High temperature plasticity of titanium carbide in the low area of homogeneity

This paper deals with the study of structural and mechanical properties of a ceramic material which is composed on the basis of nonstoichiometric titanium carbide in the low area of homogeneity. Material for the investigation was produced by means of self-propagating high temperature synthesis (SHS) with subsequent high effort compaction. It was established that under tensile and compression straining the material had unusually high plasticity at temperatures above the toughbrittle transition (TBT) temperature. The strain rate and stress pattern strongly affect the TBT temperature. Optical and scanning electron microscopy as well as x-ray structural and energy dispersion analysis helped to establish the regularities of titanium carbide structure evolution under various conditions of sample deformation. The paper presents data on plasticity and mechanical properties of the nonstoichiometric titanium carbide. At high temperatures and low strain rates, the nonstoichiometric titanium carbide displays features of superplastic flow. At relatively high strain rates, dynamic recrystallization occurs in the titanium carbide, which results in considerable refining of microstructure, which, under certain temperature rate conditions, also results in the transition to superplastic state.     

Microstructure Evolution and Pinning of Boundaries by Precipitates in a 9 pct Cr Heat Resistant Steel During Creep

Structural changes in a 9 pct Cr martensitic steel during a creep test at 923 K (720 °C) under the applied stress of 118 MPa were examined. The tempered martensite lath structure (TMLS) was characterized by M₂₃C₆-type carbide particles with an average size of about 110 nm and MX-type carbonitrides with a size of 40 nm. The M₂₃C₆ particles were located on the packet/block/lath boundaries, whereas the MX precipitates were distributed homogeneously throughout TMLS. TMLS in the grip portion of the crept specimen changed scarcely during the tests. In contrast, the structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. The formation of a well-defined subgrain structure in the gauge section was accompanied by the coarsening of M₂₃C₆ carbides and precipitations of Laves phase during creep. The most pronounced structural changes occurred just at the beginning of the tertiary creep regime, which was interpreted as a result of the change in the mechanism of grain boundary pinning by precipitates.     

Effect of Co on Creep Behavior of a P911?Steel

The microstructure and creep behavior of a 3 pct Co modified P911 steel and standard P911 steel were examined. It was shown that the nanoscale M₂₃C₆ carbides and MX carbonitrides in the 3 pct Co modified P911 steel are not susceptible to significant coarsening under creep conditions. Also, coarsening simulations of M₂₃C₆ particles were performed for both steels. The rates of lath and particle coarsening in the P911 + 3 pct Co steel are remarkably lower than those in the P911. Increased stability of a tempered martensite lath structure in the 3 pct Co modified P911 steel provides enhanced creep resistance at an exceptionally high temperature of 923 K (650 °C).     

Electrical output parameters of efficient cylindrical thermionic energy converters

Electrical output parameters are presented for four one-element cylindrical TEC operating with cesium plasmas and electrode gaps from 0.3 to 0.8 mm. The cylindrical emitters have been made by vapor-phase epitaxial deposition of tungsten on cylindrical molybdenum substrates having [111] axial orientation under the conditions where the deposit is automatically formed faced with (110) planes. The average vacuum work function is up to 5.3 eV. The collector is made of a molybdenum-ruthernium alloys. Tests have been made on how the vacuum work function of the emitter influences the output parameters. Calculations combined with experiment have been used to estimate the work functions and electron temperatures in the plasma. Translated from Atomnaya énergiya, Vol. 87, No. 2, pp. 129–133, August, 1999.     

Effect of subgrain structure on the creep strength of alloy 1207

The effect of severe plastic deformation (SPD) on the creep resistance of the Al-6%Cu-0.48Mn-0.52Mg-0.3Sc-0.1Zr alloy has been examined in a temperature range of 125–180°C. It has been shown that SPD performed by the method of equal-channel angular pressing at 300°C to a true strain of ～1 leads to the formation of a well-defined subgrain structure, which is retained upon solution treatment before quenching because of the presence in the alloy of ultradisperse Al₃(Sc, Zr) particles with coherent boundaries. It was established that the creep strength at 125–150°C of the as-cast alloy and of the deformed material is approximately the same. At 180°C, the creep rate of the deformed aluminum alloy is almost an order of magnitude lower than that of the as-cast alloy. The reasons for the influence of the subgrain structure on the creep strength of the Al-Cu-Sc-Zr alloy are discussed.     

Fluctuation model of the superplasticity reserve

The macroscopic effect of necking, which determines the reserve of superplasticity, has been studied based on the concept of a deformation mechanism on a mesoscopic level. It has been found that the characteristics of the ultimate plasticity can be described in terms of the fluctuationally formed (during deformation) inhomogeneity of the spatial distribution of bands of cooperated grain-boundary sliding and that there is no need in the introduction into the sample of a macroscopic inhomogeneity—a nucleus of a neck.     

Features of molecular beam epitaxy of the GaN (0001) and GaN (000$$ \bar 1 $$) layers with the use of different methods of activation of nitrogen

The results of comparative studies of the growth kinetics of the GaN layers of different polarity during ammonia molecular beam epitaxy and plasma-assisted molecular beam epitaxy (PA MBE) of nitrogen with the use of sapphire substrates and GaN(000\(\bar 1\))/c-Al₂O₃ templates grown by gas-phase epitaxy from metalorganic compounds are presented. The possibility is shown of obtaining the GaN layers with an atomically smooth surface during molecular beam epitaxy with plasma activation of nitrogen. For this purpose, it is suggested to carry out the growth in conditions enriched with metal near the mode of formation of the Ga drops at a temperature close to the decomposition temperature of GaN (TS ≈ 760°C). The conclusion is made that an increase in the growth temperature positively affects the structural, optical, and electrical properties of the GaN (000\(\bar 1\)) layers. A high quality of the GaN (0001) films grown by the PA MBE method at a low temperature of ～700°C on the GaN/c-Al₂O₃ templates is shown.