In this study, the formation mechanism of deformation bands (DBs) observed in AA1100 after plane forging (PF) and return-plane forging (R-PF) during multi-axis diagonal forging (MADF) was unraveled via microtexture analysis using a rate-sensitive model and crystal plasticity finite element method (CPFEM). Blurry bands with an orientation relationship of mainly LAGBs were developed in the deformed grains in the PF specimen. The heterogeneity of the orientation stability (Q) that occurred within the blurry bands and their surrounding regions contributed to the development of the DBs in the deformed grains in the R-PF specimen. Microstructure-based CPFEM successfully simulated the development of DBs within deformed grains during R-PF. CPFEM revealed that heterogeneity of the crystallographic orientation that occurred within the blurry bands and their surrounding regions inside the PF specimen was the main cause of the formation of DBs in the deformed grains during R-PF.

In this study, the deformation bands (DBs) in AA1100 after plane forging (PF) and return-plane forging (R-PF) during multi-axis diagonal forging (MADF) were observed using microtexture analysis. Typical texture components (plane-strain texture components) observed in face-centered cubic (FCC) materials deformed by plane-strain compression were developed in the PF specimen. Blurry bands with an orientation relationship of mainly LAGBs were developed in the deformed grains in the PF specimen. Slip trace analysis and Schmid factor (SF) calculations revealed the correlation between the blurry bands and the slip behavior. Regions of matrix and DBs inside the deformed grains of the R-PF specimen were distinguished mostly by their HAGBs. The heterogeneity of the Taylor factor (M) values that occurred within the blurry bands and their surrounding regions induced the development of the DBs in the R-PF specimen. The heterogeneities of the deformation behavior between the inside and the outside of the blurry bands that developed in the grains of the PF specimen during the R-PF were a direct reason for the generation of DBs in the deformed grains of the R-PF specimen. The transmission-EBSD (t-EBSD) technique revealed that the average misorientation angle inside the DBs was higher than that inside the matrix.

In this study, sound workpieces made of interstitial-free (IF) steel were manufactured by performing up to 4 cycles without the occurrence of forging defects via the use of a new multi-axis diagonal forging (MADF-ver2) process at room temperature. The microstructure that was developed in the deformed specimens was experimentally analyzed and compared. As the number of cycles increased, the average grain size gradually decreased, and both the average misorientation angle and the fraction of high-angle grain boundaries (HAGBs) continued to increase. As the number of cycles increased, the change in average geometrically necessary dislocation (GND) density was insignificant due to the occurrence of dynamic recovery. The effects of microstructural factors on the hardness, yield strength, and total elongation of the deformed specimens were investigated. The ultrafine-grained structure formed by MADF-ver2 continuously increased the hardness and yield strength with increases in the number of cycles. On the other hand, total elongation was not dependent on the number of cycles, and this independence was mainly the result of insignificant changes in the average GND density due to the dynamic recovery that occurred throughout the specimen.

In this study, we tried to improve the oxidation resistance of Nb-12Si (wt%) alloys at 1200 °C or higher through pack cementation coatings. Nb-12Si (wt%) alloys were prepared by arc-melting under Ar atmosphere. When the alloys were coated using pack powder mixtures composed of Si, Al2O3 and NaF, two silicide layers composed of NbSi2 and Nb5Si3 phases were successfully produced on the substrate. The Si-pack coatings were performed with various heat treatment temperatures and time conditions. The microstructures and thickness changes of the coating layers were analyzed to determine the growth behaviors of the coating layer. The growth constant of 8.4 10–9 cm2/sec was obtained with a diffusion growth mode. In addition, in order to examine the resistance of the Si-pack coated alloys, isothermal static oxidation tests were performed at 1200 °C and higher temperatures. As a result, the oxidation resistance of the alloys was determined by protecting the surface of the alloys with silicide oxide layers formed by the silicide coatings. The uncoated specimens exhibited an abnormal weight increase due to the formation of Nb oxide. The coated specimen showed excellent oxidation resistance at 1200 °C for up to 12 hrs, while the previous reports on the same alloy verified oxidation resistance only up to 1100 °C. It appears that the excellent oxidation resistance is closely related to the NbSi2 coating layer thickness. The oxidation behaviors of the coating layers after the oxidation tests were discussed in terms of microstructural and phase analyses.

This study evaluated the thermal degradation mode of Nb-silicide-based in situ composite (Nb-19Ti-16Si-4Hf-13Cr-2Al-4B at.%) when exposed to temperatures up to 1600 degrees C in Ar atmosphere. For evaluation, the Rockwell hardness test, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) equipped with an energy-dispersive spectroscopy (EDS) were used. The as-manufactured specimen was composed of Nb solid solution (Nbss), Laves phase, and two types of silicides ((Nb,Ti,Hf)(5)Si-3) that have hexagonal and tetragonal crystal structures, respectively. Exposed to ultrahigh temperatures, the resulting degradation and transformation of silicides led to the precipitation of face centered cubic (FCC)-Ti phases, which occurred predominantly after 40 h exposure. The decrease in volume fraction of silicide seems to be attributable to decomposition of the silicide, by which phase separation between the silicide and Ti phases occurred owing to the high vapor pressure of Si. As a result, the hardness (HRC) decreased from about 60 to 30. The results show that the evaporation of elemental Si needs to be considered for achieving good thermal oxidation resistance in Nb silicide composites. The results further suggest that one solution for improving the fracture toughness of Nb silicide composites is via control of the microstructure. (C) 2019 Elsevier B.V. All rights reserved.

The deformation behavior and microstructural evolution of ultrafine- (UFG) and fine-grained (FG) Cu during the dynamic tensile extrusion (DTE) process were investigated. The DTE tests were conducted with identical projectile velocities using an all-vacuum gas gun. The DTE ductility increases as the grain size increases, in contrast to the previous outcome with coarse-grained Cu exhibiting that the ductility increased with a decrease in the grain size. The fragments were softly recovered and were examined by a micro-Vickers hardness test and microstructural characterization assessments. The hardness profiles of the fragments exhibit a drastic decrease in the UFG-B remnant, while hardness variations are found in the middle fragments for FG-200. A strong dual <001> + <111> texture is developed during the DTE regardless of the UFG and FG sizes. The UFG-B fragments show that the <111> fibers are replaced by the <001> fibers as a result of meta-dynamic recrystallization (mDRX), while the <111> fibers in FG-200 saturate without any extensive reduction. Evidence of the mDRX was found in an analysis of the misorientation-angle distribution, grain morphology, and grain orientation spread. A numerical simulation reveals that the mDRX can occur due to adiabatic heating, with the faster kinetics in the UFG-B originated from the accumulation of higher levels of deformation energy during the DTE process. The present study also confirms that occurrence of mDRX in the UFG and FG Cu triggers a ductile failure and local necking with a decrease in the DTE ductility.

The purpose of this study is to investigate the densification behavior and the corresponding microstructural evolution of tantalum and tantalum-tungsten alloy powders for explosively formed liners. The inherent inhomogeneous microstructures of tantalum manufactured by an ingot metallurgy might degrade the capability of the warhead. Therefore, to overcome such drawbacks, powder metallurgy was incorporated into the near-net shape process in this study. Spark plasma-sintered tantalum and its alloys with finer particle sizes exhibited higher densities and lower grain sizes. However, they were contaminated from the graphite mold during sintering. Higher compaction pressures in die and isostatic compaction techniques also enhanced the sinterability of the tantalum powders; however, a full densification could not be achieved. On the other hand, the powders exhibited full densification after being subjected to hot isostatic pressing over two times. Consequently, it was found that the hot isostatic-pressed tantalum might exhibit a lower grain size and a higher density as compared to those obtained in previous studies.

The present study investigated the effects that different types of forging exert on the deformation heterogeneities developed in AA1100 during multi-axial diagonal forging. To measure the deformation heterogeneities of deformed workpieces, the values for hardness and Kernel average misorientation were measured at the center section following each forging process. Type-D forging that consists of diagonal forging and return-diagonal forging was relatively advantageous compared with Type-P forging that includes plane forging and return-plane forging for minimizing the non-uniformity of deformation developed in workpieces. The effective strain developed in a workpiece during the 2 types of forging was simulated using 3-D FEA. FEA revealed that the positions and degrees of occurrence for soft and hard-zones in workpieces vary greatly depending on the forging type. Type-D forging was relatively advantageous compared with Type-P forging for minimizing the non-uniformity of effective strain developed in workpieces.

Shaped charge (SC) ammunition is a weapon designed to penetrate armor. The functions of the liner, which is a core component of the SC, are penetration and explosion. Because of the increasing necessity for the liner with both penetrating and demolition functions, a double-layer liner was produced which could cause explosion after penetration (penetration–explosion effect, PE). A double-layer liner was designed to increase the explosive power of the SC and to enhance the SC’s performance by PE effect. Reactive Al and Al-Ni coatings were successfully deposited on the conical Cu liner via kinetic spray to fabricate the double-layer liner. A ballistic test was conducted to compare the explosive power between a single Cu liner and double-layer liners and to identify the PE effect. By measuring the penetration area of the target plate and the number of broken support bolts, the liner’s explosive power was compared and determined. The explosive power of double-layer liner was stronger than that of the single Cu liner. Alumina and an intermetallic compound of Al-Ni, both of which are evidence of exothermic reactions, were investigated using EDS and XRD techniques.

Refractory metal alloys, such as Mo–Si–B systems, can extend high-temperature capability more than commercial Ni-based superalloys, but Mo–Si–B alloys require surface coatings to improve their poor oxidation resistance. In this study, aluminide coating layers were created on Mo–3Si–1B (wt%) alloys by pack cementation with NH4Cl, Al and Al2O3 powder. The aluminide coating layers consisted of MoAl4, Mo3Al8, and precipitates. The growth kinetics of the coating layers were estimated by identifying diffusion behaviors. The aluminide coating layer growth constant (k0) was estimated as ~ 741.3 μm/h0.5, and the activation energy (Q) for the growth of the diffusion coating layer was evaluated as ~ 44.2 kJ/mol for the examined coating temperatures of 800, 900 and 1000 °C. The thicknesses of the coating layers calculated by an estimated kinetic equation were compared with the experimental results. The coating layer successfully protected the Mo–Si–B substrate during isothermal oxidation at 1400 °C under an air atmosphere. The growth kinetics of the coated layer and oxidation behaviors were discussed in terms of microstructural analyses.

The effect that forging routes (3 routes, A-C) exert on the development of deformation heterogeneity in AA1100 was theoretically performed using finite element analysis. Route A corresponded to a forging process that involved from 1 to 6 passes through multi-axial diagonal forging (MADF); Route B corresponded to a forging process that involved from 1 to 12 passes through the MADF; and, Route C corresponded to a conventional multi-axial forging process in which the operations from 1 to 6 passes of the MADF were simply repeated twice. From the aspect of strain uniformity developed on AA1100, Route B, which combined forging operations to the direction perpendicular to the face of the workpiece and forging operations to the diagonal direction of the face of the workpiece, was relatively advantageous compared with Routes A and C that only involved forging operations to the direction perpendicular to the face of the workpiece. Routes A and C showed a tendency toward regions where both the low and the high effective strains were restricted to specific regions of the workpiece. The low and high effective strains created by Route B, on the other hand, tended to be distributed over various regions of the workpiece.

In this study, we tried to improve the oxidation resistance of Mo–3Si–1B (wt%) alloy at intermediate (800 °C) and high temperature (1300 °C) after applying Si/Al pack cementation coatings. In addition, we examined structural changes and phase change in the intermediate and high temperature oxidizing atmospheres to observe oxidation behaviors. The diffusion pack coatings were carried out with an NH4F activator and both Si and Al powder mixtures. Mo–Si–B alloys were subjected to diffusion coatings of Si/Al powders at various temperatures in various time conditions at a temperature of 1100 °C. When the pack coatings were carried out for 48 h, Mo(Si, Al)2 and Mo3Al8 layers together with a T2 (Mo5SiB2) layer were successfully synthesized on the substrate. After oxidation tests, an Al2O3 protective oxide layer was formed by the diffusion of Al in an ambient atmosphere. The Si/Al pack cementation coatings provided excellent oxidation resistance through the formation of an Al2O3 oxide layer. The enhanced oxidation behaviors are discussed in terms of microstructural evolution at high temperatures.

Scandate dispenser cathode with hundreds of ampere per square centimeter has been developed using metal injection molding (MIM) technique for terahertz vacuum electron devices. We have investigated scandate thermionic cathode surface by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) before surface treatment. And then, scandate cathode was fabricated with 1 mm diameter. The performance was verified by beam transmission test after assembling electron gun using microfabricated scandate cathode. The beam transmission rate was 99.8% for 0.5 mm radius drift tube with 0.8 T magnetic field condition.

A method was developed to fabricate spherical Mo5Si3 powder by milling and spheroidizing using inductively coupled thermal plasma. A Mo5Si3 alloy ingot was fabricated by vacuum arc melting, after which it was easily pulverized into powder by milling due to its brittle nature. The milled powders had an irregular shape, but after being spheroidized by the thermal plasma treatment, they had a spherical shape. Sphericity was increased with increasing plasma power. After plasma treatment, the percentage of the Mo3Si phase had increased due to Si evaporation. The possibility of Si evaporation was thermodynamically analyzed based on the vapor pressure of Mo and Si in the Mo5Si3 liquid mixture. By this process, spherical Mo silicide powders with high purity could be fabricated successfully.

In this research, we investigated the effects of reduction atmospheres on the creation of the Mo-Si-B intermetallic compounds (IMC) during the heat treatments. For outstanding anti-oxidation and elevated mechanical strength at the ultrahigh temperature, we fabricated the uniformly dispersed IMC powders such as Mo5SiB2 (T2) and Mo3Si (A15) phases using the two steps of chemical reactions. Especially, in the second procedure, we studied the influence of the atmospheres (e.g. vacuum, argon, and hydrogen) on the synthesis of IMCs during the reduction. Furthermore, the newly produced IMCs were observed by SEM, XRD, and EDS to identify the phase of the compounds. We also calculated an amount of IMCs in the reduced powders depending on the atmosphere using the Reitveld refinement method. Consequently, it is found that hydrogen atmosphere was suitable for fabrication of IMC without other IMC phases.

Shaped charge is a weapon that is devised to penetrate armor. The explosion performance of it was relatively low. The liner, which is the core component of shaped charge, is generally formed as Cu material because of good penetration ability. The double-layer liner was designed to increase the explosive power of the shaped charge and to enhance the performance of the weapon through the explosion after penetration. Reactive Al and Al-Ni coating layers were successfully deposited on the cone-shaped Cu liner via kinetic spray to fabricate a double-layer liner. A ballistic test was conducted to compare the explosive power between single Cu liner and double layer liners. Alumina and intermetallic compound of Al-Ni, which are evidence of the exothermic reaction, were investigated by EDAX, XRD techniques. Al2O3 and AlNi were detected on 4th target plate, which means that an explosion occurred after penetration. The explosive power of the liners was compared by measuring the penetration area of target plate and the number of broken support bolts. The explosive power of double layer liner was larger than that of the single Cu liner.

Coating properties and oxidation behaviors of Si pack cementation-coated TZM (Mo-0.5Ti-0.1Zr-0.02C) alloys were investigated in order to understand the stability of the coating layer at high temperatures up to 1350 °C in an ambient atmosphere. After the pack cementation coatings, MoSi2 and Mo5Si3 layers were formed. When MoSi2-coated TZM alloys were oxidized in air at high temperatures, the Si in the outer MoSi2 layer diffused and formed SiO2. Also, due to the diffusion of Si, the MoSi2 layer was transformed into a columnar shaped Mo5Si3 phase. During isothermal oxidation, the Mo5Si3 phase was formed both within the coated MoSi2 layer and between the MoSi2 and the substrate. The coating properties and the oxidation behavior of the Si pack-coated TZM alloys were discussed along with the identification of growth kinetics.

The fabrication of the intermetallic phase T2-Mo3Si with continuous matrix of α-Mo was attempted with the combination process of high energy ball milling, pulverization of arc-melted ingot, addition of Mo by hydrogen reduction of MoO3 and spark plasma sintering processes. High energy ball milling or arc melting of Mo-16.7Si-16.7B (at %) powders were performed to obtain to intermetallic phase T2 and Mo3Si. The Mo phase of 57vol% distributed intermetallic compound powders were prepared by hydrogen reduction of MoO3 and further mixing of elemental Mo powders. X-ray diffractometry analysis revealed that the intermetallic phase T2-Mo3Si can be produced by the pulverization process of arc-melted ingot. Hydrogen reduction of 1vol% MoO3 mixed intermetallic powder followed by further addition of Mo powders was a more adequate method enabling the homogeneous distribution of the Mo phase than that of added MoO3 powders with total amount. The powder mixture was successfully consolidated by spark plasma sintering yielding a sound microstructure comprising the intermetallic phase T2-Mo3Si uniformly distributed in a continuous matrix of α-Mo.

It is known that the fine grain size of a copper shaped charge liner improves penetration performance. To obtain fine microstructure in copper, recrystallization after plastic deformation is usually used. In this study, ultrafine grain of copper was obtained by a severe plastic deformation technique called equal channel angular pressing (ECAP). Conical liners were fabricated from ECAP processed billets. Mechanical properties and penetration performances of ultrafine-grained copper shaped charge liners have been compared with those of conventionally forged and recrystallized ones.