Strengthening in MULTIPHASE(MP35N) alloy: Part II. elevated temperature tensile and creep deformation

The strengthening mechanisms in the cobalt alloy MP35N have been investigated by tensile and creep deformation at elevated temperatures and by transmission electron microscopy (TEM) of the deformed alloy. A high ultimate tensile strength (UTS) of 800 to 900 MPa was maintained at all test temperatures from 300 to 873 K due to the maintenance of high strain hardening. On straining, there was the usual initial fall of the strain-hardening rate with stress, but above a critical stress of about 500 MPa, the strain-hardening rate stopped falling and was held almost constant at about 2000 MPa. At 973 and 1073 K, this high strain-hardening rate suddenly ceased during the test, while at 1123 K, negligible strain hardening was seen. At temperatures between 673 and 1073 K, tensile load drops were seen whose magnitude increased with strain, and thus stress, at a fixed temperature. The load drops also increased with increased temperature. How-ever, in conditions when the strain-hardening rate fell to a low value, the load drops ceased. In a tensile test in which small increments of strain were applied as the temperature was increased in 10 K intervals, a steadily rising flow stress with temperature was seen up to a critical temperature of 1073 K, beyond which the flow stress fell and the load drops also ceased. In conditions where the high rate of strain hardening was found, fine platelike structures were seen by transmission electron microscopy (TEM) to form on {111} planes in the face-centered cubic (fcc) matrix. Diffraction evidence showed that these were faulted hexagonal close-packed (hep) plates. Creep tests carried out above and below the critical temperature of 1073 K showed very different behavior. At 1098 K, the sample showed conventional creep behavior, while at a temperature of 973 K, the material showed sigmoidal creep strain rate. At low strains, up to a strain of about 0.02, there was an initial steady-state creep rate. The creep strain rate then increased and fell back to a second steady-state creep rate. During the accelerated creep stage, hexagonal plates were again seen to form. Strain-induced martensite forming at temperatures up to and including 1073 K, but not at higher temperatures, appears to account satisfactorily for all of the behavior seen in this study. It is proposed that the hexagonal plates form martensitically at high speed, but as proposed in Part I,^[1] solute partitioning occurs between the closely spaced fcc and hcp phases. At 1025 and 1073 K, the end of both the load drops and the high strain hardening during a tensile test may be explained by the stabilization of the remaining fcc matrix by loss of hexagonal phase stabilizing solute. The critical temperature of approximately 1073 K seen in this study is close to the critical softening temperature of 1083 K, above which recrystallization of cold-worked MP35N occurs readily (Part I^[1])- The critical temperature is proposed to be close to the transus temperature, above which single-phase fcc is the stable structure of MP35N.     

Modification of tempeh with the addition of bakla (Vicia faba Linn)

Bakla ( Vicia faba Linn), an indigenous pulse, was subjected to fermentation by the strains of Rhizopus oligosporus either alone or blended with soybean ( Glycine max ). Mycelial growth as viewed on the surface of the fermented mass was best obtained when strain NRRL 3271 was used. Increase in moisture content and pH during fermentation was highest in the case of strain NRRL 2710 irrespective of composition of bakla-soybean mixture. The tempeh, the fermented product, in each case had a mushroom-like odour, which was independent of the strain used. Bakla and bakla-soybean (up to 1:1 ratio) tempeh was free of beany flavour but this flavour increased as the soybean content of the blend was further raised and was perceptibly high when soybean content reached 75%. Bakla tempeh was more crisp than soybean tempeh. The crispness decreased with increasing soybean content.     

A Huge Effect of Weak dc Electrical Fields on Grain Growth in Zirconia

We show that the application of a modest dc electrical field, about 4 V/cm, can significantly reduce grain growth in yttria-stabilized polycrystalline zirconia. These measurements were made by annealing samples, for 10 h at 1300°C, with and without an electrical field. The finding adds a new dimension to the role of applied electrical fields in sintering and superplasticity, phenomena that are critical to the net-shape processing of ceramics. Grain-growth retardation will considerably enhance the rates of sintering and superplasticity, leading to significant energy efficiencies in the processing of ceramics.     

Semiconductive Behavior of Polymer‐Derived SiCN Ceramics for Hydrogen Sensing

A low‐cost sensing mechanism of hydrogen gas is developed using polymer‐derived ceramic, a liquid organic precursor, polysilazane with the addition of 5 wt% of photoinitiator, 2,2 Dimethoxy‐2‐phenyl acephenone. UV photopolymerization is utilized to partially cross‐link the H‐shaped free standing specimen, and then pyrolyzed at 1400°C in hot isostatic press under nitrogen gas to convert the partially cross‐linked polymer into conducting and amorphous ceramic, silicon carbonitride. This work presents the preparation of free standing silicon carbonitride specimens as the sensor body for sensing hydrogen gas, depending on the semiconductive behavior of polymer‐derived ceramics in high‐temperature environments. The band gap of silicon carbonitride would be varied from adsorbing hydrogen molecules on the surface of the H‐shaped free standing specimen with two different thicknesses. An amenable specimen‐geometry for the four‐point test of measuring resistance is developed in a furnace filled with pure hydrogen and vacuumed environments.     

A Model for the Nanodomains in Polymer-Derived SiCO

The polymer-based synthesis of ceramics such as SiCO (and SiCN) leads to the incorporation of significant amounts of carbon into their molecular structure. A key feature of the nanostructure of these polymer-derived ceramics is the revelation of persistent, 1–5 nm size domains by small-angle X-ray scattering. Here we present a model for these nanodomains, which is consistent with the nuclear magnetic resonance (NMR) data and with the phenomenological properties of SiCO (high resistance to creep and viscoelastic behavior). The model consists of clusters of silica tetrahedra encased within an interdomain wall constituted from mixed bonds of SiCO, and from a network of sp² carbon. The model predicts the domain size as a function of the carbon content. These predictions are in reasonable agreement with the measurements of the nanodomains in SiCO synthesized with varying carbon contents (the domain size decreases with higher carbon). Simple maps are developed for easy reading of the domain size and the width of the interdomain boundary in the composition diagrams.     

Flaw Generation During Constrained Sintering of Metal-Ceramic and Metal–Glass Multilayer Films

Sintering of symmetrical multilayer films has been studied theoretically and experimentally. Experiments were conducted on ceramic/metal/ceramic and glass/metal/glass films. The sintering rate of free films was compared to the sintering rate of multilayers. The origin of processing flaws was examined. The following results were obtained: (1) Differential sintering rates of the components in the multilayer give rise to in-plane tensile and compressive stresses. The film that is stressed in tension in the early stage of sintering is most susceptible to fracture. Experiments with ceramic/metal/ceramic multilayer are in agreement with this prediction. (2) A theoretical prediction that the glass/metal/glass multilayer will not develop defects because of a high value of the shear relaxation factor in glass is confirmed by experiments. (3) The likelihood of developing a tensile stress in the multilayer depends only on geometry, the green density, and the ratio of the intrinsic sintering pressures. (4) The in-plane shrinkage of the multilayer depends on the difference in the free-sintering rates and the shear relaxation factors, and is reasonably well predicted by the analysis. (5) We have evidence that the metal layer deforms plastically when it is placed in tension by differential sintering.     

Flash sintering of a three‐phase alumina,spinel, and yttria‐stabilized zirconia composite

Three‐phase ceramic composites constituted from equal volume fractions of α‐Al₂O₃, MgAl₂O₄ spinel, and cubic 8 mol% Y₂O₃‐stabilized ZrO₂ (8YSZ) were flash‐sintered under the influence of DC electric fields. The temperature for the onset of rapid densification (flash sintering) was measured using a constant heating rate at fields of 50‐500 V/cm. The experiments were carried out by heating the furnace at a constant rate. Flash sintering occurred at a furnace temperature of 1350°C at a field of 100 V/cm, which dropped to 1150°C at a field of 500 V/cm. The sintered densities ranged from 90% to 96%. Higher electric fields inhibited grain growth due to the lowering of the flash temperature and an accelerated sintering rate. During flash sintering, alumina reacted with the spinel phase to form a high‐alumina spinel solid solution, identified by electron dispersive spectroscopy and from a decrease in the spinel lattice parameter as measured by X‐ray diffraction. It is proposed that the solid solution reaction was promoted by a combination of electrical field and Joule heating.     

Double dispersed bioconvective Casson nanofluid fluid flow over a nonlinear convective stretching sheet in suspension of gyrotactic microorganism

Microorganisms play a vital role in understanding the ecological system. The motions of micororganisms are self‐propelled while the impact of thermophoresis and Brownian motion property of nanoparticle shows more challenges in biotechnological and medical applications. The present problem is based on the understanding of double‐dispensed bioconvection for a Casson nanofluid flow over a stretching sheet. Suction phenomenon is introduced at the surface of the stretching sheet along with the convective boundary condition. The convection and movement of the microorganisms are assisted by an applied magnetic field, nonlinear thermal radiation, and first‐order chemical reaction. The governing equations are highly coupled and thus we used the spectral quasilinearization method to solve the governing equations. The study of the residual errors on the systemic parameters had given a confidence with the present results. The final outcomes are displayed through graphs and tables. The thermal dispersion coefficient shows a positive response in the temperature while a similar response is observed for the concentration with solutal dispersion coefficient. The response is reversible for the heat transfer rate at the surface with thermal dispersion coefficient. The density of the motile microorganism at the surface decreases with increase in the Casson number, thermal dispersion coefficient, and solute dispersion coefficient, while an opposite phenomenon was observed with increase in the density ratio of the motile microorganism.