The influence of critical points and structure and microstructural evolution in iron rich FeSi alloys

The evolution of microstructure in Fe-8.7 to 14.2 at.% Si alloys in the neighbourhood of higher order transition lines has been investigated. Systematics of various features of the FeSi phase diagram, highlighting the multicritical points, have been presented. The possible influence of these critical points on the kinetic pathways for the microstructural evolution has been discussed and compared with experimental results. It is shown that below the metastable tricritical point the alloy undergoes continuous ordering and a conditional spinodal followed by the renucleation of B2 phase in the DO₃ matrix. The latter process leads to the anomalous contrast in the B2 superlattice darkfield images. It is also shown that above the metastable (extrapolated) higher order B2 → DO₃ transition line a small domain exists in the two phase field where decomposition to two phase B2 + DO₃ takes place by classical nucleation of fine DO₃ domains in B2 matrix. No evidence for DO₃ ordering could be obtained in alloy containing less than 11 at.% Si.     

The martensitic phases and their stability in CuZn and CuZnAl alloys—II. The transformation between the close packed martensitic phases

The stability between the close packed martensitic structures 6R, 18R and 2H in CuZnAl single crystals is analysed. Experimental results for high electron concentrations (e/a > 1.50) and data from the literature for lower e/a are used, covering a wide range of compositions. Numerical values for the stacking fault energies and the differences of enthalpy are reported. The thermodynamic stability of the orthorhombic 18R structure with respect to 6R and 2H in a wide composition range is established.     

The martensitic phases and their stability in CuZn and CuZnAl alloys—III. The transformation between the high temperature phase and the 2H martensite

The stability between the high temperature β phase and the 2H martensite is analyzed in CuZnAl single crystals of a high electron concentration around 1.52. The results are compared with those deduced from previous reports, and the equilibrium temperatures and entropy differences are determined. A phenomenological model for the stress induced transformation explains well the observed crystallography.     

The martensitic phases and their stability in CuZn and CuZnAl alloys—I. The transformation between the high temperature β phase and the 18R martensite

The stability between the high temperature β phase and the orthorhombic martensite in CuZn and CuZnAl single crystals is analyzed. Experimental results for high electron concentrations (e/a > 1.50) and data from the literature for lower e/a are used, covering a wide range of compositions. Numerical values for the differences of entropy and enthalpy are reported. Also, the contribution of L2₁ order to the phase stability is evaluated and an estimation of pair interchange energies is obtained.     

Shape memory effect and related transformation behavior in FeNiC alloys

Shape memory effect in two representative FeNiC alloys, 31% Ni-0.4% C and 27% Ni-0.8% C, with low Ms temperatures has been studied in detail. The shape memory test was conducted not only on as-austenitized specimens but also on ausformed specimens. The effect of an external load during reverse transformation was also examined. The reverse martensitic transformation behavior related with the shape memory effect was observed in situ, using a high temperature optical microscope. The results are (1) about 50% shape recovery is observed for up to 5% initial tensile strain, while 75–95% shape recovery is obtained for 1–2% initial bending strain. (2) An ausformed specimen shows a better shape memory effect probably owing to the increased austenite strength. (3) The austenite-martensite interface moves backward on heating not only for a plate-type martensite but also for a curved or irregular shaped martensite in a specimen strengthened by ausforming. (4) The shape recovery decreases with increasing an applied load, but even in this case the reversible interface movement occurs.     

Kinetics of the amorphous to icosahedral phase transformation in AlCuV alloys

Single phase icosahedral samples are obtained by annealing melt-spun Al₇₅Cu₁₅V₁₀ amorphous alloys. The kinetics of this amorphous to icosahedral phase transformation were measured isothermally and nonisotheramally by differential scanning calorimetry and from changes in the electrical resistivity. Transmission electron microscopy and energy dispersive X-ray analysis indicate that the transformation proceeds polymorphically by nucleation and growth, ruling out a “micro-quasicrystal” model of the glass in this system. A standard Johnson-Mehl-Avrami analysis of the isothermal, transformation data yields Avrami exponents in the range 2–2.5, which are inconsistent with a polymorphic transformation. These anomalous Avrami exponent arise from an inhomogeneous distribution of quenched-in nuclei. Fits are made to a kinetic model assuming a constant nucleation rate and growth on these quenched-in nuclei. An analysis of the nucleation rates obtained from these fits gives an estimate for the interfacial energy between the icosahedral phase and the glass of 0.002 J/m² ⩽ α ⩽ 0.015 J/m², demonstrating that the short range order must be similar on both sides of the interface.     

Computational modelling of heat⧹mass transfer near the LIQUID–SOLID interface during rapid solidification of binary metal alloys under laser treatment

This paper presents the results of an investigation into the problem of planar solid–liquid interface stability during rapid solidification of binary metal alloys under laser treatment. A new quantitative model is proposed. This model describes the self-organized development of stable spatially-periodic vortices in the melt near the solid–liquid interface due to concentration- (or thermal) capillary effectsfn2 together with effects due to the influence of normal concentration or temperature gradients directed from the interface towards the melt. These vortices give rise to a cellular structure at the solid–liquid interface of rapidly frozen melts.A computer code was developed to solve the set of second-order linear differential equations which describe heat and mass transfer at the liquid–solid interface. This model allows calculation of the liquid phase velocity field, the second component concentration field in the melt, as well as the temperature field in the liquid and solid phases near the solid–liquid interface at a given solidification rate.     

Thermodynamic prediction of MS and driving force for martensitic transformation in FeMnC alloys

The M_S temperatures of FeMnC alloys have been calculated by the application of LFG model of ΔG^γ→α with Mogutnov's ΔG^γ→α_Fe, Hsu-(A) model with Orr-Chipman's ΔG^γ→α_Fe and Hsu-(B) model with Orr-Chipman's ΔG^γ→α_Fe and are in good agreement with the experimental values. Through the mathematical treatment, the relationship between M_S and the composition of FeMnC alloys can be obtained as M_S(K) = 817.4 − 7513.4X_C− 4141.9X_Mn− 32083.5X_CX_Mn (LFG with Mogutnov et al.'s ΔG^γ→tα_Fe); M_S(K) = 829.9 − 7580.5X_C− 4146.0X_Mn− 15727.8X_CX_Mn [Hsu-(A) with Orr-Chipman's ΔG^γ→α_Fe]; M_S(K) = 829.2 − 7276.1X_C− 2915.4X_Mn− 43825.7X_CX_Mn [Hsu-(B) with Orr-Chipman's ΔG^γ→α_Fe]. Where X_C = atom fraction of carbon and X_Mn = atom fraction of manganese. The linear correlation coefficients of these relations are larger than 0.992. Both carbon and manganese depress M_S linearly and the effect of carbon is almost two-fold stronger than that of manganese. Introduction of an interaction term (X_CX_Mn) between alloying elements in the present treatment shows that carbon and manganese enhance the effect upon M_S of each other. The driving force for transformation increases monotonically with carbon and manganese content and there is no singularity. The calculated M_S and the driving force largely depend on the ΔG^γ→α model and ΔG^γ→α_Fe values adopted.     

The solidification behaviour of undercooled AlCu alloys in contact with the primary phases

The near eutectic AlCu liquid is undercooled in contact with both α-Al and θ-Al₂ Cu phase by entrained droplet technique to study and understand the evolution of solidification microstructures at different undercoolings and for different substrates. Four different types of microstructures could be observed in droplets in contact with α-Al and two distinct morphologies could be observed in the droplets in contact with θ-Al₂ Cu. The results can be qualitatively explained with the help of an extrapolated metastable phase diagram and present understanding of solidification in undercooled melt. Finally the knowledge gained is extended to analyse the solidification behaviour of AlCu eutectic melt pool obtained by pulsed laser processing.     

A transformation kinetic model and its application to CuZnAl shape memory alloys—I. Isothermal conditions

By generalizing the impingement factor, an isothermal transformation kinetic model dy/dt = nk(1 − y)^{c + 1} (kt)^{n − 1} with a kinetic parameter of the impingement exponent, c, has been developed. The correlation between the existing isothermal kinetic functions, such as the Johnson-Mehl-Avrami equation and the Austin-Rickett equation, has been explained in terms of difference in the impingement exponent c. This paper describes a procedure for the simultaneous determination of the kinetic exponents, n and c, and their effects on the shape of the (y vs 1n t) kinetic curves. It has shown in the isothermal case that the use of the Arrhenius type equation of activation can be justified when activation energy is independent of the fraction transformed y. This modified kinetic equation has been applied to the analyses of the isothermal aging kinetics of three CuZnAl shape memory alloys with different aging products. These experimental results provide a further demonstration of validity of this modified kinetic equation for analyzing the heterogeneous solid state transformation kinetics.     

A high resolution electron microscope study of the omega transformation in ZrNb alloys

High resolution direct lattice imaging and dark field electron microscopl lere used to examine the omega phase transformation in lr-Nb allols. Direct lattice imaging demonstrated the existence of subvariants lithin an omega variant. The existence of an ordered sequence of subvariants, lhich is the basic premise of recent diffuse intensitl calculations lhich seek to explain diffuse diffraction observations in high Nb content allols, could not be checked because of the small sile of the omega regions. In the lol Nb content allols dark field electron microscopl was used to shol that the λ phase consists of large domains (100–200 Å dia.) the interior of lhich contains features that are 3–6 Å dia. As the Nb content is increased the omega domains decrease in sile until onll 3–5 Å images are observed in allols containing 15lt.% Nb or more. The isolated images are present over the range of composition from 8 to 30lt.% Nb. Time sequence dark field micrographs shol that these small images change lith time. The diffuse λ reflections are believed due in part to the existence of a 〈111〉 linear detect, consisting of groups of 〈111〉 rols of atoms lhich are displaced from b.c.c. to λ positions for short periods of time.     

Investigation of the martensitic transformation in TiFe alloys by means of the mössbauer effect

In the present work we have investigated the β to α′ martensitic transformation in Ti-Fe alloys by means of Mössbauer Spectroscopy and at lesser extent scanning Electron Microscopy. We found that the critical concentration above which the β-Ti(Fe) phase is fully retained by quenching to room temperature is 7.4 at.% Fe. For samples slowly cooled from the β-region of the phase diagram to room temperature, the isomer shift of the β-phase does not show any dependence on iron concentration. This result is explained in terms of the transformation β→β′ + α, where β′ has the eutectoid concentration. The α-phase was not present in the Mössbauer spectra due to its lower iron content as seen by scanning electron microscope.A new phase, denoted by α_p′ was found to form as a result of cold rolling of β-Ti(Fe) alloys for iron concentrations up to 13 at.% Fe. This new phase, α_p′, has an isomer shift which is positive in contrast to that of the α′-phase.A comparison with results found in the literature is also discussed.     

Grain coarsening in FeNiCr alloys and the influence of second phase particles

The effects of second phase particles, e.g. M₂₃C₆, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined. Various theoretical models on grain coarsening have been compared. The grain size at all stages of grain coarsening was dependent on the undissolved carbide particle size (r), the volume fraction (f), and the nature of the carbides. The nature of M₂₃C₆ carbides varied, since Fe, Ni and Mo entered the M₂₃C₆ unit cell; and complex carbides such as (Cr₁₅Fe₄Ni₂Mo₂)C₆ were formed. Gladman's equation was verified for predicting the observed grain size values to a significant degree, and other models were less successful.