Role of temperature gradient in liquid／solid phase solution-diffusion bonding

The liquid-film solution-diffusion bonding of ZCuBe2. 5 alloys was conducted using Cu-based alloy powders. The tensile strength of the joint is up to 318 MPa. With the increase of temperature gradient, the bonding time decreases and the interlace migration velocity increases remarkably. The appropriate temperature gradient is 5 -40 K/cm. Under fixed bonding time, the thickness of diffusion layer increases with the increase of temperature gradient, and this tendency becomes more remarkable with the prolonging of bonding time.     

Shear along grain boundaries in aluminum bicrystals

Aluminum bicrystals 99.99 pct pure having a pure tilt boundary about the <110> axis were subjected to creep stresses along their boundary at various temperatures. The initial rate of grain boundary gliding was found to be almost linear and a plot of the logarithm of the initial rate for various stresses vs the reciprocal of the absolute temperature resulted in a series of parallel straight lines. The usual rate equation was used to calculate the activation energy for grain boundary shear. It was found to increase continuously as the boundary angle increased, and approached a maximum value for the maximum angle of misorientation. The mechanism for grain boundary shear is considered to be a combination process involving slip and grain boundary self-diffusion.     

Inclination dependence of grain boundary energy and its impact on the faceting and kinetics of tilt grain boundaries in aluminum

The shape evolution and migration of <1 0 0> and <1 1 1> tilt grain boundaries with rotation angles θ in the range between 6° and 24° were investigated in situ in a scanning electron microscope at elevated temperatures. The results revealed that boundaries with misorientation θ < 15° did not attain a continuously curved shape in the entire temperature range up to the melting point and, thus, did not move under a capillary driving force. Instead, they remained straight or formed several facets which were inclined to the initial boundary orientation. Molecular statics simulations suggest that the observed behavior of low-angle boundaries is due to the anisotropy of grain boundary energy with respect to boundary inclination. This anisotropy diminishes with increasing misorientation angle, and high-angle boundaries assume a continuously curved shape and move steadily under the curvature driving force.     

Strains and stresses caused by penetrative wetting of grain boundaries by the liquid phase

We demonstrate that the dependence of grain boundary energy on composition leads to generation of normal stresses during grain boundary interdiffusion process. These self-generated stresses facilitate grain boundary embrittlement and rapid penetration of liquid phase along the grain boundaries.     

Abnormal grain growth and grain boundary faceting in a model Ni-base superalloy

Normal or abnormal grain growth in a model Ni-base superalloy is observed to depend on the grain boundary structure when heat-treated in a solid solution temperature range above the solvus temperature (1150°C) of the γ′ phase. When heat-treated at 1200°C abnormal grain growth occurs and most of the grain boundaries are observed to be faceted by optical microscopy, transmission electron microscopy, and scanning electron microscopy at the intergranular fracture surface. Some of the grain boundary facet planes are expected to be singular corresponding to the cusps in the polar plot of the boundary energy against the inclination angle, and it is proposed that if these boundary segments move by a boundary step mechanism, the abnormal grain growth can occur. When heat-treated at 1300°C normal grain growth occurs, the grain boundaries are defaceted, and hence atomically rough. Normal growth is expected if the migration rate of the rough grain boundaries increases linearly with the driving force arising from the grain size difference. The correlation between the grain boundary structural transition and the growth behavior thus appears to be general in pure metals and solid solution alloys.     

Crystallography of Fe2Al5 phase at the interface between solid Fe and liquid Al

In this study, the microstructures and crystallographic features of a η-Fe₂Al₅ phase formed on pure Fe hot-dipped in a pure Al melt at 750 °C were examined in order to understand the η phase layer formation having a saw-tooth morphology. A number of the columnar η grains (forming the η phase layer) grow towards the solid Fe (α-Fe) side along the [001] direction, resulting in a significant saw-tooth morphology at the interface between the η and α-Fe phases. The neighboring η grains have high-angle boundaries with a common [001] axis. In the η phase layer, the low-angle boundaries develop close to the liquid Al side, and their density becomes higher with longer dipping times, resulting in the development of a fine dislocation substructure in the η phase. In the α-Fe phase, fine substructure consisting of a high density of low-angle boundaries develops around the growth tips of the columnar η grains. These substructure developments are likely responsible for the α → η transformation strain. A possible mechanism for the formation of this η phase layer having a saw-tooth morphology will be discussed in terms of the stress field caused by the α → η transformation.     

Penetration of oxygen along grain boundaries during oxidation of alloys and intermetallics

Fast penetration of oxygen into grain boundaries and intergranular oxidation of -NiAl has been observed. Since the solubility of oxygen in NiAl is virtually nil, special ways of oxygen ingress at grain boundaries have to be presumed. Selective intergranular oxidation of binary alloys and fast penetration of oxygen along grain boundaries were analyzed by computer simulation. Interdiffusion caused by consumption of the less-noble component by oxidation at the metal-oxide interface leads to deviation of the alloy composition from the original value. When the diffusivity of the less-noble component is higher than the diffusivity of the other component, a grain-boundary Kirkendall effect may lead to void-chain formation. Experimental evidence for this phenomenon is presented. The deviation in composition and void formation were considered as processes influencing the effective oxygen diffusivity. Both processes were found to allow penetration of oxygen as fast as grain-boundary interdiffusion occurs. In addition, oxygen penetration during intergranular internal oxidation when oxides form at voids beneath the metal-oxide interface was analyzed and treated as a self-propagating process. In this case, fast oxygen penetration is accompanied by fast internal oxide formation and fast displacement of the metal-oxide interface.     

Behavior of particles in front of metallic solid/liquid interface in electromagnetic field

1　INTRODUCTIONTheinteractionbetweentheparticlesandtheso lidifyinginterfaceisofteninvolvedintheprocessingofmetallicmaterials .Forexample ,duringthesolidi ficationofthemetalcasting ,theinclusioninthemoltenmetal(includingforeigninclusionandsec ondaryinclusion)ispossibletobeengulfedbysolidi fyinginterfacetostayinthecasting ,alsoitispossibletobepushedbytheinterfacetoretaininthemoltenmetal.Anothercaseis ,thereinforcingparticleswillbeengulfedtoenterthesolidandpossiblypushedbytheinterface ,whichw…     

Steady-state migration of the liquid film along a grain boundary during melting of alloys

We discuss melting of alloys along grain boundaries as a free boundary problem for two moving solid–liquid interfaces. One of them is the melting front and the other is the solidification front. The presence of the triple junction plays an important role in controlling the velocity of this process. The interfaces interact strongly via the diffusion field in the thin liquid layer between them. In the liquid film migration (LFM) mechanism the system chooses a more efficient kinetic path, which is controlled by diffusion in the liquid film over relatively short distances. However, only weak coherency strain energy is the effective driving force for LFM in the case of melting of one-phase alloys. The process with only one melting front would be controlled by the very slow diffusion in the mother solid phase over relatively large distances.     

Utilizing the SIMS technique in the study of grain boundary diffusion along twist grain boundaries in the Cu(Ni) system

The secondary ion mass spectrometry (SIMS) technique was used to study grain boundary diffusion along (100) twist grain boundaries in the Cu(Ni) system. Concentration profiles of Ni down Cu twist grain boundaries with nominal disorientation angles of 10°, Σ5 (36.87°), and 45°, were measured using the SIMS technique. The average activation energy for grain boundary diffusion, Q_b, was found to be 245±22, 140±10, and 102±15 kJ/mol, for the 10°, Σ5, and 45° twist grain boundaries, respectively. The average grain boundary diffusion pre-exponential term, sδD_bo, was found to be 9.6±1.24×10⁻⁹, 1.1±0.17×10⁻¹⁴, and 1.3±0.36×10⁻¹⁶ m³/s, for the 10°, Σ5, and 45° twist grain boundaries, respectively.     

Sliding of copper [001]-twist grain boundaries detected by shear deformation of liquid B2O3 particles on the grain boundaries

From a Cu–0.065mass%B solid solution alloy, bicrystals having [001]-twist grain boundaries with various misorientation angles were prepared by the Bridgman method. Cu bicrystals containing B₂O₃ particles were made from the bicrystals by internal oxidation treatment. The Cu–B₂O₃ bicrystals were tensile tested at high temperatures, where B₂O₃ particles become liquid. The amount of grain-boundary sliding was measured from the amount of shear deformation of the liquid B₂O₃ particles on a sliding grain boundary. Shear deformation of voids where the B₂O₃ particles was embedded was also utilized to measure the amount of grain-boundary sliding. The temperature and misorientation-angle dependence of the grain-boundary sliding is discussed for the Cu–B₂O₃ bicrystals. Since the liquid B₂O₃ particles easily deform at high temperatures, the measured amount of grain-boundary sliding is considered to reflect the inherent nature of the Cu [001]-twist grain boundaries. Using the present method of measurements, up to 2 μm grain-boundary sliding can be measured with an accuracy of about 0.1 μm.     

Phase transformation studies at the solid/liquid interface by directional solidification of Alloy 718

The phase transformation temperatures are especially important in alloy 718 because it requires careful heattreatment cycles. The DTA method is generally used to estimate the characteristic temperatures of phase transformations. However, DTA is an indirect method to confirm phase transformations, and there are always discrepancies in analyzing the DTA thermograms. In this study, the directional solidification and quenching technique has been applied to estimate the phase transformation temperatures near the solid/liquid interface temperatures by measuring temperatures directly during directional solidification. These phase transformation temperatures were confirmed in the quenched solid/liquid interface. Solidification behavior at the solid/liquid interface with solidification rate and temperature gradient were also discussed.     

Diffusion of Zn along tilt grain boundaries in Al:: pressure and orientation dependence

For the first time, a systematic investigation has been carried out on the pressure and orientation dependence of diffusion along grain boundaries. Zn diffusion has been measured in the B-kinetic regime in different sets of Al bicrystals at 553 K. Pressures up to 1.2 GPa have been applied. Three sets of symmetrical <001>, <011> and <111> tilt grain boundaries have been chosen with tilt angles around those, which correspond to a low coincidence lattice site parameter. The Zn concentration depth profiles have been measured by electron probe microanalysis and have been analysed using the Whipple equation. The orientation dependence of the diffusivity shows minima for the high coincidence grain boundaries at all applied pressures. Using known data for the bulk diffusion of Zn in Al, the activation volume for the triple product of grain boundary diffusion of Zn in Al has been determined for each grain boundary under investigation. The values of the activation volume range from 0.26 to 0.47 of the atomic volume of Al and show slight maxima for the high coincidence grain boundaries.