Size effect on IMC growth induced by Cu concentration gradient and pinning of Ag3Sn particles during multiple reflows

Size effect on growth kinetics of interfacial intermetallic compound (IMC), induced by Cu concentration gradient and pinning effect of Ag₃Sn particles during multiple reflows, was investigated in this article. The simulation results, for Cu distributions in solder bulks of different volumes after a single reflow for 60 s at 250 °C, show that Cu concentration gradient in liquid increases with the growing size of solder bump. On the contrary, resistive pressure of nano particles decreases gradually with the increasing bump size. In conclusion, the pinning effect of Ag₃Sn particles on IMC grains plays a dominant role in small samples, whereas the inhibiting effect of Cu concentration gradient is mainly functional in big samples. Combining the two factors, solder bump in an intermediate diameter of 800 μm benefits most and has the largest IMC thickness during multiple reflows.     

Microstructural evolution and mechanical properties of niobium processed by equal channel angular extrusion up to 24 passes

We have systematically investigated the microstructural evolution of niobium (Nb) subjected to severe plastic deformation via equal channel angular extrusion (ECAE) up to 24 passes. The starting Nb billet material consists of a centimeter-scale grain size with a columnar structure. We have found that the grain size reduction of the Nb is almost saturated at ∼300 nm after eight passes of ECAE. However, the population of high-angle grain boundaries continues to increase with further ECAE, and no saturation appears to have been reached at 24 passes. We have evaluated the mechanical properties of the samples with different number of ECAE passes over a wide range of strain rates, from quasi-static to high strain rates. We have used strain-rate jump tests to examine the strain-rate sensitivity (SRS) of the processed samples and found that the SRS of the ECAE-processed Nb is ∼0.012, which is a factor of three smaller than that of the coarse-grained counterpart. The activation volume derived for plastic deformation indicates that the double-kink formation of screw dislocations is still the predominant deformation mechanism in the ECAE-processed Nb. Quasi-static true stress-strain curves exhibit elastic-nearly perfectly plastic behavior. The quasi-static yield strength is also nearly saturated after eight passes of ECAE. High-strain-rate compressive true stress-strain curves show uniform flow softening. However, the dynamic peak stress keeps rising with an increased number of ECAE passes, suggesting a strong grain boundary contribution to dynamic strengthening. Scanning electron microscopy of post-loaded surfaces displays a morphology of diffuse shear bands accompanying highly compressed grains. In our report, we demonstrate that grain boundaries of severely deformed metals play different roles at low, quasi-static vs. high-strain rates of mechanical loading. The difference is primarily determined by the strength of grain boundaries acting as dislocation barriers at different loading rates. This discovery is significant for the understanding of the effect of the microstructure as a function of the applied loading rate.     

Investigation of crystallisation kinetics of Bi0·5Se99·5-xZnx glasses by differential scanning calorimetry

Abstract

The kinetics of crystallisation in Bi_0·5Se_99·5-xZn_x (x=0, 0·1, 0·2, 0·5, 1·0) glasses are studied by non-isothermal method using differential scanning calorimetry. Differential thermal analysis was performed at different heating rates of 5, 10, 15, and 20 K min^-1. The values of glass transition and crystallisation temperatures are found to be composition and heating rate dependent. From the heating rate dependence of the glass transition and crystallisation temperatures, the activation energy of crystallisation ΔE _c, the order parameter n and the enthalpy released ΔH _c are calculated. The thermal stability (T _c–T _g) was found to be maximum for Bi_0·5Se_99·5 glass, which suggest that this glass can be considered as a critical composition at which the system becomes chemically ordered. The crystallisation enthalpy ΔH _c is maximum for Bi_0·5Se₉₉Zn_0·5 glass, hence it is the least stable glass in the present system.     

Impact of the Zr-substitution on phase composition,structure, magnetic,and microwave properties of the BaM hexaferrite

Rapid developments in information technologies and a large rise in electrical and electronic equipment have generated different forms of electronic environmental contamination. Microwave absorption materials are important to avoid the damage that can be caused by electromagnetic radiation. A necessary condition for the absorption of the most incoming radiation is balanced wave impedance at the air/shield interface, which depends on the studied materials' magnetic and electrical properties. The paper introduces the preparation of BaFe_12-xZr_xO₁₉ (x = 0.1, 0.3, 0.6, 0.9, and 1.2) using a solid-state reaction technique. The studied samples were examined by X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer, and a vector network analyzer. The studied samples showed that controlling the grain size and the prepared samples' magnetic properties play an important role in enhancing the microwave radiation absorption. The examined samples can be a promising absorption material in electromagnetic shielding applications.     

A constitutive equation for hot deformation range of 304 stainless steel considering grain sizes

A general constitutive equation based on the framework of invariant theory by consideration of hot deformation key variables and also the properties of the material such as initial grain size is presented in the current work. Soundness of the considered parameters to be used in the developed formula was initially verified based on the important axioms such as objectivity, entropy principle, and thermodynamics stability. To access the prediction ability of the method, the formula was simplified for the simple hot compression test. To evaluate the simplified formula, single-hit hot compression tests were carried out at the temperature range of 900–1100 °C under true strain rate of 0.01–1 s⁻¹ on a AISI 304 stainless steel. The capability of proposed formula for reproducing the variation of flow stress with strain and the strain hardening rate with stress for the resultant flow stress data was examined. The good agreement between model predictions and actual results signified the applicability of this method as a general constitutive equation in hot deformation studies.     

Effect of Solidifcation Rate on Grain Structure Evolution During Directional Solidifcation of a Ni-based Superalloy

The effect of solidification rate on grain structure evolution during directional solidification(DS) of a Ni-based superalloy was explored.It was found that a high solidification rate led to sharper <001> texture and smaller grain size in the DS samples.One of the most important findings in this work was that such result was not in accordance with the general concept,and the sharper <001> texture was accompanied by the larger grain size.To explain the contradiction,the modeling samples with five grains were produced and the effect of solidification rate on the evolution of grain texture was illustrated based on the modeling samples.     

Numerical approach of cyclic behaviour of 316LN stainless steel based on a polycrystal modelling including strain gradients

A non-local polycrystal approach, taking into account strain gradients, is proposed to simulate the 316LN stainless steel fatigue life curve in the hardening stage. Material parameters identification is performed on tensile curves corresponding to several 316LN polycrystals presenting different grain sizes. Applied to an actual 3D aggregate of 316LN stainless steel of 1200 grains, this model leads to an accurate prediction of cyclic curves. Geometrical Necessary Dislocation densities related to the computed strain gradient are added to the micro-plasticity laws. Compared to standard models, this model predicts a decrease of the local stresses as well as a grain size effect.     

The effect of homogeneously dispersed few-layer graphene on microstructure and mechanical properties of Al2O3 nanocomposites

Fully dense few-layer graphene (FG)/Al₂O₃ nanocomposites with homogeneously dispersed FG in matrix are prepared by using a heteroaggregation method followed by spark plasma sintering. It is found that the two dimensional FG has great ability to restrain grain growth in comparison to other inclusions. In addition, the morphology of grain in composite is modified by the addition of FG during densification process compared with monolithic alumina. Thanks to the greatly decreased grain size, the composites are almost as hard as monolithic alumina at low sintering temperature (1573 K) even if graphene content is as high as 1.2 vol.%. However, at higher sintering temperature (1673 K), the hardness of composites decreases further but the change in elastic modulus is very limited. The decline of hardness and elastic modulus mainly arises from the sliding feature of FG, low modulus of reduced graphene oxide in both in-plane and out-of-plane directions.     

Origin of significantly enhanced dielectric response and nonlinear electrical behavior in Ni2+-doped CaCu3Ti4O12: Influence of DC bias on electrical properties of grain boundary and associated giant dielectric properties

CaCu_3-xNi_xTi₄O₁₂ (x?=?0, 0.05, and 0.10) powders were synthesized using a solid state reaction method. Phase structure and microstructure analyses revealed that all sintered CaCu_3-xNi_xTi₄O₁₂ ceramics were of a pure phase. The CaCu₃Ti₄O₁₂ ceramics had a dense microstructure and grain sizes were enlarged by doping with Ni²⁺. Interestingly, the dielectric permittivity was significantly enhanced, whereas the loss tangent was greatly suppressed to ～0.046–0.034 at 1?kHz. All sintered ceramics exhibited non-Ohmic characteristics. Clarification of the influences of DC bias showed that the dielectric permittivity and loss tangent values were increased by DC bias. The resistance of grain boundaries and the associated conduction activation energy of CaCu_3-xNi_xTi₄O₁₂ ceramics were reduced as the DC bias voltage increased. Therefore, the observed non-Ohmic behavior and significantly enhanced dielectric properties should be closely related to variation in the Schottky barriers at the grain boundaries.