Thermo-mechanical stability of bulk (Al1–xCrx)2O3 solid solution (original) (raw)
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The effect of microstructure on thermal expansion coefficients in powder-processed Al2Mo3O12
Journal of Materials Science, 2013
Orthorhombic Al 2 Mo 3 O 12 was investigated as a model anisotropic phase to understand the influence of powder preparation routes and bulk microstructure (mean grain size) on the bulk coefficient of thermal expansion (CTE) and to compare it to the intrinsic CTE of powder samples. A co-precipitation route was used for the synthesis of pure single-phase nanopowders, while a polyvinyl alcohol-assisted sol-gel method was utilized for the synthesis of micron-sized powders. Sintered samples prepared from both powders exhibited different microstructures in terms of mean crystal sizes and porosity. Bulk samples obtained from nanopowders were highly porous and contained crystals of approximately 100-nm diameter, while the bulk pieces produced from the micron-sized powders were denser, contained crystals larger than 5 lm, and showed occasional intergranular and transgranular microcracks. Such different microstructures hugely impact the bulk CTE: the nanometric sample possesses a bulk CTE (0.9 9 10 -6°C-1 , from 200 to 700°C) closer to the instrinsic CTE (2.4 9 10 -6°C-1 ) than for the micrometric sample, which showed a negative CTE (-2.2 9 10 -6°C-1 ) from 200 to 620°C, and an even more negative CTE above 620°C (-35 9 10 -6°C-1 ). A finite element analysis showed that the local maximum thermal tensile stresses could be as high as 220 MPa when simulating a temperature drop of 700°C as an example of thermal treatment following sintering. This tensile stress is expected to exceed the tensile strength of Al 2 Mo 3 O 12 , explaining the origin of microcracks in bulk samples prepared from the micron-sized powders. The thermal behavior of the microcracks leads to differences between the intrinsic and bulk thermal expansion; we show experimentally that such differences can be reduced by nanostructuring.
Advanced Engineering Materials, 2007
Solid solutions of (Al 1-x Cr x ) 2 O 3 attract interest for their potential to form corundum-type structures for a wide range of compositions between pure chromia of the eskolaite structure (which is isostructural with corundum) and pure alumina, and for the promise that it can be synthesized by PVD at much lower deposition temperatures than in CVD. It can be expected that such solid solutions of corundum-type structure, especially for high aluminum concentrations, show properties similar to a-alumina which is known for its outstanding high temperature hardness and chemical inertnessindispensable properties to replace corundum coatings in high temperature cutting tool applications.
On the thermal stability of ultrafine-grained Al stabilized by in-situ amorphous Al2O3 network
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2015
Bulk Al materials with average grain sizes of 0.47 and 2.4 mm, were fabricated by quasi-isostatic forging consolidation of two types of Al powders with average particle sizes of 1.3 and 8.9 μm, respectively. By utilizing the native amorphous Al 2 O 3 (am-Al 2 O 3) film on the Al powders surfaces, a continuous, ∼7 nm thick, am-Al 2 O 3 network was formed in situ in the Al specimens. Systematic investigation of the changes to the am-Al 2 O 3 network embedded in the Al matrix upon heating and annealing up to 600°C was performed by transmission electron microscopy (TEM). At the same time, the stability of the Al grain structure was studied by transmission Kikuchi diffraction (TKD), electron back-scatter diffraction (EBSD), and TEM. The am-Al 2 O 3 network remained stable after annealing at 400°C for 24 h. In-situ TEM studies revealed that at temperatures Z 450°C, phase transformation of the am-Al 2 O 3 network to crystalline γ-Al 2 O 3 particles occurred. After annealing at 600°C for 24 h the transformation was completed, whereby only nanometric γ-Al 2 O 3 particles with an average size of 28 nm resided on the high angle grain boundaries of Al. Due to the pinning effect of γ-Al 2 O 3 , the Al grain and subgrain structures remained unchanged during annealing up to 600°C for 24 h. The effect of the am-Al 2 O 3-γ-Al 2 O 3 transformation on the mechanical properties of ultrafine-and fine-grained Al is discussed from the standpoint of the underlying mechanisms. & 2015 Elsevier B.V. All rights reserved. transform to crystalline γ-Al 2 O 3 (or transitional γ`-Al 2 O 3) in the range 450-550°C [4,10]. The γ-Al 2 O 3 crystals grow epitaxially in respect to an Al substrate on am-Al 2 O 3 /Al interface [11] and later they develop preferentially along the grain boundaries [12]. The growth of γ-Al 2 O 3 films is limited by the rate of inward oxygen diffusion through the films. The γ-Al 2 O 3 on the surface of loose powder remains thermodynamically stable during thermal exposures to temperatures normally associated with Al PM Contents lists available at ScienceDirect
EÄect of Cr 2 O 3 addition on microstructural evolution and mechanical properties of Al2O3
2000
The eects of chromia (Cr 2 O 3) additions on the microstructural evolution and the mechanical properties of alumina (Al 2 O 3) were investigated. When small amounts (<5 mol%) of Cr 2 O 3 were added in samples hot pressed at 1500 C, the grain size distribution became bimodal; large platelike grains were dispersed in a relatively small grained matrix. The large grains were composed of a core region that is free of Cr and a surrounding shell region rich in Cr. The interface between the core and the shell was composed of mis®t dislocations. The high diusion rate of Cr ions through the surface of alumina was attributed to this microstructural evolution. The mechanical properties of the specimens were strongly in¯uenced by this microstructural change. The fracture toughness and the¯aw tolerance (R-curve behavior) of Al 2 O 3 were improved markedly by the formation of the large platelike grains. The hardness and the elastic modulus also increased, however, the fracture strength decreased by the addition of Cr 2 O 3 .
Ceramics International, 2011
An ultra-fine alumina powder was doped with yttrium or zirconium chloride to produce Al 2 O 3 -5 vol.%ZrO 2 (AZ-5) and Al 2 O 3 -5 vol.%YAG (AY-5) composite powders. Composite samples and pure alumina, used as a reference, were submitted to dilatometric analyses up to 1500-1550 8C at 2, 5 and 10 8C/min for supplying the data required for the modeling of their sintering behaviour. The best fit for the three samples was obtained by applying an Avrami-Erofeev nucleation and growth model (An) and a subsequent power law reaction (AnFn). The evolution of the activation energy with densification is given for the three samples, as well as the prediction of their best sintering conditions. Finally, densification mechanism for the immiscible (AZ-5) and miscible (AY-5) systems are hypothesized and correlated with their final microstructures. #
High purity nanocrystalline α-alumina powder was mixed with 20 wt% ZrO 2 by slurry method sintered at temperature (T s) 1450°C, 1500°C, 1550°C and 1600°C for 2 hour. The density, porosity, structural properties and mechanical properties of Al 2 O 3 -ZrO 2 composites with respect to sintering temperature have been explored in the present work. The XRD spectra indicate that α-Al 2 O 3 , t and m-ZrO 2 are the crystalline phases present in 20 wt% ZTA composites for all sintering temperature. It is observed that with higher sintering temperature the intensity of m-ZrO 2 phases increases and the t-ZrO 2 phases decreases. However, t-ZrO 2 retention becomes much easier to trigger the transformation to monoclinic. Higher density of 20 wt% ZTA has been achieved at 1600ºC, whereas the highest porosity was obtained for sintering temperature 1450°C.The microstructures of the samples was studied by using SEM which represents highly homogeneous and finer structure at 1600ºC. The effect of sinterin...
Microstructural and thermal characteristics of the sintered Al-Fe2O3 composites
Engineering review, 2020
This work has as an objective a study of evolution of characteristic properties of crystalline microstructure and mechanical hardening of aluminum by iron oxide (III), (hematite α-Fe2O3) nan energetic material known as thermite, samples of massive alloys, Al (base)-X wt% Fe2O3 (X =2, 4, 16 and 40) were studied.Al-Fe2O3 composite was developed by a sintering technique from the mixtures of compacted powders of Al high purity and α-Fe2O3 under a temperature of 700 °C for 1 hour and then slowly cooled. We have not noted the formation of thermite as foreseen by the chemical reaction due to the mixture of aluminum with hematite. The evolution of crystalline microstructures and the morphologies of surface were determined by means of X-ray diffraction, thermal analysis and optical metallography. The mechanical behavior was characterized by the tests of Vickers indentation and corrosion resistance by electrochemical tests.
Study of High Temperature Sintering Process on Characteristics of Al2O3–ZrO2–TiO2 Ceramics Systems
Materials Focus, 2014
In this study, the effect of sintering temperatures on the microstructure and mechanical properties of Al 2 O 3 -30 wt.% ZrO 2 -5 wt.% TiO 2 composite has been investigated. The samples were obtained by uniaxial pressing (210 MPa) and green compact sintered in air at 1550, 1600 and 1650 C for 3 h. The relative density, average grain size, Vickers hardness, flexural strength and microstructural features were examined to correlate densification with grain growth and secondary phase formation at higher sintering temperature. The average grain size of the sintered composite tends to be larger and suppresses the densification at high temperature sintering process. A relative density of 98.6% of the composite was observed at 1550 C. At higher sintering temperature and TiO 2 addition, the formation of secondary phase of Al 2 TiO 5 , which has lower elastic modulus and consisted of larger grain size, reduces the flexural strength and Vickers hardness of the sintered bulk from 60.55 MPa to 34.40 MPa and 10.80 GPa to 5.73 GPa, respectively.
Materials Science Forum, 2004
Purpose: The purpose of paper is to determinate thermal stability and mechanical properties of sputtered chromium-molybdenum-nitride (CrMoN) coatings. Design/methodology/approach: We have deposited 1.8 m-thick ternary Cr 0.5 Mo 0.5 N 1.0 films on a CoCrMo alloy using a RF dual magnetron sputtering system, with Cr and Mo targets and N 2 as the reactive gas. These films were subjected to various thermal treatments in Ar, air, and microwave plasma. The hardness, Young's modulus, surface roughness, microstructure, and composition of films were studied by nanoindentation, AFM, x-ray diffraction, and x-ray photoelectron spectroscopy. Findings: The as-prepared CrMoN films consist of an amorphous Cr-rich nitride matrix with Mo-rich nitride crystalline grains, about 15 nm in size. These films are thermally stable up to 600ºC in air. Thermal annealing in the air at 800ºC resulted in an increase in surface roughness and hardness, due to film oxidation, with Cr 2 O 3 as the main crystalline phase. Plasma treatment in a H 2 /N 2 gas mixture, at 800ºC, did not lead to grain growth. Instead, the existing grains were reduced to about 10 nm and a new nanocrystalline phase has been formed. This leads to a decrease in the surface roughness, and an increase in the film hardness. In addition, we have further modified the film properties through a combined thermal treatment process. Thermal annealing in the air at 800ºC, followed by microwave plasma treatment at 800ºC resulted in a film with decreased surface roughness, and improved mechanical properties. Reversing the order of the thermal treatments resulted in a further decrease in surface roughness, but it shows a reduction in the mechanical properties. Research limitations/implications: The present investigation was carried out with only one composition, Cr 0.5 Mo 0.5 N 1.0 , of ternary thin-film system. Originality/value: The combination of thermal and plasma treatments can be used to control the microstructure, surface topography, and mechanical properties of ternary CrMoN films. Such post-deposition treatments can further improve the materials properties for desired application, and to produce new nanocomposite materials with technologically important combination of properties.
International Journal of Applied Ceramic Technology, 2021
The mechanical behavior of low (and negative) thermal expansion and low stiffness Al 2 TiO 5 materials and Al 2 TiO 5-3Al 2 O 3 .2SiO 2-ZrTiO 4 composite materials was studied by diametral compression test at room temperature 400 and 800 • C. The effect of both temperature and composition was analyzed. Stressstrain curves were obtained and, from them, apparent elastic modulus (E app) and mechanical strength (σ F) were determined. Fracture mechanisms and fracture patterns were also analyzed. All materials showed a brittle behavior up to 800 • C. The thermal variation of σ F , that was even higher as testing temperature increased, was interpreted based on the microcracks behavior. A double linear correlation of E app was found with temperature (T) and zircon content ([Z]), with a fitting coefficient > .9. The particular low stiffness and the mechanical and thermal behavior of the studied materials suggest that they would be able to withstand thermal stresses.