sr and Ca

2011 ◽  
Vol 79 ◽  
pp. 134-139
Author(s):  
Jing Bai ◽  
Yang Shan Sun ◽  
Feng Xue
Keyword(s):  
Activation Energy ◽  
Grain Boundary ◽  
Ternary Phase ◽  
Creep Behaviour ◽  
Creep Property ◽  
Grain Boundary Sliding ◽  
Experimental Conditions ◽  
Lamellar Eutectic ◽  
Boundary Sliding ◽  
Almost Continuous

Microstructures and creep property of the Mg-4Al based alloy with addition of 2% Sr and 1%Ca were investigated. The as-cast microstructures of the present alloy consist of dendritic α-Mg and two major intermetallics: lamellar eutectic C14-Mg2Ca and bulky type Mg-Al-Sr ternary phase. These intermetallics mainly distribute along grain or cell boundaries and form an almost continuous network. The alloy studied shows an excellent creep resistance under the experimental conditions. This is primarily attributed to formation of the thermostable intermetallics with addition of Sr and Ca to Mg-Al based alloy. The values of stress exponent, n, and creep activation energy, Q, imply that both dislocaiton motion and grain boundary sliding contribute to the creep deformation.

Creep Behavior of Multi-Cation α-SiAlON Partially Stabilized Produced with an Yttrium-Rare Earth Oxide Mixture(CRE2O3)

2005 ◽  
Vol 498-499 ◽  
pp. 575-580
Author(s):  
Claudinei dos Santos ◽  
Kurt Strecker ◽  
M.J.R. Barboza ◽  
Sandro Aparecido Baldacim ◽  
Francisco Piorino Neto ◽  
...  
Keyword(s):  
Solid Solution ◽  
Rare Earth ◽  
High Temperature ◽  
Liquid Phase ◽  
Creep Behavior ◽  
Rare Earth Oxide ◽  
Si3n4 Ceramic ◽  
Creep Tests ◽  
Oxide Mixture ◽  
Metallic Cations

a−SiAlON (a’) is a solid solution of a−Si3N4, where Si and N are substituted by Al and O, respectively. The principal stabilizers of the a’-phase are Mg, Ca, Y and rare earth cations. In this way, the possible use of the yttrium-rare earth oxide mixture, CRE2O3, produced at FAENQUIL, in obtaining these SiAlONs was investigated. Samples were sintered by hotpressing at 17500C, for 30 minutes, using a sintering pressure of 20 MPa. Creep behavior of the hot-pressed CRE-a-SiAlON/b-Si3N4 ceramic was investigated, using compressive creep tests, in air, at 1280 to 1340 0C, under stresses of 200 to 350 MPa, for 70 hours. This type of ceramic exhibited high creep and oxidation resistance. Its improved high-temperature properties are mainly due to the absence or reduced amount of intergranular phases, because of the incorporation of the metallic cations from the liquid phase formed during sintering into the Si3N4 structure, forming a a’/b composite.

scholarly journals Creep Behaviour Under Compressive Stresses of Calcium and Barium Containing Mg-Al-based Die Casting Alloys

2013 ◽  
Vol 765 ◽  
pp. 69-73 ◽  
Author(s):  
Hajo Dieringa ◽  
Daniela Zander ◽  
Mark A. Gibson
Keyword(s):  
Creep Behaviour ◽  
Die Casting ◽  
Creep Tests ◽  
Alloying Additions ◽  
Casting Alloys ◽  
Compression Creep ◽  
Compressive Stresses ◽  
Property Evaluation ◽  
Pressure Die Casting ◽  
Applied Stresses

The development of creep resistant high pressure die casting (HPDC) alloys is one of the main focuses in magnesium research. Alloying elements like rare earths, calcium, strontium or scandium added to the necessary aluminium for die casting have already been introduced. Newly developed barium containing magnesium alloys with three levels of alloying additions were processed via HPDC and their compression creep response at 200 °C was evaluated. DieMag633 (Mg-6Al-3Ba-3Ca) displays the best creep resistance followed by DieMag422 (Mg-4Al-2Ba-2Ca) and then DieMag211 (Mg-2Al-1Ba-1Ca). Stress exponents from tests at different applied stresses were calculated. The creep tests were also accompanied by microstructural investigations and mechanical property evaluation.

A formulation for anisotropy in diffusional creep

1992 ◽  
Vol 436 (1896) ◽  
pp. 187-196 ◽  
Keyword(s):  
Grain Boundary ◽  
Elastic Anisotropy ◽  
Creep Behaviour ◽  
Grain Boundary Sliding ◽  
Diffusional Creep ◽  
Sources And Sinks ◽  
Boundary Sliding ◽  
Compliance Matrices

Grain shape can introduce anisotropy in creep which depends on the diffusion of vacancies between grain boundary sources and sinks. Such anisotropy is examined to determine the rate of creep under multiaxial stresses both for lattice and grain boundary diffusion. Noting the role of grain boundary sliding in this form of creep it is shown that, with some approximations that only become significant in an identified case, complete and fully self-consistent formulae can be derived for the rate of creep in terms of grain dimensions. The results are presented in the form of compliance matrices which are analogous to those that have a well-established role in the characterization of elastic anisotropy. A comparable usefulness of these ‘creep compliance coefficients’ is envisaged in evaluating anisotropic diffusional creep behaviour and a similar approach can be extended to more general cases where creep rates may be interface controlled.

Microstructural Evolution and Creep of an Al-0.2wt.%Sc Alloy after Equal-Channel Angular Pressing

2008 ◽  
Vol 584-586 ◽  
pp. 846-851 ◽  
Author(s):  
Petr Král ◽  
Jiří Dvořák ◽  
Vàclav Sklenička
Keyword(s):  
Electron Microscopy ◽  
Applied Stress ◽  
Creep Resistance ◽  
Creep Behaviour ◽  
Shear Bands ◽  
Synergetic Effect ◽  
Grain Boundary Sliding ◽  
Creep Testing ◽  
Creep Tests ◽  
Angular Pressing

Experiments were conducted on an Al-0.2wt.%Sc alloy to evaluate the effect of equalchannel angular pressing (ECAP) on its creep behaviour. ECAP was conducted at room temperature with a die that had an internal angle of 90° between the two parts of the channel. The subsequent extrusion passes were performed by route BC up to 8 ECAP passes. Creep tests in tension were performed on the as-pressed samples at 473 K under an applied stress range between 10 to 50 MPa. For comparison purposes, some creep tests were performed also on the unpressed alloy. Following ECAP and creep testing, samples were prepared for examination by means of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) equipped with an electron back scattering diffraction (EBSD) unit. The observation of the surface of the ECAPed samples after creep exposure showed the occurrence of mesoscopic shear bands. The EBSD data reveal that these bands are separated by high angle grain boundaries. The creep resistance of an alloy is a little decreased after one ECAP pass. However, successive ECAP pressing lead to a noticeable decrease of the creep properties. Thus, the Al-0.2wt.%Sc alloy processed by 8 ECAP passes exhibited faster creep rate by about two and/or three orders of magnitude than the unpressed alloy when creep testing at 473 K and at the same applied stress. The detrimental effect of ECAP on the creep resistance is probably a consequence of a synergetic effect of mesoscopic sliding of groups of grains along shear bands, more intensive grain boundary sliding and creep cavitation in creep of the ultrafine-grained material.

High and Room Temperature Mechanical Evaluation of SiC Ceramics with Alumina and Rare Earth Oxides Additions

2008 ◽  
Vol 591-593 ◽  
pp. 593-597 ◽  
Author(s):  
M.K. Hwang ◽  
Cosme Roberto Moreira Silva ◽  
Maria do Carmo de Andrade Nono ◽  
Claudinei dos Santos ◽  
Francisco Piorino Neto
Keyword(s):  
Rare Earth ◽  
Room Temperature ◽  
Grain Boundary Sliding ◽  
Rare Earth Oxides ◽  
Argon Atmosphere ◽  
Oxidation Reactions ◽  
Creep Tests ◽  
Creep Mechanisms ◽  
Sic Ceramics ◽  
Boundary Sliding

In this work, the mechanical properties of SiC samples with (5 wt%) alumina and (5 wt%) rare earth oxides (REO) additions were investigated at room and high temperatures. The possibility of using REO instead of pure yttria could bring cost reductions to obtain sintered SiC ceramics. The powders were mixed, milled, sieved and hot-pressed at 1800°C in argon atmosphere. Hardness and fracture toughness were evaluated at room temperature. The compressive creep behavior was carried out under stress range from 150 to 300MPa and temperatures of 1300, 1350 and 1400°C. After creep tests, the surface analysis of the samples by XRD and SEM showed indications of oxidation reactions, cavity and cracking. The possible creep mechanisms activated in this system were diffusion, grain boundary sliding and cavitation

Grain Boundary Characters and Sliding of [0001] Symmetric Tilt Boundaries in Alumina

2003 ◽  
Vol 778 ◽  
Author(s):  
Katsuyuki Matsunaga ◽  
Hitoshi Nishimura ◽  
Hiroyuki Muto ◽  
Takahisa Yamamoto ◽  
Yuichi Ikuhara
Keyword(s):  
Grain Boundary ◽  
Grain Boundary Sliding ◽  
Creep Tests ◽  
Open Spaces ◽  
Atomic Structures ◽  
Transmission Electron ◽  
Symmetric Tilt ◽  
Tilt Boundaries ◽  
Boundary Sliding ◽  
Boundary Core

AbstractBicrystal experiments were performed to investigate atomic structures and high-temperature creep properties of [0001] symmetric tilt grain boundaries in Al2O3. Al2O3 bicrystals with Σ7, Σ31 and Σ39 boundaries were fabricated by a diffusion bonding technique, and their atomic arrangements at the grain boundary cores were analyzed by high-resolution transmission electron microscopy (HRTEM), in combination with static lattice calculations based on two-body ionic potentials. Compressive creep tests were also conducted to examine the behavior of grain boundary sliding for the above bicrystals. It was found that the behavior of grain boundary sliding depends on the grain boundary characters, whereas the trend of grain boundary sliding was not related to their Σ values. In contrast, HRTEM observations showed that the Σ31 boundary exhibiting the highest sliding rate has open spaces at the boundary core. Since grain boundary diffusion is expected to accommodate strains at grain boundary cores during sliding, it is likely that such open spaces give rise to high diffusivity at the grain boundary core, which results in the rapid grain boundary sliding of Σ31.

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