S.P. Gupta

Intermetallic compound formation in Fe–Al–Si ternary system: Part II

Abstract The phase equilibria in the Fe–Al–Si ternary system have been studied in the temperature range 600–900 °C using ternary diffusion couple experiments with pure Fe and high purity Al–Si eutectic alloy. A number of binary and ternary intermetallic compounds have been observed. Except for the FeAl3 binary phase, the remaining intermetallics of the system, namely, Fe2Al5, FeAl and Fe3Al, are observed to form as solid layers next to pure iron. The FeAl3 phase was observed in association with the liquid which solidified into secondary and tertiary crystallization products. A number of ternary phases, namely, τ1, τ2, τ4, τ5 and τ6 have been observed. These phases have crystallized either directly from the liquid or through the invariant or monovariant reactions. The composition of the phases and associated microstructure of each phase has been presented.

Intermetallic compounds in diffusion couples of Ti with an Al–Si eutectic alloy

Abstract The phase equilibria in the Ti–Al–Si System have been studied in the temperature range 700–1000 °C using ternary diffusion couple experiment with pure titanium and high-purity Al–Si eutectic alloy. A number of binary and ternary intermetallic compounds have been observed. The intermetallics TiAl 2 , Ti 9 Al 23 , TiAl and Ti 3 Al form as solid layers. The intermetallics Ti 5 Si 4 and Ti 5 Si 3 are observed as two-phase layers associated with TiAl 3 . Two ternary intermetallic compounds τ 1 and τ 2 with compositions close to Ti 3 Al 2 Si 5 and Ti 3 AlSi 5 are observed. The microstructure of binary and ternary phases has been reported.

Intermetallic compound formation in the Zr–Al–Si ternary system

Abstract The microstructure and composition of binary and ternary intermetallic compounds have been studied in a ternary diffusion couple of Zr and Al–Si eutectic alloy. The τ1 and τ3 phases always formed in the liquid part of the diffusion couple and τ2 phase formed as a layer structure. Two intermetallics, ZrSi and ZrSi2, of the Zr–Si binary system have been observed. A number of intermetallics, namely, ZrAl3, ZrAl2, Zr2Al3, ZrAl, Zr3Al2, Zr5Al4, Zr5Al3, Zr2Al and Zr3Al, of the Zr–Al system have been observed. The solubility of Si in the ZrAl3 and ZrAl intermetallics is very small. In the other intermetallics, the solubility of Si is negligible. The results of this investigation also show two ternary invariant reactions. The microstructure of a number of phases has been presented.

Kinetics of Discontinuous Precipitation and Dissolution in Cu–Ag Alloys

AbstractThe kinetics of discontinuous precipitation and dissolution of the cellular precipitate have been studied in Cu-3 at.% Ag and Cu-4 at.% Ag alloys. The growth rates of the cells were measured using optical microscopy. The interlamellar spacings of the primary cells and compositions of the depleted matrix were measured using scanning electron microscope and X-ray diffraction, respectively. The Cu-Ag alloys were observed to decompose into a lamellar structure consisting of alternate lamellae of the α (Cu-rich) and β (Ag-rich) phases when a solid solution of the alloy was aged below the solvus temperature. The rod shaped morphology of the β phase dispersed in the matrix of α was observed at all temperatures. The primary cell growth data were analysed using the theories of Cahn, Hillert, Sundquist, TUfllbull and Petermann and Hornbogen. From the diffusivity values, it has been shown that the growth of primary cells occurs by the diffusion of Ag along the grain boundaries. The results are consistent wit...

Kinetics of cellular dissolution in a CuIn alloy

The dissolution of lamellar precipitates by cell boundary migration has been studied in a Cu-15 wt.% In alloy in the temperature range 778–853 K, primarily by optical microscopy. Microstructural observations indicated that the process of dissolution in this alloy is a cellular mode of transformation. By approximating the tip radius at the α-δ interface as equal to half the width of a δ lamella, the interface composition during dissolution was determined using the Gibbs-Thomson relation. The boundary diffusivities were calculated using the theories of Tu and Turnbull and of Petermann and Hornbogen modified for dissolution and a formulation applying balance of fluxes at the edge of a receding cell boundary similar to the formulation of Aaronson and Liu for the cellular precipitation reaction. The diffusivity values calculated from the experimental data fall within a range of two orders of magnitude for these theories but in all cases are four to five orders of magnitude higher than the volume diffusivity. For this alloy, there exists a critical temperature at which the driving forces for cellular precipitation and dissolution are equal. The value lies approximately 23 K below the equilibrium solvus temperature. An activation energy in the range 148–159 kJ mol−1 for different models has been obtained which agrees well with the Q values reported for cellular precipitation in Cuue5f8In alloys.

Cellular precipitation and precipitate coarsening in a Mg-Al alloy

Abstract The morphology and growth kinetics of the cellular precipitate as well as its discontinuous coarsening have been studied in the temperature range 440–580 K. Optical microscopy and X-ray diffraction were used to characterize the cellular transformation. A rapidly solidified metastable Mg-7 at.% Al alloy was observed to decompose completely via a process which has been termed “cellular precipitation” into a lamellar structure consisting of the δ and γ phases at all aging temperatures used in this investigation. The fine lamellar structure of the primary cells subsequently decomposed into a coarse lamellar structure consisting of the same two phases. Lattice parameter measurements have indicated that the depleted matrix of the δ phase associated with the initial cells was richer in solute than the equilibrium solvus, δ/(δ+γ) . The solute concentration in the depleted matrix associated with the coarsened material was less than the published equilibrium solvus. Analysis of the growth kinetics of both the primary cellular reaction, and its subsequent coarsening stage, has indicated that the transformation is controlled by diffusion of aluminum through the cell boundaries.

Coarsening kinetics of vanadium carbide in a high strength, low alloy steel

Abstract The coarsening kinetics of vanadium carbide were studied in the temperature range 715–810°C. The steel was aged for periods ranging from 1 h to approximately 15 days at six different temperatures. Quantitative electron microscopy was carried out on carbon extraction replicas prepared from bulk specimens. Particle size distributions (PSDs) were determined for various aging times at the six aging temperatures. The coarsening of vanadium carbide can be described by a volume diffusion control process obeying the t 1 3 law. The diffusion parameters Q and D0 agree with the corresponding published values for volume diffusion of vanadium in iron. A normal distribution rather than a log-normal distribution was observed to approximate the experimental PSDs; this gave some credence to the Lifshitz-Slyozov encounter-modified theory.

Discontinuous precipitation and coarsening of lamellar cellular precipitate in Fe-Zn alloys☆

Abstract The initiation of cellular reaction, formation of cells, and their development have been studied in two Fe-Zn alloys containing 20 and 25 wt.% zinc. Optical microscopy was utilized to study the cellular structure developed during aging in the temperature range 400–520°C. Deflection of the grain boundary was observed to be associated with the nucleation of soluterich phase. From the observation of grain boundary migration into puckered configuration and nucleation of new lamellae at such positions of the boundary, it is evident that the cellular phase transformation occurs in this alloy by the pucker mechanism proposed by Tu and Turnbull [1]. Multiplication of the lamellae occurred by nucleation of the new lamellae in the recess of the grain boundary and by branching. Aging for longer periods resulted in a second cellular reaction or discontinuous coarsening, which decomposed the primary lamellar product into a coarse lamellar structure of the same two phases. The secondary cells were observed to nucleate at three different regions; namely, at the original position of the grain boundary, at the impinged region of two primary cells, and at the interface between the migrating cell boundary and original position of the grain boundaries.

A Comparative Study of the Kinetics of Interface Diffusion Controlled Transformations In Fe-Zn Alloys

Abstract A comparative study of the kinetics of discontinuous precipitation, coarsening and dissolution, diffusion induced grain boundary migration and grain growth has been carried out in Fe-Zn alloys. Except for cells of the discontinuous coarsening reaction, rates of migration of the cell boundaries of precipitation and dissolution and diffusion induced grain boundary migration (DIGM) rates lie within an order of magnitude of each other. The rate of cell boundary migration during discontinuous coarsening is two orders of magnitude smaller than those for the four transformation processes mentioned above. The kinetics (of minimum rate) of grain growth which occurs after long annealing times is comparable to the discontinuous precipitation and coarsening in the Fe-Zn alloy. The kinetics (of maximum rate) of grain growth, which corresponds to short annealing times, are comparable to the discontinuous dissolution and DIGM. The diffusivity values lie within an order of magnitude of the boundary diffusivity during grain growth when considered at the same temperature (extrapolated). Although the driving force differs by four orders of magnitude from as low as 0.08 Jmol−1 for grain growth to 1420 Jmol−1 for DIGM at 923 °K, the small difference in the diffusivity (kDbδ) predicts that the conduit for solute transport is the same for the above transformations. On a effectué une étude comparative de la cinétique de précipitation discontinue, de grossissement et de dissolution, de migration de joints de grain induite par diffusion (DIGM) et de croissance de grain, dalliages de Fe-Zn. À lexception des cellules de réaction de grossissement discontinu, le taux de migration des joints de cellules de précipitation, de dissolution ainsi que le taux de migration de joints de grain induite par diffusion se situent à un ordre de grandeur lun de lautre. Le taux de migration du joint de la cellule lors du grossissement discontinu est de deux ordres de grandeur plus petit que celui des quatre procédés de transformation mentionnés ci-dessus. La cinétique (taux minimum) de croissance de grain qui se produit après une longue durée de recuit est comparable à la précipitation discontinue et au grossissement de lalliage Fe-Zn. La cinétique (taux maximum) de croissance de grain qui correspond à de courtes périodes de recuit est comparable à la dissolution discontinue et à “DIGM”. Les valeurs de diffusivité se situent à un ordre de grandeur de la diffusivité du joint lors de la croissance de grain lorsque considérée à la même température (extrapolée). Bien que la force directrice diffère par quatre ordres de grandeur, allant de 0.08 Jmol−1 pour la croissance de grain jusquà 1420 Jmol−1 à 923 °K pour “DGIM”, la petite différence de diffusivité (kDbδ) prédit que le conduit pour le transport de soluté est le même pour les transformations ci-dessus.

Persistent slip bands in underaged Cu-2at.% Co single crystals during fatigue

Abstract The development of dislocation microstructure in fatigued single crystals of Cu-2at.%Co in the underaged condition was followed using transmission electron microscopy. Long screw dislocations form initially and then condense into bands of increasing dislocation density as deformation progresses. The deformation is inhomogeneous. The dislocation bands collapse at large strains and give way to the appearance of persistent slip bands (PSBs) which are narrow regions 0.1–1 μm wide. The width of the PSBs on the (1 2 1) plane and their density depend on the applied constant plastic strain amplitude, γpl being larger for higher γpl. The PSBs have a very low density of precipitates as revealed from the contrast obtained under a number of different operating diffraction conditions of imaging. It is proposed that the softening of the underaged Cu-2at.%Co alloys during fatigue is due to the dissolution and/or reduction in size of the cobalt clusters within the PSBs.