Grain boundary migration with precipitation and dissolution of a liquid phase in MoNi alloy

When 85Mo15Ni (by weight) alloys prepared by liquid phase sintering at 1380°C are heat-treated at 1520°C, the grain boundaries and liquid films between the grains migrate, leaving behind them a new MoNi solid solution with Ni content higher than that in the initial solid formed during the liquid phase sintering treatment. The grain boundaries migrating during this discontinuous dissolution of the liquid phase have a flat shape. When the temperature change is reversed by first sintering at 1520°C and subsequently heat-treating at 1380°C, the grain boundaries and liquid films again migrate, with the composition change of the solid reversed. Liquid precipitates form at grain boundaries and migrate with them, their size and total volume increasing during the migration. The grain boundaries have a curved shape and their initial migration rate is considerably higher than that during the discontinuous dissolution. This contrasting migration behaviour between the discontinuous dissolution and precipitation is attributed to the coherency strain energy operating locally on both grain boundary segments and liquid droplets, and to the high mobility of fine liquid droplets.     

Observations of the effect of an applied stress on the morphology of discontinuous precipitation in a CuCd alloy

The influence of an applied tensile stress on the morphology and growth rate of the discontinuous precipitation product in a Cu−3.8 wt% Cd alloy was first reported by Sulonen. In this contribution, we investigate and further quantify a number of factors associated with the role of applied stress in this alloy, including absolute and relative growth rates of interfaces with normals parallel to and perpendicular to the tensile axis, and the morphological stability of discontinuous precipitation front as a function of the applied stress. Once morphological instability has occurred, the amplitude and wavelength of the protuberences formed depends on the value of the applied stress, and on the angle between the average interface normal and the tensile axis. It is suggested that the process is best viewed as one of transformation-assisted viscoelastic deformation.     

The suppression of chemically induced liquid film migration in CoCu at high temperature

When a Co-20 wt% Cu alloy prepared by liquid phase sintering at 1150°C is heat-treated at 1300°C, some intergranular liquid films initially migrate and reverse their directions to return to their initial locations, while others do not migrate at all. Previously, extensive liquid film migration (LFM) has been observed when the sintering and the heat-treatment temperatures have been reversed. When the same alloy sintered at 1150°C is heat-treated at 1150°C in contact with a Cu-10Co-10Sn (wt%) powder mixture, extensive LFM occurs, producing Cu-depleted solid behind the migrating liquid films. When the same experiment is carried out at 1300°C, very little LFM is observed and the grain composition is equilibrated by lattice diffusion. Upon increasing the Sn content in the powder mixture to 15 and 20 wt%, extensive LFM occurs at both 1150 and 1300°C. These observations are consistent with the prediction of the coherency strain theory that there are high temperature limit and minimum critical driving force for LFM. At high temperatures and low driving forces, the ratio of the solute lattice diffusivity to the migration velocity is so large that the coherency in the frontal diffusion zone is expected to be broken, eliminating the driving force for LFM. The same principle should apply to the grain boundary migration.     

The effect of external stress on the discontinuous precipitation in an AlZn alloy at high and low temperatures

The effect of external tensile stress on continuous precipitation (DP) has been investigated in an Al21.8 at.% Zn alloy at high (215°C) and low (75 and 50°C) temperatures. The ratio of the macroscopic lattice diffusivity, D, to the DP boundary velocity, v, (D/v) is estimated to be larger than the interatomic spacing, λ, at the high temperature, and smaller than λ at the low temperatures. Under tensile stresses, the DP rates are enhanced at the grain boundary segments oriented transverse to the stress direction and suppressed at those oriented parallel to it at both high and low temperatures. Furthermore, Yi and Park show the DP rate changing continuously with temperature over the range where D/v increased from values much smaller than λ to those much larger. These results show that the diffusional coherency strain is the major driving force for DP even at low temperatures where, with D/v < λ, no solute diffusion is usually assumed to occur in front of the moving DP boundaries.     

Precipitation in a liquid phase sintered WNiFe alloy

The precipitate phases formed within samples of a W-5 wt% Ni-5 wt% Fe alloy following sintering and controlled, post-sintering heat treatments have been characterized using techniques of transmission electron microscopy. Interphase boundary precipitation of a complex eta carbide has been confirmed in an as-sintered, commercial alloy of high impurity carbon content, and reproduced on a reduced scale in an experimental alloy of reduced carbon content by isothermal ageing (10 h, 900–950°C of solution treated (1 h, 1350°C) and quenched specimens. The experimental alloy contained no evidence of third phase precipitation in the as-sintered condition, but fine particles were noted along tungsten grain boundaries in solution treated and quenched specimens and were found to have a structure similar to that of the matrix γ phase. Isothermal ageing of solution treated specimens at temperatures of 750–850°C produced local colonies of cellular precipitation within the matrix phase, the discontinuous phase of essentially pure tungsten arising in response to a retained supersaturation of tungsten in matrix solid solution in the quenched material. A competitive precipitation of the eta carbide, (Ni, Fe)₆W₆C, was observed in the matrix phase under similar heat treatment conditions, with the most common form of the precipitate being Widmanstätten laths of (111)₇ habit plane. The present study produced no evidence for the formation of an intermetallic phase in the WNiFe alloy containing a weight ratio of Ni:Fe of 1:1.     

The discontinuous precipitation of a liquid phase in MoNi induced by diffusional coherency strain

When a liquid phase sintered MoNi alloy is heat-treated at a temperature lower than that used for sintering where the solid and liquid phases coexist, the liquid films and grain boundaries between the grains migrate, leaving behind a new solid solution somewhat depleted of Ni. Since some liquid is produced during this migration, it resembles a discontinuous precipitation of the liquid phase. It is demonstrated experimentally that the driving force for this discontinuous precipitation arises from the coherency strain produced by Ni atom diffusion out of the grains. When two solute atom species simultaneously diffuse into or out of crystals in a ternary system, the resulting coherency strain depends on the size and concentration of the diffusing atoms and can be varied independently of the free energy of mixing. In particular, the coherency strain can be reduced to zero when the strain effects of the two atomic species exactly cancel each other. In this study, series of 90Mo10Ni alloy (by wt%) have been prepared by liquid phase sintering at temperatures between 1400 and 1520°C in order to produce solid MoNi grains of varying Ni concentration that are in equilibrium with the surrounding liquid matrix. The migration of liquid films and grain boundaries in the sintered specimens is induced by heat-treating them at 1400°C after adding various amounts of Fe to the liquid matrix. The migration does not occur when the estimated coherency strain is close to 0, although the free energy of mixing is finite. This result is a definitive demonstration that the driving force for the steady state migration is the coherency strain energy. The evidence that the grain boundaries exist in the sintered specimens and remain as grain boundaries during the migration is discussed.     

Coherent precipitation effect on thermo-power of AlCu alloys

In situ thermoelectric measurements of AlCu alloys are described for several types of heat treatments. The thermoelectric power shows deviations from the solid solution matrix during the sequence of precipitations occurring in these alloys (i.e. GP zones, θ′',θ′ and θ). The sign and the amplitude of these deviations can be modeled on the basis of the HEW theory for the resistivity maximum. Extension to the diffusion thermopower displays a size and morphology (isotropic, platelike) effect in the electron Bragg scattering by the coherent precipitates, corresponding rather well with the experimental data.     

Effect of Cu contents on aging precipitation behaviors of Cu-rich phase in Fe-Cr-Ni alloy

 Abstract: The precipitation characteristics and effect on strengthening mechanism of Cu-rich phases during short-time and long-time aging in the Super 304H steels with different Cu contents were investigated by the use of X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results show that the size of Cu-rich phase particles increases, the distance of Cu-rich phase particles decreases and the density of Cu-rich phases increases with the increase of Cu content during short-time aging (~ 800 h) at 650℃ in the Super 304H steels. During long-time aging (>2000 h) at 650℃, Cu-rich phases precipitate sufficiently and the strengthening effect of Cu-rich phases is preferable in the Super 304H steel with 4% Cu. The strengthening effect of Cu-rich phases in the Super 304H steels with 2.2 % Cu or 5 % Cu is weaker than that with 4% Cu during long-time aging (>2000 h).     

The effect of plastic deformation on fracture morphology of the sigma phase in the NbTiAl system

It has been observed that in bulk specimens indentation-induced plastic deformation at room temperature causes intergranular fracture in the σ-phse of the NbTiAl system. The TEM study of the indented regions has revealed that only intergranular microcracks, which develop when two slip bands impinge on both sides of a boundary or when a slip band arrives at a grain boundary triple point, are resulted by the dislocation—grain boundary interaction. Consequently, upon subsequent loading of the indented speciments the crack propagates intergranularly in these regions. In the undeformed σ-phase, the accidental cracks that develop during the thin foil preparation have been found to follower a predominantly transgranular path, similar to the bulk specimen, with no dislocation activity at their tips. In contrast, in the plastically deformed regions the accidental cracks follow the path along slip bands. This phenomenon is believed to be a thin-foil effect.     

The effect of temperature on strain rate sensitivity in an AlMgSi alloy

The effect of temperature on strain rate sensitivity has been studied in an AlMgSi alloy in the range of dynamic strain ageing. Two regimes of behaviour were observed which depended on the temperature. At low temperatures (T <- 303 K) it was possible to correlate all strain rate sensitivity measurements with a single strain/strain rate/temperature parameter. In the high temperature regime the temperature and strain rate dependence of the strain rate sensitivity deviated significantly from that measured at low temperatures. The two regimes of behaviour were found to correspond with the normal and inverse Portevin-Le Chatelier effect.     

Effect of T1 precipitate on the anisotropy of AlLi alloy 2090

A study has been made of the effect of T₁ precipitate on the anisotropy of an AlLi alloy 2090. The critical role of T₁ precipitates has been identified by comparing the behavior of the alloy with various microstructures. These include (1) T8E41 tempered specimen with T₁, θ′ and δ′, (2) reverted specimen with T₁, (3) re-solutioning specimen with very fine δ′, and (4) under-aged specimen with fine δ′ and θ′. It has been shown that the {110}〈112〉 crystallographic texture developed during thermomechanical treatment has a direct effect on the anistropy. It also has an indirect effect by causing an inhomogeneous distribution of nucleation sites for T₁ among four {111} habit planes during stretching and accordingly inhomogeneous distribution of T₁ precipitates among four possible {111} planes during aging. Such inhomogeneously distributed T₁ precipitates in both T8E41 tempered and reverted specimens affect the variation of tensile properties, particularly elongation with specimen orientation.     

Investigation of thermoelastic martensitic transformation in a CuZnAl alloy

Thermoelastic martensitic transformation in a Cu-29%Zn-3%Al alloy was investigated experimentally and theoretically. The phase structures and morphological changes occurring during transformation were studied using optical microscopy and high voltage TEM (1000–1200 kV), both equipped with combination heating and cooling stages. A “single crystal pure shear” experiment was designed to measure the relationship between M_s and shear stress, and from these data changes of enthalpy and entropy of the transformation were calculated through the Clausius-Clapeyron equation. The changes of enthalpy and entropy were also obtained by using calorimetric measurement. Both results were in good agreement. The kinetic behavior of the transformation in polycrystalline alloy as a function of applied stress was followed using electric resistance measurement. The slope of the transformation rate was constant over the range of 20–70% transformed, which corresponds physically to transformation occurring by plate growth in the unpartitioned parent phase or equivalently, the interphase boundary “moving freely.” A phenomenological theory was suggested to describe the constant slope portion of the transformation. Here, phase boundary motion is related to thermal hysteresis and quantitatively described the effect of applied stress on the transformation behavior.