Showing papers on "Microstructure published in 1994"

Method for the simultaneous determination of anisotropic residual stresses and texture by x‐ray diffraction

Abstract: A method is developed that yields the residual stress, the orientation distribution coefficients, the average crystallite dimension, the microstrain, and the crystal structure parameters from x‐ray diffraction data in a single‐step procedure. To this end, a general approach is introduced that combines the equations of micromechanics with the harmonic description of texture. All relationships are cast into a Rietveld‐like format, which incorporates a microstructure model derived from line‐broadening methods. In this manner, data collected over the whole x‐ray‐diffraction pattern at different tilting of the sample can be fitted directly. The associated fitting parameters are the crystal structure and microstructure, the texture coefficients, and the micromechanical properties and fields.

Effect of microstructure (particulate size and volume fraction) and counterface material on the sliding wear resistance of particulate-reinforced aluminum matrix composites

Abstract: The effects of microstructure (namely, particulate volume fraction and particulate size) and the counterface materials on the dry-sliding wear resistance of the aluminum matrix composites 2014A1-SiC and 6061Al-Al2O3 were studied. Experiments were performed within a load range of 0.9 to 350 N at a constant sliding velocity of 0.2 ms-1. Two types of counterface materials, SAE 52100 bearing steel and mullite, were used. At low loads, where particles act as loadbearing constituents, the wear resistance of the 2014A1 reinforced with 15.8 µm diameter SiC was superior to that of the alloy with the same volume fraction of SiC but with 2.4 µm diameter. The wear rates of the composites worn against a steel slider were lower compared with those worn against a mullite slider because of the formation of iron-rich layers that act asin situ solid lubricants in the former case. With increasing the applied load, SiC and A12O3 particles fractured and the wear rates of the composites increased to levels comparable to those of unreinforced matrix alloys. The transition to this regime was delayed to higher loads in the composites with a higher volume percentage of particles. Concurrent with particle fracture, large strains and strain gradients were generated within the aluminum layers adjacent to contact surfaces. This led to the subsurface crack growth and delamination. Because the particles and interfaces provided preferential sites for subsurface crack initiation and growth and because of the propensity of the broken particles to act as third-body abrasive elements at the contact surfaces, no improvement of the wear resistance was observed in the composites in this regime relative to unreinforced aluminum alloys. A second transition, to severe wear, occurred at higher loads when the contact surface temperature exceeded a critical value. The transition loads (and temperatures) were higher in the composites. The alloys with higher volume fraction of reinforcement provided better resistance to severe wear. Wearing the materials against a mullite counterface, which has a smaller thermal conductivity than a counterface made of steel, led to the occurrence of severe wear at lower loads.

Microstructure evolution of eutectic Sn-Ag solder joints

Abstract: Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.

Role of Eta-carbide Precipitations in the Wear Resistance Improvements of Fe-12Cr-Mo-V-1.4C Tool Steel by Cryogenic Treatment

Abstract: The wear resistance of an Fe-12.2wt%Cr-0.84wt%Mo-0.43wt%V-1.44wt%C alloy tool steel after cold treatment at 223K (subzero treatment) and after cryogenic treatment 93K (ultra-subzero treatment) has been investigated. The wear resistance of steels after cryogenic treatment is superior to that after cold treatment. The effects of cryogenic treatment on the microstructure were also studied by means of X-ray diffraction and transmission electron microscopy methods. Unlike cold treatment, cryogenic treatment improves the preferential precipitation of fine η-carbides instead of e-carbides. These fine carbide particles enhance the strength and thoughness of the martensite matrix and then increase the wear resistance. The formation mechanism of fine η-carbide is discussed.

Toughness Properties of a Silicon Carbide with an in Situ Induced Heterogeneous Grain Structure

Abstract: Toughness characteristics of a heterogeneous silicon carbide with a coarsened and elongated grain structure and an intergranular second phase are evaluated relative to a homogeneous, fine-grain control using indentation–strength data. The heterogeneous material exhibits a distinctive flaw tolerance, indicative of a pronounced toughness curve. Quantitative evaluation of the data reveals an enhanced toughness in the long-crack region, with the implication of degraded toughness in the short-crack region. The enhanced long-crack toughness is identified with crack-interface bridging. The degraded short-crack toughness is attributed to weakened grain or interface boundaries and to internal residual stresses from thermal expansion mismatch. A profound manifestation of the toughness-curve behavior is a transition in the nature of mechanical damage in Hertzian contacts, from classical single-crack cone fracture in the homogeneous control to distributed subsurface damage in the heterogeneous material.

Microstructure Control of Silicon Nitride by Seeding with Rodlike β‐Silicon Nitride Particles

Abstract: The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike β-Si3N4 singlecrystal particles with a diameter of 1 μm and a length of 4 μm as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of β-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation Dn–D0n=kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4–8.7 MPa·m1/2, retaining high strength levels of about 1 GPa.

Improved mechanical properties in new, Pb-free solder alloys

Abstract: The mechanical properties of solders benefit from uniform dispersion of fine precipitates and small effective grain sizes. Metallurgical methods of attaining such a beneficial microstructure have been investigated in two new, near-eutectic, Pb-free solder alloys systems—Sn-Zn-In (m.p. ∼188°C) and Sn-Ag-Zn (m.p.∼217°C). It has been found that small alloying additions of Ag dramatically improve the mechanical properties of the ternary Sn-8Zn-5In alloy. The improvement is attributed to the elimination of the coarse and nonuniform distribution of plate-like dendrites and refining the effective grain size in the solidified microstructure. Also, small amounts of Cu dramatically improve the ductility in the ternary Sn-3.5Ag-lZn alloy. The quaternary Sn-3.5Ag-lZn-0.5Cu has better mechanical properties than the binary Sn-3.5Ag alloy because it has a uniform fine dispersion of precipitates and a small effective grain size. The combination of high mechanical strength and high ductility is likely to yield improved fatigue resistance properties in the interconnection of electronic components.

Fabrication of Al matrix in situ composites via self-propagating synthesis

Abstract: Al matrix composite materials with 30 vol.% TiC, TiB2 and TiC + TiB2 ceramic reinforcements were processed in situ via self-propagating high temperature synthesis (SHS) followed by high pressure consolidation to full density. Non-steady-state oscillatory motion of the combustion wave was observed during the SHS processing, resulting in a typical layered structure of the reaction products. The microstructure and phase composition of the materials obtained were studied using X-ray diffraction, optical microscopy and scanning (SEM) and transmission (TEM) electron microscopy. Very-fine-scale ceramic particles ranging from tens of nanometers up to 1–2 μm were obtained in the Al matrix. Microstructural analysis of the reaction products showed that the TiB2/Al and (TiB2 + TiC)/Al composites contained the Al3Ti phase, indicating that full conversion of Ti had not been achieved. In the TiC/Al composite a certain amount of Al4C3 was detected. High room and elevated temperature mechanical properties (yield stress, microhardness) were obtained in the high-pressure-consolidated SHS-processed TiC/Al and TiB2/Al composites, comparable with the best rapidly solidified Al-base alloys. These high properties were attributed to the high density of the nanoscale ceramic particles and matrix grain refinement.

Effect of SiC and graphite particulates on the damping behavior of metal matrix composites

Abstract: The effect of SiC and graphite (Gr) particulates on the resultant damping behavior of 6061 A1 metal matrix composites (MMCs) was investigated in an effort to develop a high damping material. The MMCs were processed by a spray atomization and deposition technique and the damping characterization was conducted on a dynamic mechanical thermal analyzer. The damping capacity, as well as the dynamic modulus, was measured at frequencies of 0.1, 1, 10 and 30 Hz over a 30 to 250°C temperature range. The microstructural analysis was performed using scanning electron microscopy, optical microscopy and image analysis. The damping capacity of the 6061 Al/SiC and 6061 Al/Gr MMCs, with two different volume fractions of reinforcements, were compared with that of as-received 6061-T6 Al and spray deposited 6061 Al. It was shown that the damping capacity of 6061 Al could be significantly improved by the addition of either SiC or graphite particulates through spray deposition processing. Finally, the operative damping mechanisms were discussed in light of the data obtained from characterization of microstructure and damping capacity.

Use of Transmission Electron Microscopy for the Characterization of Rapid Thermally Annealed, Solution‐Gel, Lead Zirconate Titanate Films

Abstract: The microstructure and preferred orientations of rapid thermally annealed Pb(Zr0.53, Ti0.47)O3 films, deposited on Pt/Ti/SiO2/Si electrode/substrates by solution-gel spinning, have been investigated using analytical and high-resolution electron microscopy and X-ray diffraction. The temperature of pyrolysis of the PZT filMS was found to influence the preferred orientation of the film: lower temperatures (350-degrees-C) favored a (111) orientation, whereas higher temperatures (420-degrees-C) favored a (100) orientation. Excess Pb was used to control the A-site stoichiometry of the film particularly at the film surface where Pb-deficient crystals could often be observed. The absence of these crystals was shown to be correlated with an improvement in the dielectric response.

Particulate silicon nitride-based composites

Abstract: In an attempt to optimize the structure and properties of silicon nitride ceramics, a variety of novel processing techniques and materials compositions have evolved over the last 15 years. Among the most important, was the development of various silicon nitride-based composites. A review of particulate, silicon nitride-based composites other than whisker- or platelets-reinforced, is presented. Materials based on silicon nitride and SiAlONs, with additions of carbides, nitrides and borides of transition metals are described. Special emphasis is placed on TiN- and TiC-containing ceramics. The manufacture of composites by hot pressing, reaction sintering, pressureless and gas-pressure sintering is discussed. The data on properties, including conductivity, density, Young's modulus, strength, fracture toughness, hardness, thermal expansion, wear, creep and oxidation resistance are presented. Analysis of actual and potential uses of the selected composites demonstrates that the particulate composites are very promising as tool, structural and electronic materials.

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Abstract: Two types of Al2O3/Mo composites were fabricated by hot-pressing a mixture of γ- or α-Al2O3 powder and a fine molybdenum powder. For Al2O3/5 vol% Mo composite using γ-Al2O3 as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al2O3 grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m1/2). In the system using α-Al2O3 as a starting powder, nanometre sized molybdenum particles were dispersed within the Al2O3 grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al2O3/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al2O3 grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.

Hardness and Young's modulus of amorphous a -SiC thin films determined by nanoindentation and bulge tests

Abstract: Due to its interesting mechanical properties, silicon carbide is an excellent material for many applications. In this paper, we report on the mechanical properties of amorphous hydrogenated or hydrogen-free silicon carbide thin films deposited by using different deposition techniques, namely plasma enhanced chemical vapor deposition (PECVD), laser ablation deposition (LAD), and triode sputtering deposition (TSD). a-SixC1−x: H PECVD, a-SiC LAD, and a-SiC TSD thin films and corresponding free-standing membranes were mechanically investigated by using nanoindentation and bulge techniques, respectively. Hardness (H), Young’s modulus (E), and Poisson’s ratio (v) of the studied silicon carbide thin films were determined. It is shown that for hydrogenated a-SixC1−x: H PECVD films, both hardness and Young’s modulus are dependent on the film composition. The nearly stoichiometric a-SiC: H films present higher H and E values than the Si-rich a-SixC1−x: H films. For hydrogen-free a-SiC films, the hardness and Young’s modulus were as high as about 30 GPa and 240 GPa, respectively. Hydrogen-free a-SiC films present both hardness and Young’s modulus values higher by about 50% than those of hydrogenated a-SiC: H PECVD films. By using the FTIR absorption spectroscopy, we estimated the Si-C bond densities (NSiC) from the Si-C stretching absorption band (centered around 780 cm−1), and were thus able to correlate the observed mechanical behavior of a-SiC films to their microstructure. We indeed point out a constant-plus-linear variation of the hardness and Young’s modulus upon the Si-C bond density, over the NSiC investigated range [(4–18) × 1022 bond · cm−3], regardless of the film composition or the deposition technique.

Nanocomposite R2Fe14B/Fe exchange coupled magnets

Abstract: We have studied the crystallization, crystal structure, microstructure and magnetic properties of R‐Fe‐B (R=Nd,Pr,Dy,Tb) based melt‐spun ribbons consisting of a mixture of R2Fe14B and α‐Fe phases. All the samples crystallize first to α‐Fe and a metastable phase (Y3Fe62B14 for R=Nd,Pr,Dy and TbCu7 for R=Tb) before they finally transform to 2:14:1 and α‐Fe. The highest values of coercivity and reduced remanence, 4.5 and 0.63 kOe, respectively, were obtained in a Nd3.85Tb2(Fe‐Nb‐B)94.15 sample. These properties are the result of a fine grain microstructure consisting of a mixture of α‐Fe and 2:14:1 having an average grain size of 30 nm.

Reactive magnetron sputter-deposition of cnx films on si(001) substrates - film growth, microstructure and mechanical-properties

Abstract: There is currently considerable interest in producing new materials with extreme combinations of mechanical properties such as high hardnesses and moduli. One example of such a material is crystalline C3N4, which has been predicted to have a bulk modulus higher than that of diamond. In this paper we report on experiments carried out to synthesize CNx thin films. The films were grown in an unbalanced magnetron-sputtering system by reactive sputtering of C in N2 discharges. Si(001) substrates with the native oxide removed by thermal desorption and then kept at temperatures ranging from 150 to 600°C and substrate bias voltages Vs between 7.5 and -200 V were used. The films were analysed using X-ray diffraction, transmission electron microscopy (TEM). Auger electron spectroscopy, Rutherford backscattering and nano-indentation tests. Typically the films were grown at rates of 5 nm s−1 to total thicknesses of 300 nm. Owing to an extensive re-sputtering, only low negative bias voltages (−80

Mode of growth and microstructure of polycrystalline silicon obtained by solid‐phase crystallization of an amorphous silicon film

Abstract: The structure and the morphology of crystallized amorphous silicon (α‐Si) films which were deposited on glass and annealed in a conventional furnace or by rapid thermal process (RTP) are studied using transmission electron microscopy (TEM). The ellipsoidal shape of the grains is attributed to the fast solid‐state crystallization along the two mutually perpendicular 〈112〉 and 〈110〉 crystallographic directions. The growth is solely based on the twin formation. The stability of the microtwins was studied by RTP and in situ TEM heating experiments. The effect of the film thickness on the preferred orientation of the grains is discussed. Very thin films exhibit (111) preferred orientation due to the strongly anisotropic rate of growth of the nuclei, which imposes an orientation filtering due to a growth velocity competition. The mode of growth of these films is compared with poly‐Si films grown by low‐pressure chemical‐vapor deposition.

Tensile behavior of a new single-crystal nickel-based superalloy (CMSX-4) at room and elevated temperatures

Abstract: Tensile behavior of a new single-crystal nickel-based superalloy with rhenium (CMSX-4) was studied at both room and elevated temperatures. The investigation also examined the influence of γ′ precipitates (size and distribution) on the tensile behavior of the material. Tensile specimens were prepared from single-crystal CMSX-4 in [001] orientation. The test specimens had the [001] growth direction parallel to the loading axis in tension. These specimens were given three different heat treatments to produce three different γ′ precipitate sizes and distributions. Tensile testing was carried out at both room and elevated temperatures. The results of the present investigation indicate that yield strength and ultimate tensile strength of this material initially increases with temperature, reaches a peak at around 800 °C, and then starts rapidly decreasing with rise in temperature. Both yield and tensile strength increased with increase in average γ′ precipitate size. Yield strength and temperature correlated very well by an Arrhenius type of relationship. Rate-controlling process for yielding at very high temperature (T ≥ 800 °C) was found to be the dislocation climb for all three differently heat-treated materials. Thermally activated hardening occurs below 800 °C whereas above 800 °C thermally activated softening occurs in this material.

High-resolution electron microscopy studies of a microporous carbon produced by arc-evaporation

Abstract: The soot produced as a byproduct of fullerene synthesis by arc-evaporation consists of a microporous carbon with a surface area, after activation with carbon dioxide, of ca. 700 m2 g–1. Here, we investigate the structure of this material, and its appearance after electron irradiation and high-temperature heat treatment, using high-resolution electron microscopy. We show that the heat treatment transforms the new carbon into a structure containing large, tube-like pores, rather than into polycrystalline graphite. This suggests that the arc-evaporated carbon may have a novel, fullerene-related microstructure, and that it may be the precursor for nanotube formation.

Structural, optical, and electrical properties of epitaxial chalcopyrite CuIn3Se5 films

Abstract: Single crystal CuIn3Se5 epitaxial films have been synthesized on GaAs(001) by a hybrid sputtering and evaporation technique The microstructure, microchemistry, and selected electrical and optical properties of the films have been investigated by scanning electron microscopy, energy dispersive x‐ray spectroscopy, transmission electron microscopy, cathodoluminescence, optical absorption and reflection, and four‐point probe resistivity measurements The results showed that the CuIn3Se5 crystals have an ordered point defect structure, a band gap of ≥118 eV, an optical absorption coefficient of about 15 000 cm−1 at a photon energy of 135 eV, and a film resistivity of ≳105 Ω cm The results suggest the presence of band tails giving rise to subgap radiative recombination and absorption Antiphase domain boundaries, stacking faults, and nanotwins were observed in the epitaxial layers and were reduced in number by rapid thermal annealing

Effects of α-SiC versus β-SiC starting powders on microstructure and fracture toughness of SiC sintered with Al2O3-Y2O3 additives

Abstract: Dense SiC ceramics were obtained by pressureless sintering of [beta]-SiC and [alpha]-SiC powders as starting materials using Al[sub 2]O[sub 3]-Y[sub 2]O[sub 3] additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from [alpha]-SiC powder was composed of equiaxed grains, whereas SiC obtained from [beta]-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the [beta][r arrow][alpha] phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using [alpha]-SiC powder increased slightly from 4.4 to 5.7 MPa [center dot] m[sup 1/2] with holding time, that is, increased grain size. In the case of the sintered SiC using [beta]-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa [center dot] m[sup 1/2] with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.

Correlation of microstructure variability and local stress field in two-phase materials

Abstract: Improved characterization of present-day materials that are manufactured to obtain optimal property values by microstructure effects necessitates a more thorough knowledge of the microstructure pattern. Methods of spatial statistics have been used to analyse patterns of second-phase inclusions as observed on planar sections. Several parameters and functions that discriminate between different dispersions of inclusions have been investigated. The effects of inclusion patterns on the interface stresses in unidirectional fibre reinforced composite material are discussed. The range of a local geometrical disorder and a physical range of interaction between stresses and positions of inclusions are obtained for investigated patterns.

Microstructure of SnO2.

Abstract: The microstructure of ${\mathrm{SnO}}_{2}$, prepared by the sol-gel method, was studied by scanning electron microscopy, transmission electron microscopy, high-resolution electron microscopy, and x-ray diffraction. A nanosponge structure was observed. There is much surface and interface structure. The interfaces vary in type from amorphous to crystalline. The high density of defects greatly influences the physical and chemical properties of this material.

Non‐ohmic characteristics of ZnO‐V2O5 ceramics

Abstract: The densification behavior, microstructure, and electrical properties of ZnO‐V2O5 ceramics were studied with V2O5 as the only additive ranging from 0.01 to 1.0 mol %. The addition of V2O5 to zinc oxide shows a tendency to enhance the densification rate and promote grain growth. However, a microstructure that consisted of anomalously grown grains was found for the specimens containing V2O5≥0.05 mol % when sintered at 1100 °C for 2 h. The x‐ray diffraction and SEM‐EDS microanalysis revealed that the sintered specimens had a two‐phase microstructure, i.e., a vanadium‐rich intergranular phase formed between ZnO grains. The formation of the grain boundary barrier layer was confirmed by the non‐ohmic I‐V behavior and the quick drop of apparent dielectric constant with increasing frequency of the ceramics. A nonlinearity coefficient of 2.4–2.8 was obtained at a current density of 10 mA/cm2 for a series ZnO‐V2O5 ceramics, and a Schottky barrier height of 0.44–0.47 eV (at 25 °C) was determined from the I‐V and C‐V experimental data, based on the thermionic emission theory, and the model of back‐to‐back double Schottky barriers.