P. Mukhopadhyay

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 ordered ω phases in (Zr3Al)–Nb alloys

Abstract Microstructural investigations on rapidly solidified Zr3Al based alloys (binary Zr3Al and ternary Zr3Al–3Nb and Zr3Al–10Nb) have revealed some unusual phase transformation sequences. The Zr5Al3 phase (D88 structure) has been found to occur in both the rapidly solidified ternary alloys unlike in the rapidly solidified stoichiometric Zr3Al alloy in which the Zr2Al phase (B82 structure) has been found to be present. The evolution of the D88 phase, which could be regarded as one of the ordered derivatives of the ω phase, could be described in terms of a superimposition of replacive and displacive ordering waves in the β phase. The orientation relationship between the β and the D88 phases has been established. The microstructural changes occurring in the rapidly solidified Zr3Al–Nb alloys during aging have been examined. It has been found that on aging the D88 phase gets transformed into the B82 phase which, on continued aging, transforms to other metastable and equilibrium phases, depending upon the aging temperature. The observed sequence of phase transformations involving different structurally related phases has been along the direction of progressively close packed structures. The symmetry changes associated with the sequence of ω related transformations have been summarized in the form of a symmetry tree.

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.

Precipitation of Zr3 al in a Zr-4.6 wt% Al martensite

The discontinuous mode of precipitation of the equilibrium Zr3Al phase in a supersaturated Zr-4.6 wt% Al martensite has been examined by transmission electron microscopy. The orientation relationship between the matrix and the precipitate phases has been established. The structure of the α-Zr(Al)Zr3Al interfaces has been studied.

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.

Precipitation hardening in nickel-copper base alloy monel K 500

The occurrence of a significant amount of age hardening, due to the precipitation of the γ′ phase, has been demonstrated in the nickel-copper base alloy MONEL K 500. The micro-structure of the precipitation-hardened and deformed alloy has been examined in peak-aged underaged and overaged conditions. An attempt has been made to compare the observed increments in strength in these three aged conditions to those predicted on the basis of relevant theoretical 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.

Tempering of a beta quenched Zr-1.9 wt % Cr alloy

The morphology and the substructure characterizing the martensite produced on beta quenching a Zr-1.9 wt % Cr alloy have been examined by optical and transmission electron microscopy. The structural changes that occur in the martensite structure on tempering at various temperatures in the α+ZrCr2 phase field have also been investigated.

Microstructural studies on cast Zr3Al-3wt%Nb

In the binary Zr-Al system, Zr[sub 3]Al is the intermetallic phase richest in zirconium. In view of its low absorption cross section for thermal neutrons and its good strength and corrosion resistance, Zr[sub 3]Al may have possible applications as a structural material in thermal reactors. This phase has the ordered cubic L1[sub 2] structure and forms through the peritectoid reaction: [beta]-Zr + Zr[sub 2]Al[minus] > Zr[sub 3]Al, the reaction temperature being 1,292 K. Structurally Zr[sub 3]Al is quite similar to the [alpha][sub 2] or the Ti[sub 3]Al phase (ordered hexagonal DO[sub 19] structure) in the Ti-Al system, the two structures differing only in the stacking sequence of the close packed atomic layers. But for its inherent brittleness, Ti[sub 3]Al is a promising structural material for aerospace applications. It has, however, been demonstrated that niobium additions can reduce the brittleness of this phase. Studies on the Ti[sub 3]Al-Nb system have also shown that niobium stabilizes the high temperature [beta] phase (bcc structure) which, on cooling, can decompose through various phase reactions, generating some interesting microstructures. Similar studies on the Zr[sub 3]Al-Nb system have not been reported yet. The present paper describes some microstructural observations made on a cast Zr[sub 3]Al-3wt%Nb alloy.