James M. Howe

Interphase boundary structures of intragranular proeutectoid α plates in a hypoeutectoid TiCr alloy

Abstract Interphase boundary structure on the board faces of proeutectoid α plates in a retained β matrix has been studied in a Ti-6.62at. %Cr alloy by means of conventional and high resolution transmission electron microscopy. Misfit compensating ledges with b = c /2[0001] α are observed on the broad faces of α plates. Growth ledges, irregular in path, spacing and alignment, are also observed. The apparent habit plane of the broad faces of α plates and the riser directions of the misfit compensating ledges, determined by trace analysis, imply the presence of the structural ledges predicted in the companion modeling study. High-resolution TEM observation has revealed that biatomic structural ledges with ( 1 1 00) α //(2 11 ) β terraces do indeed exist on the broad faces of α plates, with exactly the characteristics predicted. This result provides the first experimental evidence for structural ledges in any b.c.c.:h.c.p. system.

Crystallographic and mechanistic aspects of growth by shear and by diffusional processes

Growth by shear and by diffusional processes, both taking place predominantly by means of ledge mechanisms, are reviewed for the purpose of distinguishing critically between them at the atomic, microscopic, and macroscopic levels. At the atomic level, diffusional growth is described as individual, poorly coordinated, thermally activated jumps occurring in the manner of biased random walk, whereas growth by shear is taken to be tightly coordinated “glide” of atoms to sites in the product phase which are “predestined” to within the radius of a shuffle. Obedience to the invariant plane strain (IPS) surface relief effect and the transformation crystallography prescribed by the phenomenological theory of martensite is shown to be an unsatisfactory means of distinguishing between these two fundamentally different atomic growth mechanisms. In substitutional alloys, continuous differences in compositionand in long-range order (LRO) from the earliest stages of growth onward are concluded to be the most useful phenomenological approach to achieving differentiation. At a more fundamental level, however, the details of interphase boundary structure are the primary determinant of the operative mechanism (when the driving force for growth is sufficient to permit either to occur). In the presence of a stacking sequence change across the boundary, terraces of ledges are immobile irrespective of their structural details during diffusional growth. Kinks on the risers of superledges are probably the primary sites for diffusional transfer of atoms across interphase boundaries. In martensitic transformations, on the other hand, terraces containing edge dislocations in glide orientation or pure screw dislocations are mobile and accomplish the lattice invariant deformation (LID), though probably only after being overrun by a transformation dislocation. Risers associated with transformation dislocations are also mobile and cause the crystal structure change during growth by shear. The successes achieved by the invariant line (IL) component of the phenomenological theory of martensite in predicting precipitate needle growth directions and precipitate plate habit planes (Dahmen and co-workers) are here ascribed to the rate of ledge formation usually being a minimum at an interface containing an IL, primarily because nuclei formed sympathetically at this boundary orientation are likely to have the highest edge energies. Since martensite plate broad faces also contain the IL, the ability of the phenomenological theory to predict the habit plane and the orientation relationships of both precipitate and martensite plates is no longer surprising. The IPS relief effect at a free surface can be generated by precipitate plates when growth ledges are generated predominantly on only one broad face and only one of several crystallographically equivalent Burgers vectors of growth ledges is operative. Both pReferences probably result from larger reductions in transformation strain energy for the particular geometry with which a given plate intercepts the free surface. Precipitate morphology often differs significantly from that of martensite even if precipitates are plate-shaped and can readily differ very greatly. Whereas martensite morphology is determined by the need to minimize shear strain energy, that of precipitates derives from the more flexible base of the interphase boundary orientation-dependence of the reciprocal of the average intergrowth ledge spacing, as modified by both the orientation-dependence of interkink spacing on growth ledge risers and the spacing/ height ratio dependence of diffusion field overlap upon growth kinetics.

Convergent-beam electron diffraction analysis of the Ω phase in an Al-4.0 Cu-0.5 Mg-0.5 Ag alloy

Abstract Convergent-beam electron diffraction (CBED) was employed to determine the point group of the Ω phase in an Al-4.0 Cu-0.5 Mg-0.5 Ag (wt%) alloy. The [100]Ω|〈112〉α CBED pattern had 4 mm symmetry and not 2 mm as expected for the orthorhombic Ω structure. The CBED symmetries along the [010]Ω|〈110〉α and [001]Ω|〈111〉α zone axes were 2 mm and the only point group consistent with these symmetries is 4/mmm. Thus, the crystal structure of the Ω phase is not hexagonal, monoclinic, or orthorhombic as proposed by others, but tetragonal. Electron diffraction patterns of the Ω phase can be indexed on the basis of a tetragonal unit-cell with parameters a = b = 0.6066 nm, c = 0.496 nm. In comparison, the equilibrium θ (Al2Cu) phase in AlCu alloys has a tetragonal structure with unit-cell parameters a = b = 0.6066 nm, c = 0.4874 nm. The 1.76% increase in the c-parameter is sufficient for the tetragonal phase (designated θM) to attain nearly perfect atomic matching on the {111}α planes and still maintain 4-fold symmetry along the [001]θM|〈112〉α direction. In addition, the number of variants, hexagonal morphology and preferred growth of the θM phase along 〈112〉α are consistent with symmetry considerations, the proposed θM crystal structure and experiment TEM observations.

Structure and deformation behavior ofT 1 precipitate plates in an Al- 2Li- 1 Cu alloy

A high-resolution TEM study was performed in order to determine the structure, transformation mechanisms, and deformation behavior ofT1 precipitate plates in an Al-2Li-lCu alloy aged to peak strength. It is shown that a possible structure for theT1 plates may be an A1BA2C... stacking of close-packed planes with the A, planes mostly Al, the B and C planes containing a mixture of Cu and Al, and the A2 planes mostly Li. Furthermore, the structural transformation necessary to achieve the A1BA2C ... stacking from the ABC ... matrix planes may be accomplished by the nu-cleation and propagation of a pair of Shockley partial dislocations on every third and fourth {111}α matrix plane. After straining 3 pct, theT1 plates are sheared by dislocations, which cause a disruption in atomic order within the precipitates. Experimental evidence also indicates that the binding between Li and Cu in Al-Li-Cu alloys may be a major factor in determining microstructural evolution in this system, and comparison of diffraction data from hexagonal precipitates on {111}α planes in a variety of Al alloys indicates that they may have similar structures.

The structure of kinks at dislocation interphase boundaries and their role in boundary migration—I. Experimental observation of kink motion

Abstract The role of kinks in dislocation ledge motion during precipitation of γ′ plates in an Al-Ag alloy was studied by conventional, high-resolution and in situ hot-stage TEM techniques. It is shown that kinks are necessary for motion of the dislocation interphase boundary and that kinks may be produced by orientation, thermal and intersection events. Analysis of the behavior of kinks at the interphase boundary utilizing isotropic dislocation theory is in reasonable agreement with experimental observations.

Analytical transmission electron microscopy analysis of Ag and Mg segregation to {111} θ precipitate plates in an Al-Cu-Mg-Ag alloy

Abstract The composition of {111} θ precipitate plates one and two unit cells thick in an Al-Cu-Mg-Ag alloy was investigated using an analytical transmission electron microscope with a field emission gun. These studies revealed that Ag and Mg segregate to the {111} habit plane interface and that some Mg may also be present within the precipitates. The effect of Ag and Mg on the nucleation behaviour of {111} θ precipitate plates is discussed.

Characterization of γ plate-shaped precipitates in An al-4.2 at.% Ag alloy : Growth kinetics, solute field, composition and modeling

Abstract The lengthening and thickening rates of γplate-shaped precipitates in an Al–4.2 at.% Ag alloy were determined by transmission electron microscopy (TEM) for aging times of 120–720 s at 400°C. The compositions of the γ plates and the solute fields surrounding them were measured quantitatively using energy-dispersive X-ray spectroscopy (EDS) and revealed qualitatively by energy-filtering TEM (EFTEM). The shape of the solute field around the plates was also revealed by employing a double-step aging treatment to produce GP zones in the supersaturated α matrix around the precipitates, but not in the solute-depleted zone surrounding the plates. The combined data obtained from these measurement were then compared with the Ham, Horvay and Cahn (HHC) model for precipitate growth. The results show that: (1) the lengthening rate of the plates ranged from 77 nm/s at 120 s aging to 15 nm/s at 720 s aging, the thickening rate ranged from 0.2 nm/s at 120 s aging to 0.09 nm/s at 720 s aging, and the aspect ratio increased rapidly with time; (2) the solute gradient around the plates is approximately two plate thicknesses wide and has a faceted-elliptical shape for all aging times examined; (3) the solute concentration in the matrix immediately adjacent to the faces of the plates is higher than adjacent to the edges, indicating inhibited growth at the faces, (4) the Ag concentration in the γ plates is significantly lower than that given by the equilibrium Al–Ag phase diagram; and (5) the HHC model for the growth of an oblate spheroidal precipitate predicted the lengthening and thickening rates of the plates reasonably well when the time dependence of the aspect ratio was included in the calculations.

Comparison between the invariant line and structural ledge theories for predicting the habit plane, orientation relationship and interphase boundary structure of plate-shaped precipitates

Abstract A comparison is made between the invariant line and structural ledge theories for predicting the habit plane, orientation relationship and (misfit) dislocation structure at interphase boundaries for various transformation strains. It is shown that the fundamental approaches of the invariant line and structural ledge theories are significantly different and that this can lead to the same or different predictions for the habit plane, orientation relationship and dislocation structure between two crystals depending on the type of transformation strain. It is also shown that the structural ledge theory is not applicable to transformations that involve expansion or contraction between orthogonal lattices.

Microstructural development and the effect of interfacial precipitation on the tensile properties of an aluminum/silicon-carbide composite

Abstract A 2124 Al/SiC whisker composite was heat-treated to the same matrix hardness in an underaged and overaged condition, and tensile tests were performed at room pressure and under hydrostatic pressure. The overaged composite had a lower ultimate tesnsile strenght and ductility, which was attributed to S-phase precipitation at the Al/SiC w interface. This interfacial precipitation leads to increased load transfer to the whiskers, resulting in whisker cracking and early failure during testing. Hydrostatic pressure did not increase the tensile ductility, indicating that failure occured by strain localization. Microstractural examination suggests that such localization is due to the combined effects of cracking of SiC whiskers, void nucleation in constrained regions between adjacent SiC whiskers, and localized shear produced by the matrix microstructure.

Studies on the deformation behaviour of interfaces in (γ + α2) titanium aluminide by high-resolution transmission electron microscopy

Abstract The deformation behaviour of two-phase (γ + α2) titanium aluminide was investigated by high-resolution transmission electron microscopy. Specimens were deformed in compression at room temperature well beyond the yield point. The α2-Ti3Al lamellae within the γ-TiAl matrix were observed to deform by shearing when intersected by twins. In addition to dislocation slip, plastic deformation in γ-TiAl was found to proceed by (a/6)<112] twinning partial dislocations, as revealed by the steps on twin interfaces, and by the creation of a three-plane (9R) deformation-induced structure parallel to pre-existing twin interfaces. Formation of the 9R structure is due to an antitwinning operation on every third plane. The core structure of (a/6)<112] twinning partial dislocation steps appears narrow compared with similar (a/6)<112] partial dislocations associated with growth ledges at the γ-α2 interface. These observations and deformation mechanisms are discussed in terms of the structure of γ-α2 and twin interfaces.