Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni-Al nanowires

Extensive molecular dynamics (MD) simulations have been performed in a B2-NiAl nanowire using an embedded atom method (EAM) potential. We show a stress induced B2 -> body-centered-tetragonal (BCT) phase transformation and a novel temperature and cross-section dependent pseudo-elastic/pseudo-plastic recovery from such an unstable BCT phase with a recoverable strain of similar to 30% as compared to 5-8% in polycrystalline materials. Such a temperature and cross-section dependent pseudo-elastic/pseudo-plastic strain recovery can be useful in various interesting applications of shape memory and strain sensing in nanoscale devices. Effects of size, temperature, and strain rate on the structural and mechanical properties have also been analyzed in detail. For a given size of the nanowire the yield stress of both the B2 and the BCT phases is found to decrease with increasing temperature, whereas for a given temperature and strain rate the yield stress of both the B2 and the BCT phase is found to increase with increase in the cross-sectional dimensions of the nanowire. A constant elastic modulus of similar to 80 GPa of the B2 phase is observed in the temperature range of 200-500 K for nanowires of cross-sectional dimensions in the range of 17.22-28.712 angstrom, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

Superplasticity in intermetallic NiAl nanowires via atomistic simulations

A novel superplastic deformation in an intermetallic B2-NiAl nanowire of cross-sectional dimensions of similar to 20 angstrom with failure strain as high as similar to 700% at 700 K temperature is reported. The minimum temperature under which the superplasticity has been observed is around 0.36 T-m, which is much lower than 0.5 T-m (T-m = melting temperature i.e. 1911 K for bulk B2-NiAl). Superplasticity is observed due to transformation from crystalline phase to amorphous phase after yielding of the nanowire. (C) 2010 Elsevier B.V. All rights reserved.

Annealing response of the intermetallic alloy Ti-22Al-25Nb

A hot rolled two-phase Ti-22Al-25Nb (at.%) alloy containing the orthorhombic (O) and beta(B2) phases was subjected to thermal treatment under different conditions. The experiment was aimed to examine the recrystallization response of the beta(B2) phase (static and dynamic) to microstructure and crystallographic texture evolution using scanning electron microscopy coupled with electron backscattered diffraction (SEM-EBSD). Specimens rolled in the two-phase (O + beta(B2)) region consisted of highly deformed beta(B2) grains. The texture was close to that of the typical bcc deformation texture with a few additional texture components. A subsequent heat treatment of these rolled specimens in single beta(B2) phase region was characterized by static recrystallized beta(B2) grains with the final texture partly inherited from as-rolled material. In contrast, specimens rolled in the single beta(B2) region produced beta(B2) grains with the texture similar to that of completely dynamic recrystallized one. (C) 2010 Elsevier Ltd. All rights reserved.

Asymmetry in structural and thermo-mechanical behavior of intermetallic NiAl nanowire under tensile/compressive loading: A molecular dynamics study

The asymmetric stress strain behavior under tension/compression in an initial < 100 > B-2-NiAl nanowire is investigated considering two different surface configurations i.e., < 100 >/(0 1 0) (0 0 1) and < 100 >/(0 1 1) (0 - 1 1). This behavior is attributed to two different deformation mechanisms namely a slip dominated deformation under compression and a known twinning dominated deformation under tension. It is also shown that B2 -> BCT (body-centered-tetragonal) phase transformation under tensile loading is independent of the surface configurations for an initial < 100 > oriented NiAl nanowire. Under tensile loading, the nanowire undergoes a stress-induced martensiticphase transformation from an initial B2 phase to BCT phase via twinning along {110} plane with failure strain of similar to 0.30. On the other hand, a compressive loading causes failure of these nanowires via brittle fracture after compressive yielding, with a maximum failure strain of similar to-0.12. Such brittle fracture under compressive loading occurs via slip along {110} plane without any phase transformations. Softening/hardening behavior is also reported for the first time in these nanowires under tensile/compressive loadings, which cause asymmetry in their yield strength behavior in the stress strain space. Result shows that a sharp increase in energy with increasing strain under compressive loading causes hardening of the nanowire, and hence, gives improved yield strength as compared to tensile loading. (C) 2010 Elsevier Ltd. All rights reserved.

Mean square displacements of hydrogen interstitial and its neighbours in the intermetallic compound Fe0.5Ti0.5

A Green's function technique is used in the scattering matrix formalism to compute the mean square displacement of hydrogen and deuterium interstitials in the intermetallic compound Fe0.5Ti0.5 for low hydrogen/deuterium concentration. The mean square amplitudes of the metal atoms surrounding the interstitial are found to be smaller than those for the host crystal. This anomalous effect is due to the stiffening of the lattice by the dissolved hydrogen or deuterium at low concentration. This type of effect is experimentally observed in the case of NbHx at low hydrogen concentration.

Intermetallic Ti2Ni/TiNi nanocomposite by rapid solidification

Nanoscale dispersions of intermetallic Ti2Ni particles in an ordered TiNi intermetallic matrix have been produced by rapid solidification processing of near equiatomic TiNi alloys containing small amount of Si utilising the principle of kinetic competition in the undercooled liquid, A detailed characterisation of the microstructures obtained by different processing conditions was carried out to establish the trend of the refinement of the scale of microstructure. The observed microstructural conditions are rationalised in terms of a metastable phase diagram and the Uhlmann, Chalmers and Jackson theory of the trapping of second phase particles by a moving interface.

Effect of Constant and Cyclic Current Stressing on the Evolution of Intermetallic Compound Layers

In this paper the effects of constant and cyclic power loads on the evolution of interfacial reaction layers in lead-free solder interconnections are presented. Firstly, the differences in the growth behavior of intermetallic compound (IMC) layers at the cathode and anode sides of the interconnections are rationalized. This is done by considering the changes in the intrinsic fluxes of elements owing to electromigration as well as taking into account the fact that the growth of Cu3Sn and Cu6Sn5 are coupled via interfacial reactions. In this way, better understanding of the effect of electron flux on the growth of each individual layer in the Cu-Sn system can be achieved. Secondly, it is shown that there is a distinct difference between steady-state current stressing (constant current, constant temperature) and power cycling with alternating on- and off-cycle periods (accompanied by a change of temperature). The reasons behind the observed differences are subsequently discussed. Finally, special care is taken to ensure that the current densities are chosen in such a way that there is no risk for even partial melting of the solder interconnections.

Role of in situ generated tribofilm on the tribological characteristics of monolith and TiB2 reinforced MoSi2 intermetallic

Wear experiments performed on steel disc with increasing load for monolithic MoSi2 of different densities and its composite with TiB2 showed three distinct wear regimes. The specimens exhibited severe wear rate below the lower and above the upper critical loads and mild wear in between the two critical loads. The increase in density of the monolith and the reinforcement of TiB2 were effective in reducing the coefficient of friction and the specific wear rate. The wear experiments have been performed in these three regimes (15, 50 and 75 N). The tribofilm formed on the pin surface was found to contain both pin and disc materials. The temperature of the pins during the sliding against EN-24 disc was calculated using one dimensional heat transfer equation at different loads for each composition. The composite experiences lower temperatures compared to the monoliths. (C) 2002 Elsevier Science B.V. All rights reserved.

Gibbs energies of formation of intermetallic phases in the systems Pt Mg, Pt Ca and Pt Ba and some applications

The standard Gibbs energies of formation of platinum-rich intermetallic compounds in the systems Pt-Mg, Pt-Ca, and Pt-Ba have been measured in the temperature range of 950 to 1200 K using solid-state galvanic cells based on MgF2, CaF2, and BaF2 as solid electrolytes. The results are summarized by the following equations: ΔG° (MgPt7) = −256,100 + 16.5T (±2000) J/mol ΔG° (MgPt3) = −217,400 + 10.7T (±2000) J/mol ΔG° (CaPt5) = −297,500 + 13.0T (±5000) J/mol ΔG° (Ca2Pt7) = −551,800 + 22.3T (±5000) J/mol ΔG° (CaPt2) = −245,400 + 9.3T (±5000) J/mol ΔG° (BaPt5) = −238,700 + 8.1T (±4000) J/mol ΔG° (BaPt2) = −197,300 + 4.0T (±4000) J/mol where solid platinum and liquid alkaline earth metals are selected as the standard states. The relatively large error estimates reflect the uncertainties in the auxiliary thermodynamic data used in the calculation. Because of the strong interaction between platinum and alkaline earth metals, it is possible to reduce oxides of Group ILA metals by hydrogen at high temperature in the presence of platinum. The alkaline earth metals can be recovered from the resulting intermetallic compounds by distillation, regenerating platinum for recycling. The platinum-slag-gas equilibration technique for the study of the activities of FeO, MnO, or Cr2O3 in slags containing MgO, CaO, or BaO is feasible provided oxygen partial pressure in the gas is maintained above that corresponding to the coexistence of Fe and “FeO.”

Microstructure evolution and metastable phase formation in laser-ablation-deposited films of Ti5Si3 intermetallic compound

Thin films of Ti62.5Si37.5 composition were deposited by the pulsed-laser ablation technique on single-crystal Nad substrates at room temperature and on ′single-crystal′ superalloy substrates at elevated temperatures. Both vapour and liquid droplets generated by pulsed-laser ablation of the target become quenched on the substrate. Amorphization had taken place in the process of quenching of vapour-plasma as well as small liquid droplets on NaCl substrates at room temperature. In addition to the formation of Ti5Si3, a metastable fcc phase (a 0 = 0.433 nm) also forms in micron-sized large droplets as well as in the medium-sized submicron droplets. The same metastable fcc phase nucleates during deposition from the vapour state at 500°C and at 600°C on a superalloy substrate as well as during crystallization of the amorphous phase. The evolution of the metastable fcc phase in the Ti-Si system during non-equilibrium processing is reported for the first time.

Incorporation of Interfacial Intermetallic Morphology in Fracture Mechanism Map for Sn-Ag-Cu Solder Joints

A fracture mechanism map (FMM) is a powerful tool which correlates the fracture behavior of a material to its microstructural characteristics in an explicit and convenient way. In the FMM for solder joints, an effective thickness of the interfacial intermetallic compound (IMC) layer (t (eff)) and the solder yield strength (sigma (ys,eff)) are used as abscissa and ordinate axes, respectively, as these two predominantly affect the fracture behavior of solder joints. Earlier, a definition of t (eff), based on the uniform thickness of IMC (t (u)) and the average height of the IMC scallops (t (s)), was proposed and shown to aptly explain the fracture behavior of solder joints on Cu. This paper presents a more general definition of t (eff) that is more widely applicable to a range of metallizations, including Cu and electroless nickel immersion gold (ENIG). Using this new definition of t (eff), mode I FMM for SAC387/Cu joints has been updated and its validity was confirmed. A preliminary FMM for SAC387/Cu joints with ENIG metallization is also presented.

From (Au5Sn + AuSn) physical mixture to phase pure AuSn and Au5Sn intermetallic nanocrystals with tailored morphology: digestive ripening assisted approach

Here we present digestive ripening facilitated interatomic diffusion for the phase controlled synthesis of homogeneous intermetallic nanocrystals of Au-Sn system. Au and Sn metal nanoparticles synthesized by a solvated metal atom dispersion (SMAD) method are employed as precursors for the fabrication of AuSn and Au5Sn which are Au-rich Au-Sn intermetallic nanocrystals. By optimizing the stoichiometry of Au and Sn in the reaction mixture, and by employing growth directing agents, the formation of phase pure intermetallic AuSn and Au5Sn nanocrystals could be realized. The as-prepared Au and Sn colloidal nanoparticles and the resulting intermetallic nanocrystals are thoroughly characterized by powder X-ray diffraction, transmission electron microscopy (TEM and STEM-EDS), and optical spectroscopy. The results obtained here demonstrate the potential of solution chemistry which allows synthesizing phase pure Au-Sn intermetallics with tailored morphology.

Digestive ripening facilitated atomic diffusion at nanosize regime: Case of AuIn2 and Ag3In intermetallic nanoparticles

Monodisperse colloidal gold-indium (AuIn2) intermetallic nanoparticles have been synthesized from Au and In colloids using the digestive ripening process. Formation of the intermetallic proceeds via digestive ripening facilitated atomic diffusion of Au and In atoms from the Au and In nanoparticles followed simultaneously by their growth in the solution. Optimization of the reaction temperature was found to be crucial for the formation of AuIn2 intermetallic from gold and indium nanoparticles. Transmission electron microscopy revealed the presence of nearly monodisperse nanoparticles of Au and AuIn2 with particle size distribution of 3.7 +/- 1.0 nm and 5.0 +/- 1.6 nm, respectively. UV-visible spectral studies brought out the absence of SPR band in pure AuIn2 intermetallic nanoparticles. Optical study and electron microscopy, in combination with powder X-ray diffraction established phase pure AuIn2 intermetallic nanoparticles unambiguously. The potential of such an unprecedented approach has been further exploited in the synthesis of Ag3In intermetallic nanoparticles with the dimension of less than 10 nm. (C) 2014 Elsevier B.V. All rights reserved.