Papers by Keyword: Intermetallic

Abstract: The reliability of QFP (Quad flat package) solder joint using Sn-8Zn-3Bi solder paste under the thermal shock test was investigated. Considering the environmental restriction such as ROHS, the QFP Cu LF (Lead-frame) was coated with lead-free materials (Sn, Sn-3Bi). To analyze the reliability under thermal shock treatment, the samples were placed in the thermal shock chamber (248K/423K, Dwell time: 30min). During the thermal shock test, the solder joint cross-sections were observed every 500 cycles. No crack initiation and propagation was observed through all type of plated Cu LF. The measured pull strength slightly decreased, as the number of cycles increased. After 1000 cycles, the pull strength of Sn and SnBi plated Cu LF reduced by 30% and 20%, respectively, compared with that of initial condition. Observing the fracture surface morphology by FE-SEM, the fracture mode changed and the IMC fractured area on the both fracture surface was increased. The IMC was identified as γ-Cu5Zn8 by chemical composition analysis using EDS. The reduced pull strength was affected by IMC fracture and fracture mode change. However, the pull strength does not decrease steeply but gradually. Consequently, the Sn-8Zn-3Bi solder joint shows the reliable solder joint strength, fracture mechanism, and compatibility with lead-free plated Cu LF during the thermal shock temperature of 248K to 423K.

Abstract: Processes of diffusion interaction between solid and liquid phases take place in such phenomena as wetting, spreading, dissolution, contact melting, liquid metal corrosion, changes of mechanical properties of metals and alloys being in contact with melts, etc. The knowledge of the mechanism and regularities of this interaction is necessary for development of such technologies, as sintering, self-propagating high temperature synthesis, impregnation, casting, welding, soldering, liquid metal cladding, etc. However, in despite of the fact that the problem of diffusion interaction between solid and liquid metals is of the paramount importance as for science as well for practice, no intelligible theory of the phenomenon is available. The paper fills the gap in our knowledge of this subject.

Abstract: Chemical ordering kinetics in L10- and B2-ordered AB binary intermetallics was simulated by means of Monte Carlo (MC) technique implemented with vacancy mechanism of atomic migration. While vacancy concentration is usually much lower than the antisite defect concentration in L10-ordered systems, triple defects are generated in particular B2–ordered systems. The latter definitely affects the chemical ordering process and requires that full thermal vacancy thermodynamics is involved in B2-ordering simulations. The study on L10-ordered binaries was dedicated to FePt thin layers considered as a material for ultra-high-density magnetic storage media. Metastability of the L10 c-variant with monoatomic planes parallel to the layer surface and off-plane easy magnetization was revealed. Thermal vacancy formation in B2-ordered binaries was modelled by implementing a mean-field Hamiltonian with a specific formalism of phase equilibria in a latticegas composed of atoms and vacancies. It was demonstrated that for particular pair-interaction energetics, equilibrium concentrations of vacancies and antisites result mutually proportional in well-defined temperature ranges. The MC simulations of B2-ordering kinetics involved the modelled equilibrium vacancy concentration and reproduced the experimentally observed low rate of the process.

Abstract: Paper describes solid state phenomena occurring during the formation of interconnections intended to the electronic industry. Diffusion soldering technology is an undemanding and functional tool to use when the growth of intermetallic phase(s) is desired and controlled in order to obtain the joint consisted solely of an intermetallic phase. Scanning and transmission electron microscopy investigations of the microstructure and sequence of appearance of the intermetallics within the joined area are discussed. The diffusion path describing the chemical composition changes across the interconnection was found to correlate with microstructure transformations. The growth kinetics of the δ′[Cu41(Sn,In)11] phase was determined in the temperature range of 300 – 350 °C.

Abstract: Average ternary interdiffusion coefficients in Ni3Al with Ir additions have been determined using solid-to-solid diffusion couples annealed at 1200°C for 5 hours. Disc shaped alloy specimens were prepared by the vacuum arc melting at compositions of Ni-24Al, Ni-25Al, Ni-26Al, Ni-23.5Al-1Ir, Ni-24.5Al-1Ir, Ni-23Al-2Ir, Ni-23Al-2Ir, Ni-24Al-2Ir, Ni-23Al-3Ir (at.%). Surfaces of alloys were polished down to 1200 grit and diffusion couples were assembled in Si3N4 jig for initial bonding heat treatment at 1200°C for 0.5 hours. Additional diffusion anneal was carried out at 1200°C for 4.5 hours outside of Si3N4 jig so that diffusion couples can be water quenched. Concentration profiles of individual components were measured by electron probe microanalysis using pure standard of Ni, Al and Ir. Interdiffusion flux of individual component was determined directly from the experimental concentration profiles, and the moments of interdiffusion flux were examined to calculate the average ternary interdiffusion coefficients, D˜ ij k either with Al or Ni as dependent component. Calculated interdiffusion coefficients suggest that Ir-alloyed Al2O3-forming oxidation resistant coatings would be beneficial to reduce the interdiffusion flux of Ni from superalloy substrates to the coating, and reduce the interdiffusion flux of Al from the coating to the superalloy substrate.

Abstract: The addition of aluminium (and of silicon) to steel increases its electrical resistivity and reduces therefore the power losses in electrical devices. There is also a favourable effect on magnetostriction. Nevertheless, these additions make the steel extremely brittle and very difficult to process through a conventional thermomechanical route. The authors developed an innovative processing route, avoiding the rolling of a brittle steel sheet. The used process consists of the hot dipping of a steel substrate in a pure aluminium bath, followed by a diffusion annealing treatment. In order to study the reaction of the aluminium with the substrates and the diffusion process during further annealing, two substrates (ultra low carbon steel (ULC) and a Fe + 3.4 m.-% Si steel) were used for immersion in a pure aluminium bath. Dipping times and temperatures were varied in the range of 700 to 750 °C and 5 to 1000 sec., respectively. The different surface layers formed during dipping and after annealing were characterised with an Elcometer, by Scanning Electron Microscopy (SEM) and by Energy Dispersive Spectroscopy (EDS). The results show that the chemical composition of the layers obtained is strongly dependant on the initial substrate composition. Diffusion gradients of Al and Si in the steel after hot dipping and diffusion annealing are shown and discussed. Samples with a concentration gradient of Si and Al over the thickness have been produced. There is only a light reduction of the power losses for the substrate with 3.4 m.-% Si. The ultra low carbon substrate presents worse power losses after the processing. Further improvement of the processing is still required.

Abstract: Laser cladding experiment was carried out with a 5 kW continuous wave CO2 laser by preplacing Ni75Si25 and Ni78Si13Ti9 powders onto Ni-based superalloy substrate. The microstructure of the specimens was monitored by using optical and scanning electron microscopy. The chemical compositions of the alloys and their phases were obtained using X-ray diffraction and energy dispersive x-ray spectroscopy. The phase transformation temperatures were determined by non-isothermal differential scanning calorimetry tests. The microhardness of the laser cladded sample was measured.

Abstract: Transmission electron microscopy was used to study various morphologies of ordered precipitates in Ni3Al-Cr intermetallic alloy. It was found that γ’ precipitates in the interdendritic regions have the microstructure of a nickel base superalloy with its characteristic features. In addition to the primary γ’ precipitates some fine secondary precipitates were found.

Abstract: The ideal tensile strength along the [111] direction in the Fe3Al and Ni3Al intermetallic compounds with the D03 structure has been calculated from the first principles using the fullpotential linearized augmented plane-wave method (FP LAPW) within the density functional theory (DFT). The strains corresponding to the maximum sustainable stresses in both materials were determined and compared. The behavior of atomic magnetic moments as a function of strain was analyzed. The tensile test simulations have been theoretically simulated employing both the local density approximation (LDA) and generalized gradient approximation (GGA) for the exchangecorrelation potential.

Abstract: Intermetallic titanium aluminide alloys are multiphase assemblies with complex microstructure and constitution, involving the phases γ(TiAl), α2(Ti3Al), β, and B2. The earlier stages of phase transformation and dynamic recrystallization occurring upon hot-working of such an alloy were investigated at the atomic scale by high-resolution electron microscopy. Accordingly, the conversion of the microstructure is triggered by heterogeneities in the deformation state and non-equilibrium phase composition. The β/B2 phase is apparently unstable under tetragonal distortion, which gives rise to the formation of the B19 phase via distinct shuffle displacements. These processes lead to a modulated microstructure, which is comprised of several stable and metastable phases. The phase transformations are accomplished by the propagation and coalescence of ledges. Large and broad ledges can apparently easily be rearranged into intermediate metastable structures, which serve as precursor for the nucleation of new grains.