Incoherent Interface Research Articles

Kinetics of alpha precipitation in Ti-6 wt % Cr and Ti-11 wt % Mo

The morphology and kinetics of the precipitation of the alpha phase produced by two different heat treatment routes, namely, (a) direct isothermal decomposition and (b)β-quenching and subsequent ageing, were studied. In isothermally decomposed samples theβ′ (supersaturated) →α + β transformation was seen to occur mainly through the discontinuous growth of the transformed zone consisting of groups of parallel side plates from the grain boundary regions towards the interior of the grain. Unlike for the case of a regular discontinuous precipitation, here the transformed regions are not separated from the untransformed by an incoherent interface and the growingα-plates do obey a fixed orientation relationship with the grain from which they are evolved. The theory of cellular reaction has been applied to explain the growth rate of the duplex (α + β) region. The overall reaction kinetics were analysed on the basis of the Johnson-Mehl formulation and were found to be consistent with that of a discontinuous precipitation reaction, where grain boundary nucleation sites were saturated at an early stage of the transformation. The structure of theβ-quenched samples showed a uniform distribution of athermal omega particles which acted as precursors to theα-precipitates. As a consequence, the reaction rate was greatly enhanced andα-precipitation in the quenched and aged samples was seen to occur continuously in the entire body of the grain.

Re-examination of the elastic strain energy of an incoherent ellipsoidal precipitate

The elastic strain energy of an incoherent ellipsoidal precipitate in either an isotropic or an anisotropic matrix is re-evaluated as a function of the ellipsoidal aspect ratio (β) using the method of Eshelby. An incoherent interface means an essentially disordered interphase boundary structure and therefore such a precipitate is stressed in a purely hydrostatic fashion along the precipitate:matrix interface. The general behavior of the strain energy found by Nabarro and by Kröner is again observed: that is, the strain energy becomes zero as β → 0 (disk), while reaching its maximum value when β equals unity (sphere). However, comparison of the results of the Kröner approach with the exact results shows a noticeable error in Kroner's results. It is found that the Kröner approach is correct only when the matrix is elastically isotropic and the precipitate shape is one of three special cases, i.e. disk, sphere and needle (β→ ∞). As an example, when β = 0.1, the error is found to vary up to 48%, depending upon the severity of the anisotropy of the matrix.

The effect of heating rate on the martensite to austenite transformation in Fe-Ni-C alloys

The martensite to austenite ( α′ → γ) transformation was studied as a function of heating rate in a series of Fe-Ni-C alloys with carbon contents of 0.004, 0.05, 0.3 and 0.6 wt. %. Observations of microstructure, fine structure, surface relief and the transformation temperatures determined from thermal arrest data for heating rates between about 3 and 28,000°C/sec showed that the α′ → γ transformation can occur by two different, often competing mechanisms. The mechanism which predominates at the higher heating rates in the three higher-C alloys (and over the entire range of rates in the 0.004C alloy) results in second generation γ plates with high dislocation densities and is accompanied by the sharp tilt surface relief characteristic of a martensitic-type transformation. At lower heating rates the α′ → γ transformation in the three higher-C alloys occurs largely by a thermally activated mechanism as indicated by a significant initial increase in the transformation finish temperature, A f , with heating rate and confirmed by the observation of newly-formed γ areas with equiaxed morphology, relatively low dislocation density and lack of sharp tilt relief. The thermally activated transformation apparently involves short-range diffusion across a curved, incoherent interface.

The loss of coherency of precipitates and the generation of dislocations

Abstract The theory of Brown, Woolhouse and Valdre (1968) is extended to include the possibility of the nucleation of interface dislocations by prismatic punching and the condensation of point defects. It is shown that neither process can occur spontaneously (i.e. without the introduction of dislocations by another process such as deformation or irradiation) unless the misfit exceeds a critical value, estimated to be 0·05. Furthermore, unless the radius of the precipitate exceeds certain critical values, the processes will not occur because the energy of the system is not thereby lowered. A comparison of the theory with experiment enables limits to be placed on the stress necessary to cause the generation of dislocations in a perfect lattice, and at an incoherent interface. In the former case the generation stress is higher than hitherto supposed; but in the latter case it appears that an incoherent interface behaves as an ideal source of dislocations, in the sense that when the energy of the system can b...

Internal Friction Due to Precipitation

AbstractInternal friction can be caused by precipitates if they produce internal strains, have elastic moduli different from the matrix or if they have incoherent or semicoherent interface boundaries. By considering the interaction energy with an externally applied stress field, the possible internal friction effects are discussed. It is shown that an internal friction peak can only be caused by interface relaxation in semi‐coherent or incoherent interface boundaries.

A Determination of the Punching Stress at the Interface of Particles during Rapid Quenching

The critical shear stress at the interface of incoherent particles when dislocation punching is first observed in quenched f.c.c. metals has been determined. The values obtained are close to the theoretical shear strengths of f.c.c. metals, assuming that the initial shear is in a 〈112〉 direction. The nucleation of dislocations is apparently much easier at an incoherent interface during quenching than at a fully coherent interface during particle growth, and possible explanations for this are considered.

Concentrations at interfaces in binary alloys

Concentration profiles across interfaces and at a free surface of a binary f.c.c. alloy are computed, using the regular nearest-neighbour approximation. The γ-Fe-Cu system serves as example, and most calculations are made for a temperature where the mutual solubilities are 4 at. % The coherent interface free energy is slightly anisotropic (but rather strongly at low temperatures) and nearly indifferent to a shift of the concentration-profile curve across the lattice planes. Computed concentration profiles at the free surface of Fe-Cu solid solutions show substantial enrichment of Cu. Incoherent interfaces are approximated by planes containing 25% vacant lattice sites. One then finds also appreciable enrichment of Cu at the grain boundaries and the incoherent interphase interface. The chemical and the crystallographic phase boundary are about 8 ápart. It is suggested that an electronic double layer is responsible for the low interface free energy found experimentally for the γ-Fe-Cu interphase.

Metallurgy and Magnetic Properties of an Fe-Co-V Alloy

A comprehensive study of the Fe-49% Co-2% V alloy has revealed many new aspects of the metallography of the alloy and led to a better understanding of its mechanical and magnetic properties. Results of the study necessitate revision of previous reports on the metallography by Greiner and Ellis and Köster and Schmidt. In addition to the well-known phase transformation of γ(fcc) to α(bcc) and the ordering of the αphase, a martensitic transformation has been established. The rapid kinetics of theγ→α transformation can be suppressed by drastic quenching in favor of a diffusionless transformation. Martensite is a metastable bcc structure which decomposes slowly into ordered α at intermediate temperatures. Ordering of the α phase is accompanied by pronounced grain refining. X-ray diffraction studies, elevated temperature measurements of saturation, and metallographic examination have not, however, confirmed the decomposition of the ordered α phase suggested by Greiner and Elli. The problem of brittleness has been broadly investigated. Cleavage fracture is a major cause of brittleness. Other causes include the gaseous (hydrogen and oxygen) embrittlement at grain boundaries and the cracking at the incoherent interface between the α phase and martensite. Unless specially treated,all bcc constituents, including disordered α, are inherently susceptible to cleavage fracture. The effect of order on the susceptibility to cleavage fracture and the multiple functions of quenchingin the attainment of ductility have been elucidated in terms of current theories of cleavage fracture and lattice imperfections. The main purpose of quenching is to produce a martensitic structure. Only when a ductile matrix has been prepared, can ordering exert an adverse effect on the ductility of the alloy. Measurements of saturation magnetic moment at elevated temperatures disclose a 2% increase caused by ordering. Room-temperature magnetization curves are compared for different microstructures. The electronic configuration outside the argon core of vanadium atoms is discussed in relation to the composition variation of the average magnetic moment of Fe-Co-V alloys.