Shear deformation at 29% solid during solidification of magnesium alloy AZ91 and aluminium alloy A356

Partially solid commercial Al-Si and Mg-Al alloys have been deformed in shear during solidification using vane rheometry. The dendritic mush was deformed for a short period at 29% solid and allowed to cool naturally after deformation. Both alloys exhibited yield point behaviour and deformation was highly localised at the surface of maximum shear stress. The short period of deformation was found to have a distinct impact on the as-cast microstructure leading to fragmented dendrites in the deformation region of both alloys. In the case of the Mg-Al alloy, a concentrated region of interdendritic porosity was also observed in the deformation region. Concentrated porosity was not observed in the Al-Si alloy. (c) 2005 Elsevier B.V. All rights reserved.

Numerical simulation of dendrite growth during solidification

A comprehensive probabilistic model for simulating dendrite morphology and investigating dendritic growth kinetics during solidification has been developed, based on a modified Cellular Automaton (mCA) for microscopic modeling of nucleation, growth of crystals and solute diffusion. The mCA model numerically calculated solute redistribution both in the solid and liquid phases, the curvature of dendrite tips and the growth anisotropy. This modeling takes account of thermal, curvature and solute diffusion effects. Therefore, it can simulate microstructure formation both on the scale of the dendrite tip length. This model was then applied for simulating dendritic solidification of an Al-7%Si alloy. Both directional and equiaxed dendritic growth has been performed to investigate the growth anisotropy and cooling rate on dendrite morphology. Furthermore, the competitive growth and selection of dendritic crystals have also investigated.

Skeletal Microstructural Abnormalities in Postmenopausal Women With Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is associated with osteoporosis and fragility fractures. The objectives of this study were to assess static and dynamic indices of cancellous and cortical bone structure in postmenopausal women with COPD. Twenty women with COPD who had not received chronic oral glucocorticoids underwent bone biopsies after double tetracycline labeling. Biopsies were analyzed by histomorphometry and mu CT and compared with age-matched controls. Distribution of the patients according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) was: Type I (15%), Type II (40%), Type III (30%), and Type IV (15%). Mean (+/-SD) cancellous bone volume (15.20 +/- 5.91 versus 21.34 +/- 5.53%, p = .01), trabecular number (1.31 +/- 0.26 versus 1.77 +/- 0.51/mm, p = .003), and trabecular thickness (141 +/- 23 versus 174 +/- 36 mu m, p = .006) were lower in patients than in controls. Connectivity density was lower in COPD (5.56 +/- 2.78 versus 7.94 +/- 3.08 mu m, p = .04), and correlated negatively with smoking (r = -0.67; p = .0005). Trabecular separation (785 +/- 183 versus 614 +/- 136 mu m, p = .01) and cortical porosity (4.11 +/- 1.02 versus 2.32 +/- 0.94 voids/mm(2); p < .0001) were higher in COPD while cortical width (458 +/- 214 versus 762 +/- 240 mu m; p < .0001) was lower. Dynamic parameters showed significantly lower mineral apposition rate in COPD (0.56 +/- 0.16 versus 0.66 +/- 0.12 mu m/day; p = .01). Patients with more severe disease, GOLD III and IV, presented lower bone formation rate than GOLDI and II (0.028 +/- 0.009 versus 0.016 +/- 0.011 mu m(3)/mu m(2)/day;p = 04). This is the first evaluation of bone microstructure and remodeling in COPD. The skeletal abnormalities seen in cancellous and cortical bone provide an explanation for the high prevalence of vertebral fractures in this disease. (C) 2010 American Society for Bone and Mineral Research.

A numerical study of effect of grain boundaries on elastic and plastic properties in nanocomposite materials

Nanocomposite materials have received considerable attention in recent years due to their novel properties. Grain boundaries are considered to play an important role in nanostructured materials. This work focuses on the finite element analysis of the effect of grain boundaries on the overall mechanical properties of aluminium/alumina composites. A grain boundary is incorporated into the commonly used unit cell model to investigate its effect on material properties. By combining the unit cell model with an indentation model, coupled with experimental indentation measurements, the ''effective'' plastic property of the grain boundary is estimated. In addition, the strengthening mechanism is also discussed based on the Estrin-Mecking model.

Effect of hardness on three very different forms of wear

Different abrasive wear tests have been applied to materials with hardnesses ranging from 80 HV (aluminium) to 1700 HV (tungsten carbide). The tests were: dry sand rubber wheel (DSRbrW); a similar test using a steel wheel (DSStlW); a new combined impact-abrasion test (FIA). The DSRbrW results were as expected, giving generally decreasing wear with increasing hardness. White cast irons and tool steels containing coarse, hard carbide particles performed better than more homogeneous materials of comparable hardness. When normalized to load and distance, the DSStlW results for the homogeneous materials were similar to the DSRbrW results. The multi-phase materials performed poorly in the DSStlW test, with volume loss for high-speed steel (880 HV) higher than that of aluminium. Within this group, wear increased with increasing hardness. These unexpected results are explained in terms of (a) differential friction coefficients of wheel and specimen, (b) increased fracture of sand, and (c) introduction of microfracture wear mechanisms. The FIA combined impact-abrasion results lacked clear correlations with hardness. The span of relative wear rates was similar to that reported for materials in ball mills. White cast irons at maximum hardness performed fairly poorly and showed evidence of microfracture. (C) 1997 Elsevier Science S.A.

Effect of milling conditions on the synthesis of chromium carbides by mechanical alloying

The synthesis of chromium carbides, Cr7C3 and Cr3C2, by mechanically allowing chromium and carbon powders has been investigated. Milling conditions were found to have a strong influence on the evolution of microstructure, with high collision energies being required to form carbide phases. Milling at intermediate energy levels resulted in the formation of an amorphous phase, and with low energy conditions only grain size refinement of Cr occurred with no evidence of any reaction between Cr and C. The amorphous phase was found to be the precursor to carbide formation. (C) 1997 Elsevier Science S.A.

Transmission electron microscopy of a transformation toughened white cast iron

Transmission electron microscopy has been used to study the microstructure of an experimental white cast iran, in which a combination of modified alloy composition and unconventional heat treatment has resulted in a fracture toughness of 40 MPa m(-1/2). Microstructural features of the alloy that contribute to the toughness improvement and hence distinguish it from conventional white irons have been investigated. In the as-cast condition the dendrites are fully austenitic and the eutectic consists of M7C3 carbides and martensite. During heat treatment at 1130 degrees C the austenite is partially destabilized by precipitation of chromium-rich M7C3 carbides. This results in a dendritic microconstituent consisting of bulk retained austenite and secondary carbides which are sheathed with martensite. The martensite sheaths, which contain interlath films of retained austenite, are irregular in shape with some laths extending into the bulk retained austenite. Emphasis has been placed on the morphology, distribution, and stability of the retained austenite and its transformation products in the dendrites. The implications of these findings on the transformation toughening mechanism in this alloy are discussed.

Analysis in vitro of the cytotoxicity of potential implant materials. I: Zirconia-titania sintered ceramics

Zirconia (ZrO(2)) is a bioinert, strong, and tough ceramic, while titania (TiO(2)) is bioactive but has poor mechanical properties. It is expected that ZrO(2)-TiO(2) mixed ceramics incorporate the individual properties of both ceramics, so that this material would exhibit better biological properties. Thus, the objective of this study was to compare the biocompatibility properties of ZrO(2)-TiO(2) mixed ceramics. Sintered ceramics pellets, obtained from powders of TiO(2), ZrO(2), and three different ZrO(2)-TiO(2) mixed oxides were used. Roughnesses, X-ray diffraction, microstructure through SEM, hardness, and DRIFT characterizations were performed. For biocompatibility analysis cultured FMM1 fibroblasts were plated on the top of disks and counted in SEM micrographs 1 and 2 days later. Data were compared by ANOVA complemented by Tukey`s test. All samples presented high densities and similar microstructure. The H(2)O content in the mixed ceramics was more evident than in pure ceramics. The number of fibroblasts attached to the disks increased significantly independently of the experimental group. The cell growth on the top of the ZrO(2)-TiO(2) samples was similar and significantly higher than those of TiO(2) and ZrO(2) samples. Our in vitro experiments showed that the ZrO(2)-TiO(2) sintered ceramics are biocompatible allowing faster cell growth than pure oxides ceramics. The improvement of hardness is proportional to the ZrO(2) content. Thus, the ZrO(2)-TiO(2) sintered ceramics could be considered as potential implant material. (C) 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 94B: 305-311, 2010.

Characterization of dimethacrylate polymeric networks: A study of the crosslinked structure formed by monomers used in dental composites

The resin phase of dental composites is mainly composed of combinations of dimethacrylate comonomers, with final polymeric network structure defined by monomer type/reactivity and degree of conversion. This fundamental study evaluates how increasing concentrations of the flexible triethylene glycol dimethacrylate (TEGDMA) influences void formation in bisphenol A diglycidyl dimethacrylate (BisGMA) co-polymerizations and correlates this aspect of network structure with reaction kinetic parameters and macroscopic volumetric shrinkage. Photopolymerization kinetics was followed in real-time by a near-infrared (NIR) spectroscopic technique, viscosity was assessed with a viscometer, volumetric shrinkage was followed with a linometer, free volume formation was determined by positron annihilation lifetime spectroscopy (PALS) and the sol-gel composition was determined by extraction with dichloromethane followed by (1)H NMR analysis. Results show that, as expected, volumetric shrinkage increases with TEGDMA concentration and monomer conversion. Extraction/(1)H NMR studies show increasing participation of the more flexible TEGDMA towards the limiting stages of conversion/crosslinking development. As the conversion progresses, either based on longer irradiation times or greater TEGDMA concentrations, the network becomes more dense, which is evidenced by the decrease in free volume and weight loss after extraction in these situations. For the same composition (BisGMA/TEGDMA 60-40 mol%) light-cured for increasing periods of time (from 10 to 600 s), free volume decreased and volumetric shrinkage increased, in a linear relationship with conversion. However, the correlation between free volume and macroscopic volumetric shrinkage was shown to be rather complex for variable compositions exposed for the same time (600 s). The addition of TEGDMA decreases free-volume up to 40 mol% (due to increased conversion), but above that concentration, in spite of the increase in conversion/crosslinking, free volume pore size increases due to the high concentration of the more flexible monomer. In those cases, the increase in volumetric shrinkage was due to higher functional group concentration, in spite of the greater free volume. Therefore, through the application of the PALS model, this study elucidates the network formation in dimethacrylates commonly used in dental materials. (C) 2010 Elsevier Ltd. All rights reserved.

Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics

Objective. To investigate the processing induced particle alignment on fracture behavior of four multiphase dental ceramics (one porcelain, two glass-ceramics and a glass-infiltrated-alumina composite). Methods. Disks (empty set12mm x 1.1 mm-thick) and bars (3 mm x 4 mm x 20 mm) of each material were processed according to manufacturer instructions, machined and polished. Fracture toughness (K(IC)) was determined by the indentation strength method using 3-point bending and biaxial flexure fixtures for the fracture of bars and disks, respectively. Microstructural and fractographic analyses were performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Results. The isotropic microstructure of the porcelain and the leucite-based glass-ceramic resulted in similar fracture toughness values regardless of the specimen geometry. On the other hand, materials containing second-phase particles with high aspect ratio (lithium disilicate glass-ceramic and glass-infiltrated-alumina composite) showed lower fracture toughness for disk specimens compared to bars. For the lithium disilicate glass-ceramic disks, it was demonstrated that the occurrence of particle alignment during the heat-pressing procedure resulted in an unfavorable pattern that created weak microstructural paths during the biaxial test. For the glass-infiltrated-alumina composite, the microstructural analysis showed that the large alumina platelets tended to align their large surfaces perpendicularly to the direction of particle deposition during slip casting of green preforms. Significance. The fracture toughness of dental ceramics with anisotropic microstructure should be determined by means of biaxial testing, since it results in lower values. (C) 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Al(2)O(3)/GdAlO(3)fiber for dental porcelain reinforcement

The aim of this study was to test the hypothesis that the addition of continuous or milled GdAlO(3)/Al(2)O(3) fibers to a dental porcelain increases its mechanical properties. Porcelain bars without reinforcement (control) were compared to those reinforced with long fibers (30 vol%). Also, disk specimens reinforced with milled fibers were produced by adding 0 (control), 5 or 10 vol% of particles. The reinforcement with continuous fibers resulted in significant increase in the uniaxial flexural strength from 91.5 to 217.4 MPa. The addition of varied amounts of milled fibers to the porcelain did not significantly affect its biaxial flexural strength compared to the control group. SEM analysis showed that the interface between the continuous fiber and the porcelain was free of defects. On the other hand, it was possible to note the presence of cracks surrounding the milled fiber/porcelain interface. In conclusion, the reinforcement of the porcelain with continuous fibers resulted in an efficient mechanism to increase its mechanical properties; however the addition of milled fibers had no significant effect on the material because the porcelain was not able to wet the ceramic particles during the firing cycle. (C) 2008 Elsevier Ltd. All rights reserved.

Subcritical crack growth in porcelains, glass-ceramics, and glass-infiltrated alumina composite for dental restorations

The objective was to compare fracture toughness (K(Ic)), stress corrosion susceptibility coefficient (n), and stress intensity factor threshold for crack propagation (K(I0)) of two porcelains [VM7/Vita (V) and d.Sign/Ivoclar (D)], two glass-ceramics [Empress/Ivolcar (E1) and Empress2/Ivlocar (E2)] and a glass-infiltrated alumina composite [In-Ceram Alumina/Vita (IC)]. Disks were constructed according to each manufacturer`s processing method, and polished before induction of cracks by a Vickers indenter. Crack lengths were measured under optical microscopy at times between 0.1 and 100 h. Specimens were stored in artificial saliva at 37A degrees C during the whole experiment. K(Ic) and n were determined using indentation fracture method. K(I0) was determined by plotting log crack velocity versus log K(I). Microstructure characterization was carried out under SEM, EDS, X-ray diffraction and X-ray fluorescence. IC and E2 presented higher K(Ic) and K(I0) compared to E1, V, and D. IC presented the highest n value, followed by E2, D, E1, and V in a decreasing order. V and D presented similar K(Ic), but porcelain V showed higher K(I0) and lower n compared to D. Microstructure features (volume fraction, size, aspect ratio of crystalline phases and chemical composition of glassy matrix) determined K(Ic). The increase of K(Ic) value favored the increases of n and K(I0).

Influence of leucite content on slow crack growth of dental porcelains

Objectives. To determine the stress corrosion susceptibility coefficient, n, of seven dental porcelains (A: Ceramco I; B: Ceramco-II; C: Ceramco-III; D: d.Sign; E: Cerabien; F: Vitadur-Alpha; and G: Ultropaline) after aging in air or artificial saliva, and correlate results with leucite content (LC). Methods. Bars were fired according to manufacturers` instructions and polished before induction of cracks by a Vickers indenter (19.6 N, 20 s). Four specimens were stored in air/room temperature, and three in saliva/37 degrees C. Five indentations were made per specimen and crack lengths measured at the following times: similar to 0; 1; 3; 10; 30; 100; 300; 1000 and 3000 h. The stress corrosion coefficient n was calculated by linear regression analysis after plotting crack length as a function of time, considering that the slope of the curve was (2/(3n + 2)]. Microstructural analysis was performed to determine LC. Results. LC of the porcelains were 22% (A and B); 6% (C); 15% (D); 0% (E and F); and 13% (G). Except for porcelains A and D, all materials showed a decrease in their n values when stored in artificial saliva. However, the decrease was more pronounced for porcelains B, F, and G. Ranking of materials varied according to storage media (in air, porcelain G showed higher n compared to A, while in saliva both showed similar coefficients). No correlation was found between n values and LC in air or saliva. Significance. Storage media influenced the n value obtained for most of the materials. LC did not affect resistance to slow crack growth regardless of the test environment. (c) 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Influence of pH on slow crack growth of dental porcelains

Objectives. To evaluate the effect of pH of storage medium on slow crack growth (SCG) parameters of dental porcelains. Methods. Two porcelains were selected: with (UD) and without (VM7) leucite particles, in order to assess if the microstructure would affect the response of the material to the pH variation. Disc specimens were produced following manufacturers` instructions. Specimens were stored in artificial saliva in pHs 3.5, 7.0 or 10.0 for 10 days and after that the fatigue parameters (n: SCG susceptibility coefficient and sigma(0): scaling parameter) were obtained by the dynamic fatigue test using the same pH of storage. Microstructural analysis of the materials was also performed. Results. For VM7, the values of n obtained in the different pHs were similar and varied from 29.9 to 31.2. The sigma(0) value obtained in pH 7.0 for VM7 was higher than that obtained in the other pHs, which were similar. For porcelain UD, n values obtained in pHs 7.0 and 10.0 were similar (40.8 and 39.6, respectively), and higher than that obtained in pH 3.5 (26.5). With respect to sigma(0), the value obtained for porcelain UD in pH 10.0 was lower than those obtained in pHs 3.5 and 7.0, which were similar. Significance. The effect of pH on the stress corrosion susceptibility (n) depended on the porcelain studied. While the n value of VM7 was not affected by the pH, UD presented lower n value in acid pH. For both porcelains, storage in acid or basic pH resulted in strength degradation. (C) 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Precursor scavenging of the resistive grain-boundary phase in 8 mol% yttria-stabilized zirconia: Effect of trace concentrations of SiO2

The influence that trace concentrations Of SiO2 have on improving grain-boundary conduction via precursor scavenging using additional heat treatment at 1200 degreesC for 40 h before sintering was investigated. At a SiO2-impurity level (SIL) less than or equal to 160 ppm by weight, the grain-boundary resistivity (p(gb)) decreased to 20% of its value, while no improvement in grain-boundary conduction was found at a SIL greater than or equal to 310 ppm. The correlation between the resistance per unit grain-boundary area, p(gb), and average grain size indicated that the inhomogeneous distribution of the siliceous phase in the sample with a SIL greater than or equal to 310 ppm. hampered the scavenging reaction.