This chapter deals with diffusional phase transformations. We first define a phase and move on to discuss various ways of classifying phase transformations. We then discuss in detail the energetics (thermodynamics) and kinetics of diffusional phase transformations. Transformations discussed include: precipitation, atomic ordering, spinodal decomposition, massive and cellular transformations in the light of the sections on energetics and kinetics. A final section of the role of symmetry in developing microstructure concludes the chapter.

This report is the first analysis of the coexistence and microstructure of the equilibrium phases in the Fe-Pd L10 + L12 eutectoid region. Coexistence of L10 + L12 is observed at higher temperatures (650 ^∘C ), resulting in L10 polytwin plates with internal boundaries that are decorated by L12. For higher Pd content, the L10 plates are embedded in an extended L12 matrix, but the L12 wetting layers still persist. For aging at low temperatures (525 ^∘C ), L1’ + L12 coexistence is observed, but the microstructure is essentially similar, except that L10 is replaced by L1’. The two-phase region is found to be much narrower than reported in published phase diagrams, of order 0.6 to 1 at pct in extent. There may be a further re-entrant narrowing below the L1’ formation temperature. This work establishes L1’ as a phase distinguishable from both L10 and L12, but does not yet prove that L1’ is an equilibrium phase. The preferred formation of L1’ at lower temperatures may relate both to stability conferred by overall ferrimagnetic interactions, and perhaps by kinetics, where L1’ should have a reduced nucleation barrier from A1 relative to L10.

Direct evidence is provided for the existence of the tetragonal L1' phase, first predicted by Shockley in 1938, in bulk Fe -62 at% Pd alloys aged at 525 & LCIRC;C. L1' existence as the dominant phase is supported by quantitative x-ray diffraction analysis. This is combined with transmission electron microscopy of the polytwinned microstructure, examining the diffracted intensities in specific superlattice reflections where the complete extinction in L10 is relaxed in L1'. Ordering to L1' appears to occur directly from the A1 parent phase at 525 & DEG;C, while aging at 650 & DEG;C only produces L10. The possibility of L1' ordering may have consequences for the ferromagnetic properties of classic and important binary alloy systems where L10 is the assumed equilibrium phase.

We report the first detailed investigation into how exchange-coupled magnetic properties evolve during the formation of the Co-Pt L1(0) + L1(2) nanochessboards, which self-assemble by a pseudo-spinodal mechanism below the eutectoid isotherm. The maximum observed coercivities exceed 3 kOe, but these values are more than 5 times lower than the largest values predicted by micromagnetics simulations of ideal, single-colony chessboards. For magnetization reversal controlled by nucleation, a simple analysis readily reconciles this discrepancy by accounting for misorientation associated with the polycrystalline structure that exists at the 50 mu m lengthscale, demagnetizing fields associated with eutectoid colonies at the 0.5 mu m lengthscale, and non-idealities in the chessboard structure at the filing lengthscale of about 20 nm.

This work correlates the evolution of magnetic properties with microstructure for aging of Co–Pt alloys with compositions bracketing the A1→L10+L12 eutectoid at about 60at% Pt. The magnetic properties of Co41.7Pt58.3 were observed to vary intricately with annealing time and temperature, corresponding to changes in microstructural lengthscales, phase fractions, and degree of ordering. This alloy exhibited a maximum coercivity of 4kOe and a maximum remanence ratio of 0.8, occurring at an early stage of transformation when strained L10 nanoparticles are embedded in the A1 matrix. For particle sizes below 40nm, robust exchange coupling is observed. Although the L10 nanoparticles are rigorously below the single-domain critical size for zero applied field, we argue that a form of domain wall pinning controls magnetization reversal in the peak coercivity sample. After extended aging that completely consumes the A1 phase, quantitative x-ray analysis indicates that the L12 phase is also present, suggesting that two-phase region is broader than expected. Samples with composition Co37.6Pt62.4 formed only soft ferromagnetic L12 upon aging.

We investigate the influence of crystallographic orientation and anisotropy on local phonon density of states, phonon transmissivity, and Kapitza conductance at interfaces between Lennard-Jones solids via classical molecular dynamics simulations. In agreement with prior works, we find that the Kapitza conductance at an interface between two face-centered cubic materials is independent of crystallographic orientation. On the other hand, at an interface between a face-centered cubic material and a tetragonal material, the Kapitza conductance is strongly dependent on the relative orientation of the tetragonal material, albeit this dependence is subject to the overlap in vibrational spectra of the cubic and tetragonal materials. Furthermore, we show that interactions between acoustic phonons in the cubic material and optical phonons in the tetragonal material can lead to the interface exhibiting greater “thermal anisotropy” as compared to that of the constituent materials. Finally, it is noted that the relative match or mismatch between the Debye temperatures of two materials comprising an interface does not serve an accurate gauge of the efficiency of interfacial thermal transport when those materials have different crystal structures.

In this paper the concept of pseudospinodal decomposition introduced by Ni and Khachaturyan [1] as a symmetry-lifting continuous phase separation, which can produce coherent nanoscale morphologies ranging from nanowires to nanolaminates, is reviewed. The term spinodal arises from the continuous change in the compositions of emerging cubic and tetragonal phases resulting in quasi-periodic microstructures stemming from the attendant transformation strain and surface energy anisotropies. It is argued here that important features of the pseudospinodal mechanism can be understood in terms of conventional classical and non-classical nucleation and that the behaviour is more general than the cubic → tetragonal transformation context articulated by its authors. Also, the possible relevance of the pseudospinodal mechanism to studies of decomposition of hypostoichiometric Fe-Pd alloys will be presented.

This study continues previous work on off-stoichiometric Fe-Pd alloys using a combined reaction strategy during thermomechanical processing [1,2]. Severe plastic deformation of the initial disordered fcc gamma phase (γ) of compostion Fe-35at.%Pd, followed by heat treatment in the two phase field produces a nano-composite ferromagnet comprised of soft alpha phase/ferrite (α) in a high-anisotropy L10 FePd matrix. The length scale and morphology of the transformation products have been characterized using x-ray diffraction, and scanning electron microscopy. The transformed microstructures exhibit strong texture retention similar to the stoichiometric alloy suggesting a massive ordering mode. The alloy has shown a proclivity to exchange couple at a length scale not in agreement with proposed theories of exchange coupling [3,4]. The magnetic properties were measured using standard vibrating sample magnetometry (VSM). This research has been supported by the National Science Foundation (NSF-DMR).

Hypostoichiometric Fe-Pd binary alloys (35-45 at% Pd) were severely deformed (>90%) and subsequently aged to induce concomitant recrystallization, precipitation, and ordering. This thermomechanical processing strategy was articulated by Hornbogen [1] over thirty years ago. The resulting exchange-coupled ferromagnets contain ferrite precipitates and a complex metastable two-phase lamellar transformation product comprised of ordered L1(0) and a metastable FCC phase. The later duplex microconstituent is suggested to form in conjunction with a so-called pseudospinodal reaction [2] involving emerging cubic and tetragonal phases, whereby phase separation and ordering result from continuous changes in composition and a reduction in symmetry, cubic to tetragonal. The deformation texture of the parent austenite is substantially retained in the transformation product, resulting in anisotropy of the magnetic properties as determined by magnetometry (VSM). This paper presents electron microscopy results elucidating the crystallography and morphology of the phase mixtures including HREM. Magnetic field annealing is also included as a branch of our thermomechanical processing strategy, and we discuss the influence of the external fields on recrystallization, precipitation, and ordering.

The tetragonal lattice relaxation has been included in the thermodynamics of the fcc→L10 ordering to produce a first-order character of the transition within the mean field description of the binary solution energetics. In view of growing interest in such systems e.g. Fe-Pd and Co-Pt alloys, which display a wide range of applications relevant to current and futuristic technologies, the fcc→L10 two-phase field is re-examined utilizing a generalized Bragg-Williams approach including first and second nearest neighbor interactions. The thermodynamic behavior is examined in the limit of T→0K and discussed in terms of the implications of the Third Law of Thermodynamics.

With its fast write and read, small cell size, non-volatility and excellent endurance, Spin Transfer Torque-RAM (STT-RAM) has a high potential of dominating the embedded and standalone memory world in the near future. In this paper, the suitability of different classes of magnetic materials constituting the STT-RAM free layer is reviewed for faster switching and thermal stability. We identify the following material classes for faster switching in the thermally stable free-layer of a STT-RAM: (a) In-plane materials with high HK and low MS. While the high HK deters the magnetization during the easy to hard axis switching, it helps with switching past the equator, making the switching speeds for high and low HK materials comparable. However, high HK materials benefit from higher thermal stability. (b) Perpendicular materials with low damping have the same switching speed as in-plane materials but greater switching probability because of a lower critical current. The demagnetization field helps the free layer to start switching to the hard axis, but hinders it from switching further to the easy axis beyond the equator. (c) Anti-ferromagnetically capped partially-perpendicular materials. Capping with a Va layer decreases the demagnetization field, which promotes faster switching.

In this article, some views on the nature of incoherent interphase interfaces, and their role in the nucleation and growth processes governing the evolution of microstructure in solid-state diffusional transformations (reconstructive transformations), are explored. It is argued that essentially incoherent interfaces can be involved in the initiation and propagation of polymorphic transformations and massive transformations as well as in various precipitation phenomena in metallic and ceramic systems. Similar views have already been advanced earlier in connection with studies of massive transformations. Faceting along the interphase interface during nucleation and growth can derive from thermodynamic, kinetic, and crystallographic factors independent of the bicrystallography of the conjugate phases. This idiomorphic behavior can be relevant to both intergranular and intragranular phase formation. The concept of one-dimensional (1-D) commensuration of phases through plane edge-to-edge/row matching is an interesting extension of the classic ideas of coherency and bicrystallography and potentially important in characterizing the behavior of certain types of boundaries. However, the general importance of these geometrical relations in real and reciprocal space will depend on the depth of the energy wells in orientation space associated with these special boundaries.

Dynamic in-situ heating and post-mortem TEM studies have been performed to investigate the mechanisms of the transformation of hcp e-phase to Llo-ordered t-phase in a Mn-Al-C base alloy. A new hybrid displacive-diffusional mechanism has been identified and the synergistic role of the nucleation interfaces of the massively formed t-phase has been discussed.

The vacancy walk in intermetallics of type L1(2) and B2 was studied by using Monte-Carlo simulation with a residence time algorithm. To decide between a homogeneous or heterogeneous state of the system, a coarse graining procedure was introduced, and the mean square deviation of the local order parameter was taken as a measure of heterogeneity. It was found that a sufficient fraction of lattice sites must be visited by a vacancy so that a phase transformation can be observed as homogeneous throughout the volume of a crystal. It is explained why this criterion should be preferred over an earlier one where overlap of the mean quadratic random walk distances of vacancy migration was considered.

Cu-Ti alloys have significant high-strength and high-conductivity and may replace conventional Cu-Be alloys in numerous applications. However, overaging in these alloys is associated with the formation of a coarse lamellar microconstituent (cellular regions) which nucleates at the grain boundaries of the parent matrix phase. The growth of these cells consumes the metastable, fine-scale coherent/semicoherent phase mixtures leading to a rapid degradation of mechanical properties. It is therefore important to control the nucleation and growth of the cellular or "discontinuous" precipitation reaction, in order to optimize the physical properties of the Cu-Ti alloys. In this paper we investigate the identity and stability of the phases present in the cellular regions of the overaged alloys. We find that the beta-Cu4Ti (Au4Zr) phase is present in the cellular regions at temperatures as low as 450 degrees C.

Nearly equiatomic FePd (Fe-52at.%Pd) alloys have been deformed by cold-rolling to 60thickness reduction. Ordering and disordering was studied during isochronal and isothermal annealing by residual resistometry (REST) in the deformed as well as in the recrystallized state. In both cases a first order phase transition with a broad thermal hysteresis is observed. Resistivity values corresponding to thermal equilibrium of LRO-states, however, result lower in the deformed case. This is interpreted as a consequence of internal stresses leading to a preference of one variant of ordered domains and consequently to a higher degree of LRO. A detailed study by an isothermal small-step annealing treatment yields two counteracting exponential processes during order-order relaxation with an activation energy of 2.7eV and 2.4eV, respectively.

Nearly equiatomic FePd (Fe-52at.%Pd) alloys have been deformed by cold-rolling to 60thickness reduction. Ordering and disordering was studied during isochronal and isothermal annealing by residual resistometry (REST) in the deformed as well as in the recrystallized state. In both cases a first order phase transition with a broad thermal hysteresis is observed. Resistivity values corresponding to thermal equilibrium of LRO-states, however, result lower in the deformed case. This is interpreted as a consequence of internal stresses leading to a preference of one variant of ordered domains and consequently to a higher degree of LRO. A detailed study by an isothermal small-step annealing treatment yields two counteracting exponential processes during order-order relaxation with an activation energy of 2.7eV and 2.4eV, respectively.