Extension of solid solubility during massive transformations

Recent studies of massive transformations in systems based on copper or silver have shown that the degree to which the primary solid solution,a, may be extended into the two-phase field,α+β), during a composition invariant massive transformation, varies from system to system, and depends upon a number of factors. A systematic study of this point was performed in the Cu-Zn, Cu-Al, and Ag-Cd systems, using quenching and heating techniques. In each system a series of alloys, increasing by small intervals (0.1 at. pct) of solute content, was examined. Particular care was taken in the control of composition and homogeneity of these alloys. The results show that the highest solute content for which the massively-formed phase can be identified metallographically exceeds the equilibriuma-phase boundary value by about 0.45 and 1.2 at. pct in the Cu-Zn and Cu-Al systems, respectively, and by several atomic percent in the Ag-Cd system. In the latter case, extension of solid solubility into the two-phase region occurs both from the fcca and the hcp ζ equilibrium phase fields. It is suggested that these findings may be interpreted in terms of local nonequilibrium conditions at the massive transformation interface. Formerly Fellow, Metal Physics Laboratory, Mellon Institute of Science, Carnegie-Mellon Univ.     

Distinguishing features of massive transformations

As is the case with all solid state transformations, massive transformations have some characteristic distinguishing features. They are diffusional transformations controlled mainly by diffusion of atoms at the transformation interfaces. There is universal agreement that composition-invariance is one characteristic feature of massive transformations and that a relatively rapid movement of the transformation interfaces is another. Other features involved in massive transformations are the subject of some controversy. Among these is the question of orientation relationships, nucleation features, and the mode of growth. In this article, some of the controversial aspects are discussed with reference to recent publications. This paper is based upon a presentation made at a symposium on The Massive Transformation, held at the Pittsburgh meeting of The Metallurgical Society of AIME and the Materials Science Division of ASM, October 9, 1980, under the sponsorship of the MSD Phase Transformations Committee.     

On the possible diversity of massive transformations

Some main features involved in the characterization of massive transformations are considered. These features are the crystallographic orientation relationships between matrix and product phase crystals, some aspects of nucleation and growth, and the grain structure of product phases. It is shown that no particular crystallographic OR seems to exist in CuGa between a given matrix β crystal and the ζ_m product phase crystals nucleated within the bulk volume of the former. From the existing experimental information it is shown that only those nuclei having portions of non-coherent interfaces are likely to grow. This is apparent in the case of crystals nucleated at grain boundaries which often bear a rational OR with one of the adjoining matrix grains but grow within the other matrix grain (with which no special OR is held). Similarity of shapes of different (faceted) product crystals of the massive phase nucleated at a single parent matrix grain boundary is considered as being indicative that planar interfaces with definite orientations (and presumably low energy) can occur even in absence of a special OR (like the Burgers relationship between b.c.c. and h.c.p. structures). The impingement of adjacent growing product crystals is considered within a single matrix grain. A dynamic balance of surface tensions is proposed among the grain boundaries that growing massive grains form with one another, and the two interfaces they form with the matrix grain. This balance is thought to give an additional contribution to the driving force acting on the portions of migrating interfaces near a triple junction. Thus, an anisotropy in properties of the different product phase grain boundaries and migrating interfaces leads to a possible explanation (through this proposal) for such different grain structures as the bubble-shaped pattern in AgGa, and a quite irregular pattern in CuGa. It is concluded that the kinetics of atomic transfer across the interfaces seems to be the most important single factor affecting the crystallographic features of nucleation (indirectly, through selection of interface structures according to their mobilities). Also, it is concluded that the structure and energy of product phase grain boundaries is an important factor affecting the morphology of the observed transformation products.     

Features of plastic flow of powder Al-40Sn alloy during extrusion

Features of compaction by means of the extrusion of powders of Al-40Sn alloy are investigated. Compaction is carried out in a temperature range of 25–230°C at a reduction coefficient of 4.5 (ɛ = 1.5). An investigation into the structure along the length of a sample, including the butt end, has shown that the main part of its change occurs at the steps of formation of a billet and upon the placement of the latter within an operating channel of a press mold for extrusion. Under pressure operation in this period of time, a new composite material is formed which consists of aluminum particles dispersed into an unbroken soft tin matrix. As such material is forced through the die, tin strata act as an interparticle lubricant, making the mutual displacement of aluminum particles, which do not deform much as a result, easier. As a consequence, oxide films on aluminum particles remain and prevent the establishment of strong interfaces. The extruded material contains cracks along interfaces and shows low plasticity.     

Numerical simulation of three-dimensional heat transfer and plastic flow during friction stir welding

Three-dimensional visco-plastic flow of metals and the temperature fields in friction stir welding have been modeled based on the previous work on thermomechanical processing of metals. The equations of conservation of mass, momentum, and energy were solved in three dimensions using spatially variable thermophysical properties and non-Newtonian viscosity. The framework for the numerical solution of fluid flow and heat transfer was adapted from decades of previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate, temperature, and temperature-dependent material properties. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing. The heat and mass flow during welding was found to be strongly three-dimensional. Significant asymmetry of heat and mass flow, which increased with welding speed and rotational speed, was observed. Convective transport of heat was an important mechanism of heat transfer near the tool surface. The numerically simulated temperature fields, cooling rates, and the geometry of the thermomechanically affected zone agreed well with independently determined experimental values.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

Inhomogeneity of plastic flow in constrained deformation

Crystals of copper, Cu+6 wt pct Al, and Ag+4 wt pct Sn were compressed along [111] with flow restricted to [ \(\bar 1\bar 12\) ]. After deformation, four differently oriented regions were observed. Their origin is explained by the instability of the (111)[ \(\bar 1\bar 12\) ] orientation which can rotate to either (112) \(\bar 1\bar 11\) or (110)[001] during the imposed shape change. The direction of rotation is determined by which of two initially equally favored pairs of slip systems operate. Surface friction produces shear stresses which favor one pair over the other (depending on the sign of the shear stress) and thus one of the final orientations. Since the sign of the frictional stress varies systematically with position in the deforming crystal, a systematic variation of orientation results. Another orientation (001)[110] has also been observed to behave similarly. During rolling, the frictional forces drawing the crystal into the roll gap are also expected to lead to the division of the crystal into two misoriented regions. The predictions are generalized to include bcc metals of ( \(\bar 1\bar 12\) )[111] and (110)[001] orientations. Previously reported observations of rolled crystals of FeNi₃, Fe-3.5 pct Si, and Fe-2 pct Al are in accord with the present analysis.     

Calculation of the plastic flow of P/M material during the forging of axisymmetric disks

Conclusions As a result of an investigation into the process of compression of a porous annular blank, curves have been constructed of the distribution of density and of the parameter of strengthening of the material of the basis. Radial flow leads to the formation, at the inner surface of the blank, of a zone of maximum intensity of bulk and shear strains. This brings the appearance of a region of solid material (=1), whose boundary shifts during compression toward the side wall of the container. The velocities and stresses at the interface between the solid and porous materials are continuous owing to the dependence of the permissible field of velocities on density. With the results obtained it is possible to determine an optimum starting blank shape, ensuring, at a given degree of deformation, the required final dimensions and density of the part.Translated from Poroshkovaya Metallurgiya, No. 7(271), pp. 16–20, July, 1985.     

Effects of self-accommodation and plastic accommodation in martensitic transformations and morphology of martensites

The effects of self-accommodation and plastic accommodation in martensitic transformations and the displacement vector for lattice deformation are discussed. The authors propose that the formation of an invariant habit plane is connected with the self-accomodation between different martensitic variants and results in the formation of internal twinned martensites; the plastic accommodation, rather than self-accommodation, occurs between parent and new phases when the strength is low or the dislocation density is high for the parent phase and the invariant habit plane is difficult to form, resulting in the formation of dislocation martensites.     

Phase transformations during annealing of WC-Co powders

Phase transformations during the annealing of powders produced by the electrodispersion of WC-Co alloys are considered. All the powders contained various amounts of high-temperature β-WC phase. Eutectoid decomposition β-WC → α-WC + W₂C is found to result first in the formation of needle α-WC crystals and then in their destruction due to gliding.     

Phase transformations during heating of llmenite concentrates

llmenite concentrates were heated in argon and oxygen in the temperature range 700 °C to 1000 °C to study the behavior of the pseudorutile phase and other changes which occur. Pseudorutile does not persist in argon or oxygen in the temperature range studied. In argon at 700 °C, pseudorutile decomposes into hematite and rutile, while at 1000 °C, it combines with ilmenite to form ferrous-ferritic pseudobrookite solid solution. A new phase “Fe₂O₃-2TiO₂” was identified as an intermediate product during the heating of ilmenite or pseudorutile in oxygen. This compound decomposes into hematite and rutile below 800 °C and to pseudobrookite and rutile above 800 °C. The sequence of reactions during the heating of ilmenite and pseudorutile in oxygen is proposed. Formerly with the Department of Metallurgy, Imperial College of Science, Technology and Medicine, London. Formerly with the Department of Metallurgy, Imperial College of Science, Technology and Medicine, London.     

Evaluation of regional cerebral blood flow in massive intracerebral calcifications

Fahr's disease is histopathologically characterized by massive bilateral calcifications of the cerebral basal ganglia, the dentate nuclei of the cerebellum and both the cerebral and cerebellar cortices. We report a case of Fahr's disease in which a 99mTc-hexamethyl-propylenamine oxime (99mTc-HMPAO) brain SPECT study was used to evaluate regional cerebral blood flow to the calcified regions. There was markedly decreased perfusion to the basal ganglia bilaterally as well as decreased perfusion to the cerebral cortices that correlated well with the patient's clinical condition.     

A plastic flow induced fracture theory forK iscc

In this paper a theory is developed for K_iscc based on hydrogen induced fracture due to plastic flow on a localized scale within the crack-tip plastic zone. The theory is qualitatively and quantitatively consistent with experimental data on the influence of yield strength and hydrogen content on the threshold level and is also able to explain the effect of material composition and microstructure on K_iscc. Evaluation of the yield strength dependence of K_iscc. using this theory, appears to indicate that secondary cracks should initiate a few μm’s from the main crack tip, that is, at a scale usually finer than grain size, suggesting that carbide and inclusion interfaces or martensite lath boundaries could play a role in the fracture initiation. Analysis of the hydrogen concentration dependence of K_iscc by means of this theory indicates that this dependence may be a direct result of the interaction of hydrogen with hydrogen traps in the material through terms involving the binding energy of hydrogen to the traps responsible for initiating fracture. Formerly Associate Research Scientist, Henry Krumb School of Mines, Columbia University     

Localization of plastic flow: spatial vs temporal instabilities

The initiation of unstable plastic deformation associated with dynamic strain ageing in controlled load rate tests has been studied. Experimental measurements on an AlMgSi alloy show the existence of two regimes of instability: temporal and spatial. The initiation of the instability is found to occur by the growth of spatially uniform (temporal) strain rate oscillations. At a larger strain, spatial strain localization is found to develop. The existence of an intermediate regime of purely temporal instability is confirmed by a (linear) stability analysis, taking into account stress triaxiality due to non-uniform deformation. Interaction between adjacent material elements via dislocation cross-slip is also considered. Both effects are shown to cause a delay in the onset of spatial instabilities relative to purely temporal instabilities.     

Unstable plastic flow in the deformation of laminar materials

Instability in the plastic deformation of laminar materials is theoretically studied. The initial formulas in the analytical model are consistent with the theory of a third contacting body, taking account of the change in the geometric and energy instability of plastic deformation. The basic solution is in good agreement with experimental data on the deformation of laminar aluminum-copper and aluminum-steel composites.