M. Sundararaman

Some aspects of the precipitation of metastable intermetallic phases in INCONEL 718

Some aspects of the precipitation of the metastable intermetallic phases —γ″ and γ″—in the commercial nickel base superalloy, INCONEL 718, have been investigated over a wide range of aging temperatures. It has been confirmed that the spherical γ″ particles and the ellipsoidal γ″ particles evolve predominantly through homogeneous nucleation. Precipitation of the former does not appear to precede that of the latter in this alloy. The tetragonal distortion associated with the γ″ particles has been found to increase with increasing precipitate size. It has been observed that at certain temperatures, physical association between precipitates of the two types occurs frequently, leading to the development of different composite precipitate morphologies. During coarsening, the precipitate size has been found to depend linearly on the cube root of the aging time for γ′ as well as γ″ particles.

Deformation behaviour of γ″ strengthened inconel 718

Abstract This study on deformation mechanisms of a γ″ (DO 2 structure) strengthened nickel base alloy (Inconel 718) has shown a new type of precipitate shearing mechanism. When the size of γ″ particles exceeds a critical value (~ 1onm), these precipitates are sheared by the passage of true crystallographic deformation twins which do not destroy the ordered atomic arrangements within precipitate crystals. For smaller precipitates, shearing occurs by the movement of a group of dislocations which enables restoration of order. Consequent to the change in the precipitate shearing mechanism the work hardening rate drops to a lower value (work hardening exponent changing from ~0.8 to ~0.5) as the deformation twinning mode becomes operative for precipitates with radii larger than about 10 nm. Strengthening due to precipitation has been estimated as a function of the precipitate size, corresponding to different precipitate shearing and precipitate bypassing mechanisms and these results have been compared with the experimental data.

Evolution of microstructure in the stoichiometric Ni–25at.%V alloy

Abstract The disorder-to-order reaction in the stoichiometric Ni 3 V alloy is a typical cubic-to-tetragonal type (A1 to D0 22 ) transformation. Microstructural evolution has been studied at different temperatures below the ordering temperature in samples of this alloy having the same initial microstructure produced by solution treatment followed by water quenching. The ordered phase has been observed to evolve through a series of structural changes, starting with the impingement and alignment of fine ordered domains corresponding to all the three variants of the ordered phase, followed by the appearance of a two-variant lamellar structure which ultimately gets converted to a structure in which each Ni 3 V grain comprises a single variant. The lamellar structure is found to be quite stable with regard to coarsening in response to heat treatments. Evidence has been found which indicates that a discontinuous coarsening type of reaction is one of the possible mechanisms driving the two-variant lamellar to single-variant morphological transition. It has also been observed that coarsening of domains takes place in a manner similar to Ostwald ripening wherein larger domains coarsen at the expense of smaller domains.

The activation and the spreading of deformation in a fully lamellar Ti–47 at.% Al–1 at.% Cr–0.2 at.% Si Alloy

The spreading of deformation in a lamellar Ti–47 at.% Al–1 at.% Cr–0.2 at.% Si alloy deformed under compression is studied at 25°C and 600°C. This microstructure is largely dominated by twin-related variants which are separated by either twin interfaces or thin α 2 slabs. The alloy deforms at both temperatures by ordinary dislocations and twins. Deformation in a particular γ variant and its adjacent twin-related variant involves the same kind of glide system, either ordinary dislocations or twins. This property is found to be true for all twin-related lamellae. The occurrence of this correlated glide is explained by the introduction of the notion of pilot and driven orientations. The lamellar orientation in which the operating glide system is activated on the basis of Schmid factor considerations is termed the pilot orientation. It imposes its deformation system on to the twin-related lamella, called the driven orientation, whose deformation may not involve the slip system most favoured by the applied stress.

Effect of ternary Nb additions on the stability of the D022 structure of the Ni3V phase

Abstract Effect of Nb additions on the stability of the D0 22 structure of the Ni 3 V phase has been investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. Such additions are found to stabilize the D0 a phase. The result is explained on the basis of existing qualitative models.

Propagation of stacking faults across domain boundaries in Ni-V and Ni-V-Nb alloys with D022 structure

Abstract Disorder-to-order reaction in stoichiometric Ni3V and Ni75V21,5Nb3,5 alloys involve a cubic (fcc)-to-tetragonal (D022 structure) transformation. The structural changes occurring during the transformation result in a lamellar morphology of D022 domains with {110}fcc-type interfaces. The propagation of stacking faults across domain boundaries in Ni3V and Ni75V21,5Nb3.5 has been studied using transmission electron microscopy. Different modes of propagation of stacking faults across these boundaries have been observed. These modes have been attributed to the generation of appropriate glide partials at the domain boundaries. The nature and the Burgers vectors of the glide partials have been identified using visibility and invisibility criteria. Computer simulation results have been used to substantiate the results of contrast experiments. Dislocation reaction mechanisms responsible for the generation of these glide partials have been identified. In the present work, special emphasis has been laid on the propagation mode by which stacking faults cross the domain boundaries in the same glide plane.

A new method for determining the Curie temperature using a dilatometer

Dilatometry is a tool used for the study of dimensional changes in materials as a function of temperature and also to identify phase transformations including magnetic transformations. In this paper, we describe two new methods that can be employed in an inductively heated dilatometer to determine the Curie temperature in metallic ferromagnetic materials. These methods are based on the fundamental magnetic properties of materials such as hysteresis loss and anomalous thermal conductivity changes near the Curie point. These methods have been used to determine the Curie point in nickel, iron and Co?5 at% Ni alloy. The values obtained match well with those reported in the literature. The effects of the geometry of the specimen and of the push-rod material on the measurement sensitivity of the transition temperature have been discussed.

Delineating overlapping structural and magnetic phase transformations in a Fe-5.93at% Ni alloy

Traditionally, a dilatometer exploits dimensional change as a function of temperature/time to study phase transformations in materials. This limits its application where transition temperatures of two transformations overlaps. In this work, an inductively heated dilatometer has been used to delineate the magnetic transition occurring simultaneously with a structural transformation in a Fe-5.93at% Ni alloy by exploiting the fundamental principle of induction heating.

Accommodation of transformation strain at cell interfaces during cubic to tetragonal transformation in a Ni-25at.%V alloy

Abstract The ordering transformation in a stoichiometric Ni-25at.%V alloy involves a cubic (A1) to a tetragonal (DO22) transformation. The microstructure essentially comprises cells (or colonies) of transformation twins corresponding to different variants of the ordered phase. Whereas it is well established that the formation of transformation twins reduces the strain energy associated with such cubic to noncubic transformations, the role of the interface separating two contiguous cells in further reducing the strain energy is generally overlooked. This paper presents some evidences of accommodation of strain at and in the vicinity of the intercell interfaces.