Grain Boundaries Of Matrix Research Articles

Crack-healing function of metal/Al2O3 hybrid materials

Nano-Ni/Al2O3 hybrid materials have the crack-healing function by thermal oxidation process such as 1200°C for 6 h in air. In this hybrid material system, crack was filled up by an oxidation product, NiAl2O4, via outward diffusion of cations along grain boundaries of Al2O3 matrix. Ni/Al2O3 with Y2O3 doping and SiC+Ni/Al2O3 nano-hybrid materials have similar crack-healing performance with better oxidation resistance at high temperatures than Ni/Al2O3 nano-hybrid materials. Mo/Al2O3 hybrid materials were studied on a candidate with crack-healing function via thermal oxidation process at temperatures as low as 700°C.

Prediction of hot deformation behavior of Al–5.9%Cu–0.5%Mg alloys with trace additions of Sn

High temperature deformation behavior of Al–5.9wt%Cu–0.5wt%Mg alloys containing trace amounts (from 0 to 0.1 wt%) of Sn was studied by hot compression tests conducted at various temperatures and strain rates. The peak flow stress of the alloys increased with increase in strain rate and decrease in deformation temperature. The peak stress could be correlated with temperature and strain rate by a suitable hyperbolic-sine constitutive equation. The activation energy for hot deformation of the alloy without Sn content was observed to be 183.4 kJ mol−1 which increased to 225.5 kJ mol−1 due to 0.08 wt% of Sn addition. The Zener-Hollomon parameter (Z) was determined at various deforming conditions. The tendency of dynamic recrystallization increased with low Z values, corresponding to low strain rate and high temperature. The peak flow stresses at various processing conditions have been predicted by the constitutive modeling and correlated with the experimental results with fairly good accuracy. It was possible to predict 80, 75, 100, 100, 90, and 85% of the peak stress values within an error less than ±13%, for the investigated alloys. With addition of Sn content >0.04 wt%, peak flow stress increased significantly for all strain rate and temperature combinations. Scanning electron microscope revealed two types of second phases at the grain boundary of the undeformed alloy matrix, one being an Al–Cu–Si–Fe–Mn phase while the other identified as CuAl2. The high strength and flow stress value of the alloy with 0.06 wt% of Sn content, may be attributed to the variation in amount, composition, and morphology of the Al–Cu–Si–Fe–Mn phase, as well as to the lower value of activation energy for precipitation reaction, as revealed from differential scanning calorimetric studies.

Reinforcement of Hydroxyapatite with Multi-Walled Carbon Nanotubes

Multi-walled carbon nanotubes (MWNTs) reinforced hydroxyapatite (HA) composite was fabricated by in-situ method and followed by hot-pressing sintering; the influence of MWNTs’ content on the mechanical and microstructure properties was explored. The results show that adding MWNTs within a certain range could enhance the mechanical properties of HA matrix significantly. The maximal increment of the bending strength and fracture toughness of the composites, compared with the pure HA, were 157% and 171% respectively. XRD and TEM showed that the primary crystal phase of the composite was HA together with the diffraction peaks of carbon nanotube. By SEM, we found that MWNTs are homogeneously dispersed within grains or at grain boundaries of the HA matrix in composites which MWNTs’ content was not more than 15vol%, otherwise MWNTs tended to be agglomerated. The reinforcement mechanism was discussed based on the microstructure investigation. The broken nanotubes and pullout of MWNTs at interfaces were efficient in transferring the load from the HA matrix to the nanotubes, leading to the improvement of the mechanical properties.

Fabrication of Nanocomposite Thermoelectric Materials by a Pulsed Laser Deposition Method

We applied a pulsed laser deposition (PLD) technique to fabricate nanocomposite half-Heusler thermoelectrics by employing two different methods: a dry process and a wet process. First, we tried to obtain nanosized thermoelectric particles by using PLD in a liquid solvent. Nanosized (<100 nm) spherical and crystalline half-Heusler particles containing Ti, Zr, Hf, Ni, and Sn elements were obtained by this method, showing good controllability of stoichiometry. The key is to select a solvent that prevents oxidation. Second, the dry PLD process was employed to coat the thermoelectric powder with metal oxides. To this end, we developed a PLD coating apparatus. After sintering the coated powder using the spark plasma sintering (SPS) technique, we confirmed that a nanosized layer of the metal oxides was uniformly formed at the grain boundaries of the half-Heusler matrix. With these two examples, the capability of the PLD techniques to fabricate well-controlled nanocomposite thermoelectric materials is demonstrated.

Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr

Cr-doped Al2O3/TiAl in situ composites were successfully fabricated by hot-press-assisted exothermic dispersion method with elemental powder mixtures of Ti, Al, TiO2 and Cr. The effect of the Cr addition on the microstructures and mechanical properties of the Al2O3/TiAl composites were investigated. The as-synthesized composites consisted of γ-TiAl, α2-Ti3Al, Al2O3, and a small amount of Laves phase Ti(Al,Cr)2. Microstructure analysis indicates that Al2O3 particles mainly distributed on grain boundaries of the TiAl matrix. As the Cr content increases, the Al2O3 particles are dispersed more fine and homogeneously in the TiAl matrix, which resultes in the increase of the hardness, flexural strength and fracture toughness of the composites gradually. When the Cr content was 3 wt.%, the flexural strength and fracture toughness of the Al2O3/TiAl composite reaches the top values of 820 MPa and 8.3 MPa·m1/2, respectively. The reinforcement mechanism of Cr addition to Al2O3/TiAl composites was also discussed.

Comparative Study of Cu Diffusion in Ru and Ru-C Films for Cu Metallization

In this paper, the diffusivities of copper in 15 nm thick Ru and Ru–C barrier layers are determined experimentally by using sheet resistance and X-ray diffraction measurements with the Cu/barrier/Si samples. By fitting the dependence of diffusivities on temperature, the activation energy for Cu diffusion in Ru–C film is 1.11 eV, which is substantially higher than that in Ru film (0.54 eV). Microstructural analysis by transmission electron microscopy combined with energy-dispersive X-ray suggests that the higher activation energy for Cu diffusion in Ru–C is associated with the stuffing of added carbon atoms on the grain boundaries of the Ru matrix and consequently leading to the superior diffusion barrier performance of the Ru–C film.

Age-hardening by miscibility limit in a multi-purpose dental gold alloy containing platinum

This study examined the age-hardening by miscibility limit in a multi-purpose dental gold alloy containing platinum. The hardness increased rapidly in the initial stage of the aging process, reached the maximum value, then decreased continuously with aging time. The significant hardness increase resulted from the heterogeneous precipitation of the Pt-rich β phase from the grain boundary of the Au-rich α1 matrix due to the miscibility limit of Au-Pt system. With increasing aging time, the fine Pt-rich β precipitates covered almost the whole matrix, and by further aging, the precipitates grew coarse. The microstructural coarsening reduced the interface between the Au-rich α1 matrix and the Pt-rich β precipitates, which released the lattice strains between the two phases, resulting in a softening effect. In the later stage of aging process, the Au-containing Pt3In particle-like structure was transformed into the Au-depleted particle-like structure containing relatively large amounts of Cu resulting from the overlapping miscibility limit of both Au-Pt and Ag-Cu systems, which was responsible for the slow decreasing rate in hardness in the later stage of aging.

Fluctuation induced magneto-conductivity studies in YBa 2Cu 3O 7−δ + xBaZrO 3 composite high- T c superconductors

Fluctuations on the electrical conductivity of polycrystalline YBa 2Cu 3O 7−δ + xBaZrO 3 ( x = 1.0, 2.5, 5.0 and 10.0 wt.%) superconductors were investigated from the resistivity vs. temperature data for zero field and 8 T (Tesla) external magnetic fields. Attempts have been made to identify the optimum inclusion of BaZrO 3 (BZO) in YBa 2Cu 3O 7−δ (YBCO) superconductors. The phase formation, texture and grain alignments were analyzed by XRD and SEM techniques. Then the effects of superconducting fluctuations on the electrical conductivity of granular composite superconductors were studied for zero field and 8 T external magnetic fields. Though inclusions of BZO sub-micron particles are not expected to influence superconducting order-parameter fluctuation (SCOPF) much, the transition from 2D to 3D of the order parameter in the mean-field region depends on the BZO content in the composites. It has been observed that BZO residing at the grain boundary of YBCO matrix influences the tailing region without having significant change in the mean-field critical temperature. In the present work, attention has been focused mostly in the experimental domain relatively above the T c . It reveals that, 1 wt.% composite exhibits a better superconducting property in comparison with pure YBCO.

Fabrication of Alumina-Based Metal Nanocomposites by Pressureless Sintering and Their Mechanical Properties

The processing conditions to prepare nano-sized Cu and Mo dispersed Al2O3 (Al2O3/Cu and Al2O3/Mo) composites by pressureless sintering were explored. The composite powders of Al2O3/Cu and Al2O3/Mo were obtained by the hydrogen reduction of Al2O3/CuO and Al2O3/MoO3 powder mixtures and consolidated by pressureless sintering using infrared heating furnace with a heating rate of 200 degrees C/min. SEM and TEM analyses for the composite showed that the nano-sized metal particles were well distributed and situated on the grain boundaries of the Al2O3 matrix. The nanocomposites, sintered at 1300 to 1500 degrees C for 4 min, showed the relative density of above 90%. Maximum hardness of 16.1 GPa was obtained in Al2O3/Cu nanocomposites with sintering additive of 1 wt% MgO. The sintered nanocomposites exhibited the enhanced fracture toughness of above 4.5 MPa x m(1/2), compared with monolithic Al2O3. The mechanical properties were discussed in terms of observed microstructural characteristics.

Structural-phase state and physicomechanical properties of degraded D16- and V95-type aluminum alloys

We have established that the decrease in the resource mechanical characteristics of D16- and V95-type aluminum alloys after model and in-service degradation is connected with the changes in their fine structure under the influence of thermomechanical factors: the increase in the quantity of disperse (0.01 to 0.05 μm in size) second-phase precipitates, the microcracking of intermetallic inclusions (0.1 to 0.5 μm in size) formed in the course of ingot homogenization, the growth of dislocation density, and the aggregation of plasticity bands near grain boundaries of the matrix. These phenomena explain the sharp decrease in plasticity and sensitivity to the grain-boundary fracture of degraded alloys in a corrosive medium as well as the qualitatively opposite, as compared with reference data, laws of changes in their structure-sensitive physical characteristics, in particular, specific conductivity.

Microstructure and formation mechanism of cerium conversion coating on alumina borate whisker-reinforced AA6061 composite

Cerium conversion coatings, formed on alumina borate whisker-reinforced aluminum composite with pre-treatment for different time, were characterized by SEM/EDX, TEM and electrochemical method. The structure and formation mechanism of the coatings were investigated in detail. It was found that the pre-treatment prior to coatings has strong influence on formation, growth and morphology as well as granular size of coatings. Two typical coating morphologies existed at the pre-treated substrate surfaces. Cracks only presented on thick coatings because of the higher deposition rate and the greater inner stress. Cracks preferably produced at the whisker/matrix interface and the grain boundary of matrix.

Negative and linear positive magnetoresistance effect in silver-rich silver selenide

In the non-magnetic semiconductor silver selenide (Ag 2+ δ Se) with a minor heterogeneous silver excess (0.79 × 10 −4 < δ ≤ 1 × 10 −2) we measured either a saturating negative magnetoresistance (MR) effect, a linear positive MR effect or a superposition of both. This complex MR behavior depends on the amount of the silver metal excess, but is also strongly influenced by the thermal treatment of the samples. Excess silver that cannot be accommodated homogeneously in the silver selenide lattice creates structural heterogeneities and forms a microstructure which is controlled by the thermal annealing procedure. We suggest that small silver segregations at the grain boundaries of the silver selenide matrix (“nanofilms”) are responsible for the negative MR effect, whereas nanoscale silver particles within the silver selenide matrix (nanoparticles) cause a linear positive effect.

Effects of silver-based metal electroplate on fatigue properties of PZT ceramics

The effects of silver-based metal plating for electrodes on the fatigue properties of PZT ceramics were examined. The plating was created on the surface of the PZT by a firing process carried out at approximately 973 K for several hours. Due to the high-temperature of the firing process, the silver-metal infiltrated the grain boundaries of the PZT matrix. This penetration made the PZT ceramics brittle and reduced the mechanical strength of the ceramics, e.g., hardness and bending strength. The fracture characteristics of this material are also attributed to the silver-based plated electrode; fractures of transgranular-type occurred near the electrode whilst fractures were intergranular in other areas. These changes of the fracture surface were influenced by the speed of crack growth arising from the roughness of the plated surface; a rough surface gives rise to transgranular fractures due to the high growth rate of the cracks. The roughness of the electroplated surface leads to a reduction in the mechanical and fatigue strength.

Oxidation behavior of Sn–Zn solders under high-temperature and high-humidity conditions

Abstract The oxidation behavior of Sn–9Zn and Sn–8Zn–xBi (x = 1, 3, 6) solders under high-temperature and high-humidity conditions has been investigated as a function of exposure time. The evolution of surface and cross-sectional microstructures during exposure has been examined, and an oxidation model for the Sn–Zn alloys was proposed. The poor oxidation resistance of Sn–Zn alloys is attributed to the oxidation of element Zn, which diffuses to the grain boundaries of Sn matrix, forming ZnO along those boundaries. It was also found that Bi forms a solid solution in Sn with a solubility limit of 4.4 wt.%, which causes the Sn matrix to become liable to form cracks, as well as to have a high distortion energy. The solid solution of Sn(Bi) is considered to be one of the reasons for the rapid oxidation of Bi added alloys.

Effect of Al(Cr) Content on the Distribution of Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;(Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) Particles Formed by Internal Oxidation

This investigation was conducted to study the effect of Al(Cr) content in Cu-Al(Cr) pre-alloyed powders on the distribution of Al2O3(Cr2O3) particles formed by internal oxidation. The results show that the distribution of Al2O3 particles is different with the change of Al content in Cu-Al system. When the Al content is above 1.2wt.%, the Al2O3 particles tend to distribute along the grain boundaries of copper matrix , whereas when the Al content is above 2.0 wt.%, the Al2O3 particles have distributed along the grain boundaries. However, it is interesting to note in Cu-Cr system that most of the formed Cr2O3 particles prefer to distribute in the grain of copper matrix when the Cr content is less than 5wt.%, which is quite different from that of Cu-Al system.

Dilution rate and microstructure of TIG arc Ni-Al powder surfacing layer

Surfacing beads are prepared by a direct current tungsten inert gas arc nickel-aluminum (Ni-Al) powder surfacing process. With the aim of controlling the dilution rate and obtaining surfacing beads rich in intermetallic compounds, the effects of surfacing parameters on geometric parameters, dilution rate, composition, and microstructure of the bead are investigated. An assistant cooler, which can potentially reduce the temperature of the base metal, is used in the surfacing process and its effect on dilution rate and microstructure is studied. The result indicates that with the surfacing parameter combination of low current and speed, the width and penetration of the bead decrease, reinforcement increases, and dilution rate drops markedly. With the reduction of the parameter combination, the intergranular phase γ-(Fe, Ni) is formed in the grain boundaries of Ni-Al intermetallic matrix instead of the intergranular phase α-Fe, and large amount of intermetallics are obtained. With the use of an assistant cooler on a selected operation condition during the surfacing process, the reinforcement of the bead increases, penetration decreases, and dilution rate declines. The use of an assistant cooler helps obtain a surfacing bead composed of only intermetallics.

Electric current-induced abnormal Cu/γ-InSn4 interfacial reactions

The electromigration effect upon the γ-InSn4/Cu interfacial reactions have been studied by examining the γ-InSn4/Cu/γ-InSn4 couples annealed at 160 °C with and without current stressing. Scallop-type η-Cu6(Sn,In)5 phase layers are formed in the couples without current stressing and at the γ-InSn4/Cu interface where electrons are flowing from the γ-InSn4 to the Cu. The reaction path is Cu/η-Cu6(Sn,In)5/γ-InSn4. However, very large η-Cu6(Sn,In)5 compounds are found at the Cu/γ-InSn4 interface where electrons are from Cu to the γ-InSn4. Although the melting points of both γ-InSn4 and Cu are higher than 160 °C, the liquid phase is formed at 160 °C in the electrified couple at the downstream γ-InSn4 phase near the Cu/γ-InSn4 interface. The reaction path is Cu/η-Cu6(Sn,In)5/liquid/γ-InSn4. The liquid phase propagates along the grain boundaries of the γ-InSn4 matrix. The very large η-Cu6(Sn,In)5 compounds are the coupling results of the liquid phase penetration and the Cu transport enhancement due to electromigration.

Interface stability of the SiC particles/Fe matrix composite system

The interface reaction between the SiC particles (SiCp) and Fe was studied during sintering the SiCp reinforced Fe matrix composites at 1423 K for 1 h. In the composite having 3wt% (weight ratio) SiCp (the 3SiCp/Fe composite), the interface reaction products of Fe3Si, the carbon precipitates, and Fe3C or pearlite were generated. Fe3Si constructs the bright matrix of the reaction zone in the original situation of the SiCp. The carbon precipitates, are randomly embedded in the reaction zone. Fe3C or pearlite exists at the grain boundaries of the Fe matrix. As increasing the SiCp concentration in the SiCp/Fe composite, the intensity of the interface reaction between SiCp and Fe increases. After the 10SiCp/Fe composite (having 10wt. % SiCp) sintered at 1423 K for 1 h, all of SiCp are decomposed, and replaced by the reaction zone composed of Fe3Si and the carbon precipitates. No Fe3C or pearlite was generated during the reaction. The effects of the techniques of oxidizing of SiCp, coating SiCp by interaction with the Cr powder, and alloying the Fe matrix by adding the Cr element on the interface stability of the SiCp/Fe composite system were also investigated, respectively. The oxide membrane and the coating layer on SiCp can inhibit the interface reaction between SiCp and Fe by isolating SiCp from the Fe matrix during sintering. The interface reaction does not occur in the 3 SiCp/Fe-10Cr composite but in the 3 SiCp/Fe-5 Cr composite. In the SiCp/Fe−Cr alloy composites, the interface reaction between SiCp and the Fe−Cr alloys is weaker than, that between SiCp and Fe. The Cr element behaves as a diluent, it causes a reduction in the interface reaction, which is proportional to the amount of the element added.

Synthesis of YbyCo4Sb12∕Yb2O3 composites and their thermoelectric properties

Composites containing Yb-filled CoSb3 and well-distributed Yb2O3 particles are synthesized by in situ reaction method. The structural, chemical, and transport properties of the composites are studied. Some Yb2O3 particles with microsize locate at the grain boundaries of matrix and others distribute within YbyCo4Sb12 grains as nanoscale inclusions. The combination of the “rattling” of Yb ions inside the voids of CoSb3 and the phonon scattering of the oxide defects results in a remarkable reduction in the lattice thermal conductivity. The thermoelectric performance of the composites is significantly improved, and the maximum figures of merit reach 1.3 for the Yb0.25Co4Sb12∕Yb2O3 and 1.2 for the Yb0.21Co4Sb12∕Yb2O3 composites at 850K.

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al2O3). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.