Materials Science and Engineering最新文献

Fe25Cr4Al and Fe25Cr2Al (wt.%) alloys were cyclically deformed in air at various total strain amplitudes. Four different structures were observed, in order of decreasing strain: ordinary dislocation cell structures in both alloys at about 1% total strain and above; and “maze” or “labyrinth” structures at intermediate strains, again for both alloys; however, at the lowest strain rates, a typical loop patch structure was found in the 2% aluminium alloy but a precursor to the maze (labyrinth) structure was found in the 4% alloy. That precursor structure seems to be the same as that observed by Mori et al. It would thus appear that the difference between the two alloys lies in the bypassing of the loop-patch structure in the 4% alloy, with, instead, direct construction of dipolar or multipolar walls. Thus the softening observed in the 2% alloy is due to the formation of the loop patches.

An attempt has been made to study the kinetics of subgrain growth during static recovery in oxygen-free high conductivity copper at two different annealing temperatures. The changes in subgrain sizes were followed in a cold-drawn copper wire as a function of annealing time at 175 and 200°C. A linear relationship between the square D² of the subgrain diameter and the annealing time t has been established at these temperatures. The slopes K of the lines have been found to be 5.4 × 10⁻¹⁸M²s⁻¹ and 7.2 × 10⁻¹⁸m²s⁻¹ at 175°C and 200°C respectively. A large difference between the experimentally observed and theoretically calculated K values has been found.

The surface morphology of 0–40 MeV alpha-irradiated molybdenum to total doses of about 5.45 × 10¹⁷αcm⁻² and 1.14 × 10¹⁸αcm⁻² have been studied by scanning electron microscopy. Pinhole pits and surface flakes are observed and the formation mechanism for both seems to be the same. The structures show strong dependence on crystal orientation. The flakes are faceted and some of them show fivefold symmetry. The irregular shape of pinhole pits are triangular at higher doses with increase in size and decrease in concentration. At higher doses the surface erosion is high and most of the flakes are masked by sponge-like exfoliations. The size and depth of the pits are inversely proportional and indicate that the formation mechanism for helium bubble blisters and pits is not the same. At high ion energies, the compressive stress created by the lattice defects seems to play an important role in surface erosion.

Rectangular bars of commercially pure titanium were cold rolled to various strains in the range 20%–80%. Optical and electron microscopy investigations reveal that at low strains titanium deforms mainly by twinning, whereas at strains above 40% it deforms solely by slip. Texture measurements indicate that the development of texture in titanium is rapid. At strains around 20% a split rolling direction texture forms. Further deformation from 20% to 40% leads to a texture transition from split rolling direction to split transverse direction. Short-term annealing of samples rolled to a strain of 40% causes texture transition from split transverse direction to split rolling direction. It also brings about recrystallization within the primary twins and an overall reduction in the density of dislocations. Twinning is believed to be responsible for the rapid development of texture and the transitions in texture. The formation of the stable end texture is thought to be due to slip.

Ambient dielectric effects have been studied in rutile-polystyrene compacts as functions of frequency and composition to explore the possibility of their use as electronic materials and characterize them on the basis of existing theories. The systems reveal marked departures from the law of physical mixtures. The dielectric constant ϵ′ and loss ϵ″ increase with increasing titania content at each frequency; both parameters decrease with increasing frequency. The extent of interfacial polarization is substantially augmented as the frequency is reduced. Heterodispersity evidenced from compact densities is a maximum at around 60 wt. % titania where the dissimilar particles fit most loosely into one another forming fairly porous matrices. ϵ′ and (ϵ′)⁻¹ are both curvilinear in the volume fraction, but ϵ′ shows a break at 50 wt. % in the gravimetric-blend plot, reflecting a change in the pattern and degree of interactions. The system conforms to the Clausius-Mossotti equation. The situation is intermediate between the separately possible Böttcher-Bruggeman and Maxwell-Wagner models. This is ascribed to the decrease of dipole moment and ϵ′ with increasing polystyrene owing to the imposition of non-polar environments and consequent lessening of charge density and long- and short-range forces. The variation in sample preparation technique greatly modifies the dielectric properties of these compacts. The composites with a wide range of dielectric properties can be prepared by changing the composition as well as the method of preparation.

The off-state direct current I–V characteristics for glassy alloys As₄₀Se₃₀Te₃₀ and As₂₀Se₅₀Te₃₀ at different temperatures were studied. For this study of electrical conduction properties, two kinds of electrode configurations were used: a double-point contact on one surface and a sandwich device. It was found that current flow can be space-charge limited, producing non-ohmic behaviour, which in turn reflects on the electrical switching effect that the materials exhibit. The influence of the selenium content on the electrical properties was also studied. The experimental results obtained were compared with those in the literature, and the differences justified by the method of material preparation. Lastly, the dependence of ohmic resistance on temperature was analyzed, and the characteristic behaviour of intrinsic semiconduction observed.

Experiments have been carried out to study the effect of hydrogen on the austenite to martensite transformation in a 1% C low alloy steel. Hydrogen was introduced into steel just prior to transformation to martensite by soaking the steel in a 97% pure argon and 3% pure hydrogen mixture and continuously sparking it to liberate atomic hydrogen. This method of introducing hydrogen minimized the irreversible damage to the microstructure often observed after cathodic charging. The results show that hydrogen has no significant effect on the amount of retained austenite for the steel studied.

The superplastic behavior of an Al-4wt.% Ti alloy processed using rapid solidification powder metallurgy was investigated. The peak value of the strain rate sensitivity index increased with temperature and grain size. The drop in flow stress with increasing grain size and anomalously high activation energy for flow at low strain rates are explained in terms of a grain size and temperature dependent threshold stress for superplastic flow. The origin of the threshold process is interpreted to be the inhibition of grain boundary migration by the particles during superplastic flow. An increase in the activation energy for flow was observed at temperatures close to the melting point of aluminum.

Tempered martensite embrittlement (TME) in AISI 4340 steel was studied for how variations in the test temperature and grain size affect the plastic flow.

The grain size was changed by varying the austenitizing temperature in the range of 870–1200°C. For the evaluation of TME with test temperature, Charpy impact testing was performed in the range of −196-23°C.

TME occurs because of an effective activation of intergranular brittle fracture in the 300°C tempered condition where grain boundary carbides are present, the ductile-brittle transition temperature (DBTT) increases with increasing grain size and the transition to brittle fracture is attributed to the occurrence of intergranular brittle fracture. This effect of grain size on the fracture behaviour indicates that the intergranular brittle fracture is controlled by the stress concentration susceptibility, i.e. the extent of dislocation pile-up at the grain boundaries, which increases with increasing grain size.

In the 300°C tempered condition (in the presence of grain boundary carbides), the DBTT is higher by 70–150°C, compared with the 200°C tempered condition (nearly devoid of grain boundary carbides) where the transition to brittle fracture results from transgranular brittle fracture. A critical test temperature below which intergranular TME can occur is reduced with decreasing grain size.

Therefore, intergranular TME can be produced by the occurrence of intergranular brittle fracture in the presence of grain boundary carbides, which can be more effectively activated as the stress concentration susceptibility increases with increasing grain size or with decreasing test temperature.

Narrow-size-distribution, non-agglomerated TiO₂ powder was precipitated on a large scale within a continuous-flow reactor from high concentrations of titanium tetraethoxide (Ti(OEt)₄) and water in ethanol. A plug-flow reactor with static mixers enabled continuous TiO₂ production of 60 g h⁻¹ over a period of 8 h from 0.25 M alkoxide and 1.0 M water. The organic base triethylamine was added as a stabilizer to the alkoxide stream prior to synthesis. The resultant powder had an average particle diameter of 0.37 μm with about 85% singlets and a standard deviation in particle size of 16%.