Development of high performance Mg–Al2O3 composites containing Al2O3 in submicron length scale using microwave assisted rapid sintering

Abstract

In the present study, magnesium composites reinforced with different volume fraction of submicron size Al₂O₃ particulates were synthesised using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. The sintered materials were subsequently hot extruded for characterisation in terms of microstructural, physical and mechanical properties. Microstructural characterisation results revealed a reasonably uniform distribution of Al₂O₃ particulates, minimal porosity and good matrix reinforcement interfacial integrity. The average coefficient of thermal expansion (CTE) value for Mg–Al₂O₃ composites was found to decrease with increasing amount of submicron Al₂O₃ particulates. Mechanical characterisation of the composites revealed an increase in hardness, elastic modulus, 0·2% YS and ultimate tensile strength (UTS) with the increase in amount of alumina particulates. Ductility exhibited the reverse trend. An attempt is made in the present study to correlate the effect of the presence of submicron alumina and its increasing amount with the microstructural, physical and mechanical properties of magnesium.     

Thermal expansion behaviour of plasma sprayed Al–SiCp composites

Abstract

Al with 55 and 75 vol.-%SiC powders were free mechanically mixed or ball milled as feedstock. The powder feedstock was deposited onto a graphite substrate to form near net shape of Al/SiC composites by air plasma spraying. The pores and the gaps at the Al/SiC interface as well as at the boundary of Al grains exist extensively in the as sprayed composites. Coefficient of thermal expansion (CTE) of the sprayed composites was measured in the temperature range of 25–300°C. The composites plasma sprayed with Al–75SiC powder feedstock can reach a low CTE value of 8 × 10^?6 °C^?1. The effect of pore on the CTE of the composites has been discussed. The gap at Al/SiC interface has an influence on thermal expansion behaviour only at lower test temperatures. Reduction and elimination of the gap with temperature can offset the thermal expansion of the as sprayed composites, resulting in lower CTE at the beginning of the CTE test. Roughly quantitative consideration of the effect of the interfacial gaps between Al and SiC on CET was given. Linear rule of mixture (ROM), Turner and Kerner's models were used to estimate the CTE of the sprayed composites. It was found that ROM and Kerner's model give closer CTE prediction for the present composites.     

The microstructures of solvent-welded joints of irradiated poly(methyl methacrylate)

The microstructures of solvent-welded joints of irradiated poly(methyl methacrylate) (PMMA) have been investigated. The morphologies of tensile, tear, and cleavage surfaces were examined and the effects of the solubility parameter, welding temperature, and γ-ray dosage on the mechanical strength were studied. Three solvents - methanol, ethanol, and N,N-dimethyl formamide (DMF) - were used with welding temperatures at 40, 50, and 60°C. The γ-ray dosages were 100, 200, and 300 kGy. The features of the tensile fracture surfaces exhibit either long and narrow hackles on the interface of PMMA with low dosage, or lamellae in the adherent near the interface with high dosage for methanol and ethanol. For DMF, the morphologies of the tensile fracture surfaces at low and high dosages consist of many elliptical paraboloids and feathers, respectively. The morphology of the tear fracture surface of PMMA shows equiaxed hackles for methanol and ethanol and polygons for DMF. The cleavage surface appears to be chevron curves. The cleavage surface shows discontinuity of the chevron curves at the welded interface for non-irradiated specimens and continuity on irradiated specimens. The slope of the chevron curve at the interface is continuous for ethanol and DMF but is discontinuous for methanol. A solvent-affected zone was observed and its size had no correlation with the mechanical strength of the joined PMMA.     

Influence of nano–micrometre powder blends on microstructure and mechanical properties of silicon nitride

Abstract

A well known route to making tough silicon nitride compositions is to control the grain size and aspect ratio distributions. This is usually done by choosing the appropriate powder characteristics, sintering conditions, as well as sintering additives. The effect of hot pressing a blend of nano and micrometre scale silicon nitride powder is explored here. Microstructures and mechanical properties are determined for these hot pressed ceramics and are compared with a reference silicon nitride. Hardness and fracture toughness are determined at room temperature using hardness indents produced by a macro Vickers hardness indenter. Grain size and aspect ratio distributions and their impact on mechanical properties are presented. Blending of nano and micrometre scale powder is shown to result in a refined microstructure with an increase in the area/volume fraction of finer grains. Rising R curves are established for these ceramics demonstrating toughening behaviour. Crack bridging and crack path deviation are identified as possible toughening mechanisms.     

Mechanisms of inclusion formation in low alloy steels deoxidised with titanium

Abstract

In the present investigation, the conditions for inclusion formation in two Ti deoxidised steels and one Al–Ca deoxidised steel have been examined by means of optical and electron microscopy, in combination with a thermo dynamic analysis of the phase relations involved. It is concluded that the Ti containing inclusions form as a result of a series of reactions occurring in the ladle, during solidification and in the solid state. The important solid state reaction products are MnS, TiN, and MnOTiO₂ . The presence of Mn rich compounds at the surface of the inclusions is consistent with the observation of a Mn depleted zone in the surrounding steel matrix. In contrast, the primary inclusions in the Al–Ca deoxidised steel are complex oxysulphides, which are thermodynamically more stable and can therefore form in the liquid state.     

Ultrahigh strength hot rolled microalloyed steel: microstructure and properties

Abstract

A low carbon steel alloyed with Ni, Mn, Mo, Cu and microalloyed with Nb and Ti was prepared. Continuous cooling transformation behaviour of the steel was evaluated. Formation of polygonal or Widmanstätten ferrite is suppressed at high temperature and the 'C' curve is shifted to an extreme right. At lower temperatures a flat top 'C' curve with a mixed structure of bainite and martensite was obtained and the transformation temperatures do not vary much with a wide range of cooling rates. The steel was thermomechanically processed at different finishing temperatures and ultrahigh strength values were obtained as a result of austenite grain refinement, highly dislocated fine lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride at all finish rolling temperatures. The stable and large TiN/TiCN particles formed during casting have impaired the impact toughness values at ambient and at ?40°C temperatures.     

Computational simulation of microstructure evolution during solidification of ductile cast iron

Abstract

This paper presents a thermomicrostructural model for the simulation of the solidification process of an eutectic ductile cast iron. The thermal balance is written at a macroscopic level and takes into account both the structural component being cast and its mould. Models of nucleation and growth represent the evolution of the microstructure, and the microsegregation of silicon is also considered. The resulting formulation is solved using a finite element discretisation of the macrodomain, in which the evolution of the microstructure is taken into account at the Gauss integration points. The numerical results are presented in terms of cooling curves and are compared with available experimental values. Furthermore, the sensitivity of the response with respect to changes in the cooling rate and nucleation parameters are investigated. The agreement between experimental and computational values is acceptable in both quantitative and qualitative aspects. Ways to improve the computational model are suggested.     

Changes in toughness at low oxygen concentrations in steel weld metals

Abstract

Oxides in steel weld metals can initiate fracture or can improve toughness by influencing the development of beneficial microstructures. In this work, the authors conducted experiments in which the oxygen concentration was varied from 20 to 560 ppmw (parts per million by weight) in weld metals with tensile strength in the range 580–780 MPa. It is demonstrated that low and medium strength weld metals benefit from oxides up to a concentration of ～200 ppmw as consistent with previous research, because acicular ferrite is stimulated in the microstructure. By contrast, oxides are detrimental to the toughness of high strength weld deposits at low oxygen concentrations under 140 ppmw, because the microstructure remains a predominantly martensite and the oxides simply serve to nucleate fracture. In high strength weld metal, therefore, good toughness is achieved even at low oxygen concentration of 20 ppmw O.     

Tailored waste based glasses as secondary raw materials for porcelain stoneware

Abstract

The high temperature processing of porcelain stoneware products is attractive for the possibility to incorporate several inorganic wastes. However, even if recycling is an environmental benefit, it can be disadvantageous for the overall properties of the ceramic product, if wastes are not selected in terms of their chemical and physical characteristics. In the present work several kind of industrial and urban wastes, such as mining residues, lime, glass from dismantled cathode ray tubes and soda lime glass were used, after their vitrification. These newly formed glasses were used as partial substitution for natural feldspar sands (the glass content being from 5 to 7 wt-% of the total mix). Owing to the specific ('tailored') chemical formulation of the newly formed glasses, the modified products exhibited higher crystallisation, with a more homogeneous microstructure, leading to significant improvements in reliability and indentation fracture toughness.     

Microstructure and properties of AlTiNiMnBx high entropy alloys

Abstract

AlTiNiMnB _x (x=0˙1, 0˙2, 0˙4 and 0˙5) high entropy alloys are prepared by an arc furnace. The results show that the alloys have a single bcc crystal structure. Boron additions lead to distortion of the crystal lattice and alloy strengthening. The highest hardness (779 HV) is achieved with 0˙5 at.-%B.