Electron backscattered diffraction analysis of friction stir processed nanocomposites produced via spark plasma sintering

In the present study, Spark Plasma Sintered (SPSed) aluminium matrix composites were severely deformed through Friction stir processing (FSP). Pure aluminium powders and bimodal sized Al₂O₃ particles (80 nm and 25 m) were firstly mixed by ball milling and then consolidated by spark plasma sintering. The effect of the heat input as well the bimodal particle size of the alumina on the materials’ microstructure and texture development was evaluated by electron back scattered diffraction (EBSD) analysis. The EBSD analysis clearly showed that the SPSed nanocomposites possessed bimodal aluminium matrix grain structure as well as a crystallography characterised by random texture. In addition, microstructural examination revealed that the partial recrystallisation occurred during SPS for all the nanocomposites. Also, it is revealed that the Zener pinning effect of Al₂O₃ nanoparticles retarded recrystallised grain growth following recrystallisation during FSP and then leading to grain refinement of the aluminium. The results revealed that the heat generated during FSP has a remarkable effect on the grain distribution as well as on the crystallographic orientation. Also, a mixture of {112} <110> shear elements and an ideal strong B/ component were observed. The microstructural changes, occurred during FSP in the stir zone region for Al‐Al₂O₃ nanocomposites, were attributed to both the discontinuous along with the continuous recrystallisation (DDRX/CDRX). It should be pointed out that with increasing the heat input, recrystallised grains portion increased.     

High resolution transmission electron microscopy of Sm-Co based permanent magnets

Interface microstructures between rhombohedral Sm₂Co₁₇ and hexagonal SmCo₅ phases in a sintered permanent magnet of Sm(Co_0.675Fe_0.175Ni0.05Cu_0.1)₇ have been investigated by edge-on observations with the many-beam imaging technique. In the specimen annealed at 800°C for 4 h which shows a maximum magnetic coercivity, the interface boundaries were coherent accompanying the lattice distortion; the habit planes were parallel to the {101}_2:17 and {111}_1:5 planes with the orientation relationship of (0001)_2:17//(0001)_1:5 and [1010]_2:17//[1120]_1:5. In the specimen annealed for 150 h which shows a lower coercivity, the interface boundaries were characterized with regular alignments of interfacial edge dislocations. The observed interface structures were discussed in connection with the change in magnetic properties.     

High-Temperature Tribological Behavior of Al-20Si-5Fe-2Ni/ZrB2 Composites

Abstract

In this work, Al-20Si-5Fe-2Ni/ZrB₂ composites with 0–20?wt% ZrB₂ were fabricated by spark plasma sintering. The effects of ZrB₂ content on the microstructure, mechanical properties and high-temperature tribological behavior of the composites were investigated. The results indicate that Si, Al₅FeSi, and ZrB₂ particles are uniformly distributed in the aluminum matrix. The density, hardness, and compressive strength increase with increasing ZrB₂ content. The friction coefficient and wear rate are dependent on the ZrB₂ content and test temperature. At a certain temperature, the friction coefficient increases with an increase in ZrB₂ content, whereas the wear rate shows a reverse trend. Due to the improvement in thermal stability and high-temperature softening resistance, the composite shows improved wear resistance and increased transition temperature from mild wear to severe wear.     

Processing and properties of Al2O3/SiC nanocomposites

Alumina/SiC nanocomposites were produced by mechanical mixture of commercial powders. The preparation steps involved the vigorous mixing of the powders and drying under conditions where the homogeneous mixture was kept stable. Pressureless sintering of die-pressed powders achieved reasonable densities (～97% theoretical density) for 2·5wt% of SiC on sintering at 2073 K. Higher SiC contents strongly reduced the sintered density. The use of a more reactive alumina (finer matrix powder) gave similar results. Hot pressing at 1973 K/1 h/25 MPa produced high-density materials for SiC contents as high as 20 wt%. Transmission and scanning electron microscopy analysis showed that the SiC particles were well distributed and were situated both inside the grains and on the grain boundaries of the alumina matrix. The SiC strongly inhibited grain growth in the matrix in keeping with the Zener model. The bend strength increased as the SiC content increased, a result partly explained by the grain size refinement. The strength improvement of 20% over monolithic was explained in terms of the change to an intergranular fracture mode.     

EBSD and TEM investigation of the hot deformation substructure characteristics of a type 316L austenitic stainless steel

The evolution of crystallographic texture and deformation substructure was studied in a type 316L austenitic stainless steel, deformed in rolling at 900 °C to true strain levels of about 0.3 and 0.7. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used in the investigation and a comparison of the substructural characteristics obtained by these techniques was made. At the lower strain level, the deformation substructure observed by EBSD appeared to be rather poorly developed. There was considerable evidence of a rotation of the pre‐existing twin boundaries from their original orientation relationship, as well as the formation of highly distorted grain boundary regions. In TEM, at this strain level, the substructure was more clearly revealed, although it appeared rather inhomogeneously developed from grain to grain. The subgrains were frequently elongated and their boundaries often approximated to traces of {111} slip planes. The corresponding misorientations were small and largely displayed a non‐cumulative character. At the larger strain, the substructure within most grains became well developed and the corresponding misorientations increased. This resulted in better detection of sub‐boundaries by EBSD, although the percentage of indexing slightly decreased. TEM revealed splitting of some sub‐boundaries to form fine microbands, as well as the localized formation of microshear bands. The substructural characteristics observed by EBSD, in particular at the larger strain, generally appeared to compare well with those obtained using TEM. With increased strain level, the mean subgrain size became finer, the corresponding mean misorientation angle increased and both these characteristics became less dependent on a particular grain orientation. The statistically representative data obtained will assist in the development of physically based models of microstructural evolution during thermomechanical processing of austenitic stainless steels.     

Crystal orientation mapping of NiO grown on cube textured Ni tapes

Samples of cube textured Ni tapes were oxidized in flowing oxygen at different temperatures. Crystal orientation maps (COMs) of the resulting oxide layers were produced by electron backscatter diffraction. The oxide layers were also analysed by X‐ray diffraction (XRD), scanning electron microscopy and atomic force microscopy (AFM). The oxide grain size of a sample oxidized at 600 °C was similar to that of the substrate and the oxide was highly textured, both indicating epitaxial growth. The orientation relationship between the substrate and the oxide was directly observed from XRD to be (111)_NiO//(001)_Ni, [01]_NiO//[110]_Ni with four, equivalent, in‐plane variants. In each variant, the oxide has both <110>‐ and <211>‐type directions parallel to the Ni <110> directions. Differences in oxide thickness and surface roughness on neighbouring grains were revealed by AFM and these were attributed to the existence of a range of oxide growth conditions resulting from small differences in the orientation of each substrate grain. Similar macrotexture and microstructure were observed on a sample oxidized at 1300 °C, but additional, facetted oxide crystals had formed at the oxide grain boundaries. COMs showed that these crystals were either cube or 45° rotated cube orientated, a texture different to that of the large oxide grains. The grain boundary crystals were thought to form by inward diffusion of oxygen at defects in the growing oxide scale.     

Cutting performance of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> based SiC ceramic cutting tools

Composites of Si₃N₄-SiC containing up to 30 wt% of dispersed SiC particles were fabricated via hot-pressing with an oxynitride glass. To determine the effect of sintering time and SiC content on the mechanical properties and the cutting performance, the composites with fixed 8 hr-sintering time and 20 wt% SiC content were fabricated and tested. Fracture toughness of the composites increased with increasing sintering time, while the hardness increased as the SiC content increased up to 20 wt%. The hardness of the composites was relatively independent of the grain size and the sintered density. For machining heat-treated AISI4140, the insert with 20 wt% SiC sintered for 8 hr showed the longest tool life while the insert with 20 wt% SiC sintered for 12 hr showed the longest tool life for machining gray cast iron. An effort was made to relate the mechanical properties, such as hardness, fracture toughness and wear resistance coefficient with the tool life. However, no apparent relationship was found between them. It may be stated that tool life is affected by not only the mechanical properties but also other properties such as surface roughness, density, grian size and the number of the inherent defects in the inserts.     

Transmission electron microscopy of fluorapatite–gelatine composite particles prepared using focused ion beam milling

In this paper, synthetic fluorapatite–gelatine composite particles are prepared for transmission electron microscopy (TEM) studies using two methods based on focused ion beam (FIB) milling. TEM studies on the FIB‐prepared specimens are compared with TEM observations on samples prepared using an ultramicrotome. The results show that ultramicrotome slicing causes significant cracking of the apatite, whereas the ion beam can be used to make high‐quality, crack‐free specimens with no apparent ion beam‐induced damage. The TEM observations on the FIB‐prepared samples confirm that the fluorapatite composite particles are composed of elongated, preferentially orientated grains and reveal that the grain boundaries contain many small interstices filled with an amorphous phase.     

Sliding wear behaviors of in situ alumina/aluminum titanate ceramic composites

In situ alumina/aluminum titanate ceramic composites were prepared by spark plasma sintering with two kinds of alumina/titania powders, which are microsized irregular particles (referred to M powder) and microsized spherical particles composed of nanosized grains (referred to N powder). The phase constitution and microstructures of the powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD) and scanning electron microscope (SEM). The sliding wear behaviors of two alumina/aluminum titanate ceramic composites were investigated by ball-on-disc wear test with varied normal loads. The worn surfaces of ceramic composites and counterpart Si₃N₄ balls were characterized by using SEM equipped with X-ray energy dispersive spectroscopy (EDS). The results showed that the wear volume of two ceramic composites increased with increasing the normal load. Under the same normal load, the wear volume of N composite (obtained from the N powder) was higher than that of M composite (obtained from the M powder). Two different behaviors were identified: N composite showed intergranular fracture and grain pull-out; however, the surface reaction layer formed in M composite presented plastic deformation. The different behaviors are controlled by two different mechanisms, brittle fracture mechanism for N and tribochemical reaction mechanism for M. The different wear behaviors for the two ceramic composites were discussed in detail.     

Three-dimensional EBSD study on the relationship between triple junctions and columnar grains in electrodeposited Co–Ni films

Electrodeposited nanocrystalline materials are expected to have a homogeneous grain size and a narrow grain size distribution. In Co–Ni electrodeposited films, however, under certain conditions an undesired columnar grain structure is formed. Fully automated three‐dimensional (3D) orientation microscopy, consisting of a combination of precise material removal by focussed ion beam and subsequent electron backscatter diffraction (EBSD) analysis, was applied to fully characterize the grain boundaries of these columnar grains in order to gain further understanding on their formation mechanisms. Two‐dimensional orientation microscopy on these films indicated that the development of columnar grains could be related to the formation of low‐energy triple junctions. 3D EBSD allowed us to verify this suggestion and to determine the boundary planes of these triples. The triplets are formed by grain boundaries of different quality, a coherent twin on the {} plane, an incoherent twin and a large‐angle grain boundary. These three boundaries are related to each other by a rotation about the 〈〉 direction. A second particularity of the columnar grains is the occurrence of characteristic orientation gradients created by regular defects in the grain. Transmission electron microscopy was applied to investigate the character of the defects. For this purpose, a sample was prepared with the focussed ion beam from the last slice of the 3D EBSD investigation. From the TEM and 3D EBSD observations, a growth mechanism of the columnar grains is proposed.     

Single-section plane assessment in grain boundary engineered brass

The present paper reports a comparative analysis of Σ3 (in the coincidence site lattice notation) grain boundary types, in two grain boundary engineered brass specimens, by use of electron backscatter diffraction (EBSD) data coupled to the measurement of boundary traces in a single section. Although most of the data were analysed using the new single‐section technique, an analysis of boundary plane orientations in three dimensions was made in a subset of the data in order to validate the single section methodology. The single‐section trace analysis procedure, coupled with EBSD, is a viable and robust tool for analysis of Σ3 grain boundary planes. The procedure provides data which suggest that part of the enhanced strain‐to‐failure in specimen B compared to specimen A is the result of an increased proportion of mobile Σ3 boundaries, i.e. those which are displaced from the {111} symmetrical tilt configuration.     

Sub-structure characterization of experimentally and naturally deformed ice using cryo-EBSD

In this work, we present first results of high‐resolution EBSD for ice with a spatial resolution down to 0.25 μm. The study highlights the potential of EBSD to significantly increase our understanding of deformation and annealing processes associated with the build‐up of internal stresses due to strain incompatibility between grains. Two polycrystalline samples were analyzed: a natural sample of polar ice from the Vostok ice core (Antarctica) and an experimentally deformed sample of laboratory grown columnar ice. In summary, we observe the following: (1) inhomogeneous deformation through the grains is translated into lattice distortions that are concentrated mainly at grain boundaries and triple junctions (natural and experimental sample), (2) these distortions may be continuous (natural and experimental sample) or may form distinct tilt boundaries and sub‐grains of 10–50 μm size (experimental sample). These form mainly by rearrangement of basal edge dislocations into low‐energy configurations (i.e. tilt boundaries) in various prism planes. Continuous lattice distortions originate from screw or mixed edge and screw dislocations lying in the basal plane.     

氮化碳基陶瓷刀具材料的制备与力学性能研究

采用热压烧结工艺,以Ti(C, N)为添加相,以Mo、Ni和Co为金属相,成功制备了氮化碳(C₃N₄)基陶瓷刀具材料,测量了其断裂韧度、抗弯强度和维氏硬度,分析了其微观组织。结果表明,在烧结温度为1600℃、保温时间为45 min和烧结压力为32 MPa的工艺条件下,Ti(C, N)质量分数为35%、Ni-Co质量分数为8%的C₃N₄基陶瓷刀具材料力学性能最优。合适的Ti(C, N)含量能细化C₃N₄晶粒、提高烧结密度、改善力学性能,合适的Ni-Co含量能使微观组织细小均匀。     

Microstructural and mechanical properties of nanometric magnesium oxide particulate-reinforced aluminum matrix composites produced by powder metallurgy method

In this research, aluminum alloy (A356.1) matrix composites reinforced with 1.5, 2.5 and 5 Vol.% nanoscale MgO particles were fabricated via powder metallurgy method. Pure atomized aluminum powder with an average particle size of 1μm and MgO particulate with an average particle size between 60 to 80 nm were used. The specimens were pressed by Cold Isostatic Press machine (CIP), and were subsequently sintered at various sintering temperatures, viz. 575, 600 and 625°C. Optimum amount of reinforcement and sintering temperature were determined by evaluating the density, microstructure and mechanical properties of composites. The composites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Hardness and compression tests were carried out in order to identify mechanical properties. Reinforcing the Al matrix alloy with MgO particles improved the hardness and compressive strength of the alloy to a maximum value of 44 BHN and 288 MPa, respectively. The most improved compressive strength was obtained with the specimen including 2.5% of MgO sintered at 625°C. According to the experiments, a sintering temperature of 625°C showed better results than other temperatures. A good distribution of the dispersed MgO particulates in the matrix alloy was achieved.     

Microstructure and electrical properties of diborides modified by rapid thermal annealing

Diborides of Ti, Hf and Zr are thermally, mechanically and chemically stable with good thermal and electrical conductivity. We tested their properties in front‐end processes used in Si integrated circuits (IC). Films were deposited by e‐beam evaporation either on Si, for the formation of contacts to the source/drain (S/D) regions, or on Si oxides, for the formation of metal gates in p‐type metal‐oxide‐semiconductor (PMOS) transistors. We focused on their crystallization caused by rapid thermal processing (RTP) at temperatures up to 1100 °C. Transmission electron microscopy was used for identification of nanocrystallites of TiB₂, ZrB₂, and HfB₂. The grain growth was correlated with temperature and time of RTP. Of all borides, HfB₂ resulted in the most complete crystallization with little amorphous phase left. There was no crystallographic degradation of the interface with Si or dielectrics, except for extreme thermal budgets. Complementary techniques were used for monitoring chemical stability and electrical parameters of test structures to assess the role of recrystallization in device behaviour.     

An ion microprobe study of the microchemistry of Ni-base superalloy-Al2O3 metal-matrix composites

The chemical microstructure of Ni-base superalloy/Al₂O₃ metal-matrix composites (MMCs) has been studied by scanning ion microprobe microanalysis, using the secondary ion mass spectrometry (SIMS) technique. The MMCs were fabricated using the transient-liquid-phase bonding (TLP) process, with B-doped superalloy powder as an interlayer. Boron was found to diffuse rapidly throughout the matrix to form boride phases, mostly at the grain boundaries in the matrix. These borides contain excess Cr (also Mo, Si, W) in comparison with the Ni alloy-matrix, but are depleted in Ni (also in Al and Co). Carbides form at the grain boundaries as thin platelets and inside the grains as fine particles. Chemical reaction occurs between the sapphire fibre and the matrix; formation of NiAl₂O₄ spinel at the interface is suggested. This interface reaction layer is friable and parts of it peel off during consolidation to become inclusions in the matrix near the fibre/matrix interface.     

Silicon carbide/SRBSN composites

Ceramic-matrix composites have been produced using unidirectionally aligned Textron SCS-6 fibres in a sintered reaction-bonded matrix. A tape-casting technique was used to produce a prepreg sheet that could be cut and stacked to form a layup. Al₂O₃ and Y₂O₃ were used as sintering aids, final sintering being carried out in a hot press at 1700°C. Matrix, fibre and interfacial microstructure has been characterized using analytical microscopical techniques. X-ray mapping of the carbon and silicon distribution at the fibre–matrix interface was carried out, and evidence of reaction between the outer carbon-rich layer of the fibre and the matrix was found. Micromechanical behaviour of interfaces has been investigated and compared with interfacial microstructure and macromechanical properties.