High temperature thermal stability of pure copper and copper–carbon nanotube composites consolidated by High Pressure Torsion

The thermal stability of ultrafine-grained (UFG) microstructures in pure copper samples and copper–carbon nanotube (CNT) composites processed by High Pressure Torsion (HPT) was compared. The UFG microstructure in the sample consolidated from pure Cu powder exhibited better stability than that developed in a casted Cu specimen. The addition of CNTs to the Cu powder further increased the stability of the UFG microstructure in the consolidated Cu matrix by hindering recrystallization, however it also yielded a growing porosity and cracking during annealing. It was shown that the former effect was stronger than the latter one, therefore the addition of CNTs to Cu has an overall benefit to the hardness in the temperature range between 300 and 1000 K. A good agreement between the released heat measured during annealing and the calculated stored energy was found for all samples.     

Effect of consolidating temperature on strengthening mechanism in Fe–Cu alloy from rapidly solidified powder

Abstract

The strengthening mechanism of Fe-Cu alloy manufactured from rapidly solidified powder was investigated. Powders of Fe-Cu with copper content ranging from 0.5 to 5 wt-% were prepared by high pressure water atomisation and consolidated by groove rolling at 973, 1073 or 1273 K. Analysis by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) were carried out to evaluate the resulting structures. The microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Yield stress under tensile loading and hardness after aging were measured. The copper states in consolidated specimens were determined based on the results, and the states were correlated to the mechanical properties of the specimens. At each of the consolidating temperatures, the yield stress increased with an increase in copper content. However, the strengthening mechanism differed according to the temperature. Specimens consolidated at 973 and 1073 K were strengthened by microstructure refinement,whereas precipitation hardening was the main strengthening mechanism in specimens consolidated at 1273 K.     

Residual stress in Fe–Cu alloys at 0 and 600 K

The paper presents three-dimensional molecular dynamic simulations of damage and stress calculations on the atomistic level in bcc iron containing a coherent copper nano-precipitate with orientation {1 0 0}. The influence of temperature on residual stress and on damage in the Fe–Cu system is studied. At a temperature of 600 K, the level of residual stresses in the Fe–Cu system is substantially smaller than at 0 K. It influences the mechanical response of the loaded Fe–Cu crystal. As to the damage, the lowest (critical) loading is a combination of tension and compression, where the onset of phase transition to 9R structure in the Cu precipitate and incoherence at the Fe–Cu interfaces have been detected at 600 K and external load of 4 GPa.     

Growth of Fe3O4 whiskers from solid solution nanoparticles of Fe–Cu and Fe–Ag systems produced by DC plasma jet method

Fe–Cu and Fe–Ag binary systems are virtually immiscible for a whole range of composition in equilibrium. In the present study, the nanoparticles of Fe–Cu and Fe–Ag systems were produced by direct current plasma jet method. These produced nanoparticles had mean particle sizes of about 70 nm, and were a mixture of bcc and fcc phases. It was revealed by analytical high-resolution TEM observations that the nanoparticles of Fe–Cu and Fe–Ag systems were supersaturated solid solution. It has been found that numerous whiskers with a particle on their tip grow from these nanoparticles by heating above the temperature of 860 K under an Ar–O₂ atmosphere. The whiskers grow as the result of the phase separation in these solid solutions. The whiskers are composed of a Fe₃O₄ rod and a Cu₂O particle on the tip.     

Formation of metal–TiN/TiC nanocomposite powders by mechanical alloying and their consolidation

Fe–TiN, Ni–TiN, and SUS316–TiC nanocomposite powders were prepared by ball-milling Fe–Ti, Ni–Ti, and SUS316–TiC powder mixtures in a nitrogen or argon gas atmosphere. Fe–63vol.% TiN and Ni–44–64vol.% TiN milled powders were dynamically compacted by use of a propellant gun to produce bulk materials of nanocrystalline structure and having grain sizes between about 5 and 400 nm. SUS316–2.8–5.6vol.% TiC milled powders were consolidated by hot isostatic pressing (HIP) to produce bulk materials having grain sizes between about 100 and 400 nm. The possibility of using fine-dispersed TiN/TiC particles to pin grain boundaries and thereby maintain ultra-fine grained structures of grain sizes down to the nanocrystalline scale has been discussed.     

Densification of a rapidly solidified nickel aluminide powder Part II Characterization of microstructure and mechanical properties

A Ni₃Al–X intermetallic compound prepared from a rapidly solidified powder was consolidated with hot isostatic pressing (HIP). Its microstructural development during the process has been examined. It involves a change from an inhomogeneous structure (a mixture of dendritic and equiaxed structures) to a uniformly distributed equiaxed structure and the formation of a disordered network phase from a metastably ordered matrix supersaturated with chromium, as a result of non-equilibrium solidification of the powder. The resulting microstructure of the consolidated material is mainly a function of HIP temperature. Mechanical properties of the HIP material at room and elevated temperatures have been determined. The results show that both the hardness and yield strength of the material decrease, while both the ultimate tensile strength and tensile elongation increase with rising HIP temperature up to 1250°C. Scanning electron microscope examination of the fracture surfaces of tested samples reveals a transition from interparticle fracture to transparticle fracture with increasing HIP temperature.     

Application of damping mechanism model and stacking fault probability in Fe–Mn alloy

In this paper, the damping mechanism model of Fe–Mn alloy was analyzed using dislocation theory. Moreover, as an important parameter in Fe–Mn based alloy, the effect of stacking fault probability on the damping capacity of Fe–19.35Mn alloy after deep-cooling or tensile deformation was also studied. The damping capacity was measured using reversal torsion pendulum. The stacking fault probability of γ-austenite and ε-martensite was determined by means of X-ray diffraction (XRD) profile analysis. The microstructure was observed using scanning electronic microscope (SEM). The results indicated that with the strain amplitude increasing above a critical value, the damping capacity of Fe–19.35Mn alloy increased rapidly which could be explained using the breakaway model of Shockley partial dislocations. Deep-cooling and suitable tensile deformation could improve the damping capacity owning to the increasing of stacking fault probability of Fe–19.35Mn alloy.     

Ti-content and annealing temperature dependence of transformation behavior of TiXNi(92-XCu8 shape memory alloys

Ti-content and annealing temperature dependence of the transformation behavior of Ti_XNi_(92-X)Cu_8.0 (at,%) (X = 49.0–5l.0) alloys was investigated by varying the annealing temperature from 573 to 1273 K. It was found that the peak temperature of B2–B19 transformation (O^*) increases with increasing annealing temperature from 673 to 873 K for all of the alloys. With annealing at temperatures above 873 K, the influence of annealing on O^* depends on Ti-content. In the range of 50.4–51.0 at.% Ti, O^* shows little dependence on annealing temperature. In the range of 49.3–50.2 at.% Ti, O^* firstly decreases and then keeps constant with increasing annealing temperature. For the alloy of 49.0 at.% Ti, O^* continuously decreases with increasing annealing temperature from 873 to 1273 K. On the basis of the above data, a partial phase diagram of Ti-Ni-8.0Cu (at.%) was proposed. The transformation hysteresis also showed unique Ti-content and annealing temperature dependence.     

Hot dynamic consolidation of hard ceramics

Diamond and cubic boron nitride powders were shock compacted at high temperature (873 and 973 K) by using a planar impact system at 1.2 and 2.0 km s^–1. Silicon, graphite or a mixture of titanium and carbon powders were added to enhance the bonding of these superhard materials. Hot-consolidated specimens exhibited fewer surface cracks as compared with the specimens shock consolidated at room temperature. Diamond compacts having microhardness values over 55 GPa were obtained by subjecting porous mixtures of diamond crystals (4-8 m) plus 15 wt% graphite (325 mesh) to an impact velocity of 1.2 km s^–1 at 873 K. Well-consolidated c-BN samples, with microhardnesses (starting powders with 10–20 and 40–50 (m) over 53 GPa were obtained.     

Microstructure and mechanical behavior of porous sintered steels

The microstructure and mechanical properties of sintered Fe–0.85Mo–Ni steels were investigated as a function of sintered density. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young's modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young's modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure-based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels are discussed.     

Mechanical properties and thermal stability of bulk Cu cold consolidated from atomized powders by high-pressure torsion

It is difficult to densify and consolidate round-shaped metallic powders by conventional compaction techniques because powder interlocking forces are small and the powders easily slip and rotate instead of being plastically deformed and densified. In this paper, atomized Cu (99.5 % purity) powders of round shapes were cold consolidated to bulk specimens by high-pressure torsion (HPT) under 10 GPa to avoid powder slippage by the shape effect. A relative density over 98 %, high tensile strengths of 642 and 570 MPa, and moderate ductility of 7.5 % with thermally stable ultrafine grained structures are achieved after the HPT consolidation process. The specimens HPT processed at RT show higher tensile strength due to more dislocations and finer grain sizes than the specimen processed at 373 K. Higher ductility in the elevated temperature (373 K)-processed specimen than in the RT-processed specimen is attributed to good bonding between particles, decreased dislocation density, and increased grain size.     

The effect of nickel on the mechanical behavior of molybdenum P/M steels

This study has examined the effects of nickel alloying additions on the microstructural characteristics and mechanical properties of Fe–xNi–0.85Mo–0.4C-base steels that were powder processed using double-press double-sinter processing to maximize density. The steels were examined in the as-processed condition as well as in a quench-and-temper heat treated condition. Tensile behavior indicates that while nickel content (at levels of 2,4, and 6%) increased tensile strength in the as-sintered condition, it did not significantly affect tensile strength in the quenched and tempered condition. In both conditions increasing Ni content decreased elongation to fracture. The 4% Ni steel, which tended to have the smallest maximum pore size, also exhibited the greatest fatigue strength.     

Effect of strain rate and environment on the mechanical properties of the Ni–19Si–3Nb–0.15B–0.1C intermetallic alloy at high temperatures

The effect of strain rate and environment on the mechanical behavior at different temperatures of the Ni–19Si–3Nb–0.15B–0.1C alloy is investigated by atmosphere-controlled tensile testing under various conditions at different strain rates and different temperatures). The results reveal that the Ni–19Si–3Nb–0.15B–0.1C alloy exhibits ductile mechanical behavior (UTS ∼ 1250 MPa, ε ~ 14%) at temperatures below 873 K under different atmosphere conditions. However, the alloy without boron and carbon addition shows ductile mechanical behavior only when the sample is tested in vacuum. This indicates that the microalloying of boron and carbon does overcome the environmental embrittlement from water vapor at test temperatures below 873 K for the Ni–19Si–3Nb base alloy. However, the boron and carbon doped alloy still suffers from embrittlement associated with oxygen at a medium high temperature (i.e. 973 K). In parallel, both of the ultimate tensile strength and elongation exhibit quite insensitive response with respect to the loading strain rate when tests are held at temperatures below 873 K. However, the ultimate tensile strength exhibits high dependence on the strain rate in air at temperatures above 873 K, decreasing the ultimate tensile strength with decreasing strain rate.     

Normal and anormal microstructure of plasma nitrided Fe-Cr alloys

Plasma nitriding behavior of Fe-Cr alloys has been studied at temperatures in the range of 773–873 K in order to provide basic knowledge for microstructure design of nitrided layers and to improve the wear resistance. In the nitriding temperature of 773 K, typical microstructure of nitrided layers was observed as reported elsewhere. However, anormal microstructure of nitrided layers was observed under a nitriding condition, at 873 K for 176.4 ks (49 h). In Fe-13Cr alloy, nitrided layer showed stripe-pattern, each sub-layer of which has different chromium content. Nitrided layer hardness increased gradually from the specimen surface to the nitriding front before dropping drastically to the same level as matrix hardness. The stripe-pattern was also observed for Fe-3Cr alloy at the vicinity of nitriding front for the same nitriding conditions. On the other hand, nitrided layers in Fe-8Cr and Fe-19Cr alloys are composed from different sub-layers, containing different concentration of chromium. These phenomena cannot be explained only by nitrogen diffusion process during the nitriding.     

Effect of Al and C on structure and mechanical properties of Fe–Al–C alloys

Abstract

Effect of aluminium and carbon content on the microstructure and mechanical properties of Fe–Al–C alloys has been investigated. Alloys were prepared by combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot forged and hot rolled at 1373 K. As rolled alloys were examined using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand the microstructure of these alloys. The ternary Fe–Al–C alloys containing 10·5 and 13 wt-%Al showed the presence of three phases: FeAl with disordered bcc structure, Fe₃Al with ordered DO₃ structure and Fe₃AlC_0·5 precipitates with L′1₂ structure. Addition of high concentration of carbon to these alloys resulted in excellent hot workability and superior tensile at room temperature as well as tensile and creep properties at 873 K. An increase in Al content from 9 to 13 wt-% in Fe–Al–C alloys containing the same levels of carbon has no significant influence on strength and creep properties at 873 K, however resulted in significant improvement in room temperature strength accompanied by a reduction in room temperature ductility.     

Combined effects of post-weld heat treatment and aging on alloy 800/2·25Cr–1Mo steel joint

Abstract

The combined effects of post-weld heat treatment (PWHT) and aging on the interfacial microstructure and tensile properties of alloy 800/2·25Cr–1Mo steel transverse weld specimens, welded using Inconel 182, were studied to determine the optimum PWHT temperature for the joint. In the present study, the joints were subjected to PWHT for 1 h at 948, 973, 998, and 1023 K, followed by aging at 873 K for 100–5000 h. The aging treatment at 873 K is intended to provide an accelerated simulation of service exposure at 773 K. The results of the present work show that the optimum PWHT temperature for the joint investigated is 973 K.

MST/1474     

Structure and magnetic properties of nanocrystalline mechanically alloyed Fe–10% Ni and Fe–20% Ni

The mechanical alloying technique has been used to prepare nanocrystalline Fe–10 and Fe–20 wt.% Ni alloys from powder mixtures. The structure and magnetic properties were studied by using X-ray diffraction and hysteresis measurements, respectively. For both alloys studied, a disordered body centered cubic solid solution forms after 24 h milling time. The higher the milling time, the larger the lattice parameter. The steady-state grain size is ≈10 nm. The reduction of the grain size increases the saturation magnetization and decreases the coercivity. Nanocrystalline Fe–10 and Fe–20 wt.% Ni have been shown to exhibit a soft magnetic behavior.     

Microstructure, martensitic transformation and superelasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy

The aim of this study is to investigate the microstructure, martensitic transformation behavior, shape memory effect and superelastic property of Ti_49.6Ni_45.1Cu₅Cr_0.3 alloy, with Cu and Cr substituting for Ni. After annealing, the alloy showed single step A-M/M-A transformations within the whole annealing temperature range of 623 K to 1273 K even in the presence and Ti₂(Ni, Cu) precipitates. With the increase of the annealing temperature, the transformation temperatures exhibited three stages: increasing from 623 K to 873 K, decreasing from 873 K to 1023 K and unchanging from 1023 K to 1273 K. Meanwhile, the critical stress for stress induced martensitic (SIM) transformation decreased to a minimum value and increased after that, exhibiting a V shape curve. The alloy annealed at 623, 773 and 923 K exhibited shape recovery ratio more than 90% when the deformation strain was below 20%.     

Microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion

The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.     

1Cr18Ni9Ti钢的低温拉伸变形行为

对1Cr18Ni9Ti钢在室温和低温下进行拉伸试验,利用TEM分析拉伸试样断口附近的显微组织,用SEM对拉伸断口进行观察,研究了温度对1Cr18Ni9Ti钢拉伸变形行为的影响.研究表明:随着试验温度的降低,1Cr18Ni9Ti钢的抗拉强度与屈服强度及加工硬化指数单调增加;延伸率呈降低趋势,并在温度降至77 K时略有回升;拉伸断口附近显微组织中出现板条马氏体,且温度降低,板条马氏体数量增加;低温与应变共同作用诱发板条马氏体形成是影响1Cr18Ni9Ti钢低温拉伸变形行为的重要因素.