J.B. Singh

Microstructural and mechanical properties of service exposed Alloy 625 ammonia cracker tube removed after 100 000 h

Abstract The present study examined the evolution of microstructure and mechanical properties of Alloy 625 ammonia cracker tubes of heavy water production plant which had been exposed to temperatures around 600°C for 100u2009000 h. Detailed investigations revealed considerable modification of microstructure, significant increase of strength and loss of ductility and toughness during service and the extent of degradations were found to be sensitive to service temperature at different locations of the tube. While the increase in strength was due to precipitation of γ′′ and Ni2(Cr,Mo), the loss in ductility resulted from the presence of carbides and needle shaped δ particles at grain boundaries. When the exposed material was tested at elevated temperatures, the material softened with an attendant increase in ductility and toughness. A solution annealing treatment at 1160°C for 2 h was found to restore the microstructural as well as mechanical properties similar to that of virgin material.

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.

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.

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.

Order – disorder transformation in Ni – V alloys under electron irradiation

Abstract The ordering transformations in nickel-rich Ni–V alloys were investigated in situ in a high voltage electron microscope in the temperature range from 300 K to 773 K. The progress of ordering/disordering was monitored from diffraction patterns taken after different periods of time. The {1 1/2 0} ordering instability was noticed during irradiation of ordered Ni3V alloy at 300 K. The increase/decrease in superlattice intensity corresponding to long range ordered phases (Ni3V and Ni2V) in Ni-29 at.% V alloy has been attributed to the net effect of the simultaneous operation of different competing processes such as radiation induced disordering, radiation induced ordering and thermal diffusion assisted ordering.

Interfaces in ordered intermetallics

In this paper, we have examined different types of interfaces that occur between orientational/translational variants generated during the ordering process. This has been illustrated citing examples of ordering of the FCC structure into DO22, Dla and Pt2Mo type structures in some nickel base alloys. Microstructures consisting of more than one ordered structure have also been investigated. Superlattice domains of DO22 and Pt2Mo type structures have also been found to coexist in a microscopic scale of mixing in Ni-V alloys while mixed domains of Dla and Pt2Mo type structures on a much finer scale have been observed in Ni-Mo alloys. The formation of different variants (rotational and translational) of ordered structure(s) from the disordered lattice has been explained on the basis of group theoretical and symmetry considerations.

Super-Strong, Super-Modulus Materials

In many mechanical applications where rigid designs are required, a material with high modulus of elasticity combined with high strength is desired. As a result of much research in recent years, we now have a fair understanding of the ways of designing and processing materials to achieve desired strength and modulus. Elastic modulus is an intrinsic property of a solid and depends on the nature of interatomic bonds of the constituent atoms. Of the different types of bond, solids with covalent and ionic bonds exhibit a higher modulus than those with metallic bonds. The maximum attainable strength (theoretical strength) in a solid depends on modulus, and good estimates show that it can be as high as 10% of modulus. However, a strength level as high as this is not easily achievable for macroscopic components as the strength of a solid is affected by various factors at several scales of structure including the nature of interatomic bonds, atomic arrangement, defects and microstructure. In general, the strength of materials increases as the micro- and macro-structural length scale decreases, that is smaller is stronger. Further, as the strength of a solid increases, the tendency towards brittleness is enhanced. In this article an overview of the methods available to design high-strength and high-modulus materials with different types of chemical bonding is presented. The ability of materials to reach high strengths is explained in terms of refinement of length scales, either in terms of grains size, twins, ferrite size in bainitic steel, or sample size itself, and in terms of changing bond characteristics as in intermetallics. The development of carbon nanotubes and nanotube-based composites, which exhibit outstanding properties and provide the opportunity to associate high flexibility and high strength with high modulus, is discussed briefly. Reference is made to theoretical and experimental achievements, processing methodologies, characterization techniques and some important applications of super-modulus–super strong materials.

Development of Microstructure and Texture in Al5052 Alloy Processed at Room and Cryogenic Temperatures in an ECAP Die

Development of microstructure and texture in alloy Al5052 deformed at room and cryogenic temperatures in an equal channel angular pressing (ECAP) die has been investigated. Billets were deformed using the Bc route up to 14 passes. Billets pressed at room temperature showed almost a 4 fold increase in the yield strength, which increased to about 320 MPa from about 90 MPa, while it increased to about 230 MPa in billets deformed at cryogenic temperature. At two deformation temperatures, grains were refined at more or less similar size reduction rate as a function of number of passes. However, for a given amount of strain, billets pressed at the two temperatures showed subtle differences in their microstructures and texture.

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.