SCIENTIFIC CONTRIBUTIONS

I have made many original contributions to the field of mechanical metallurgy. Starting with my earlier contributions in the area of dislocation dynamics, polycrystal work hardening and grain size strengthening in hexagonal close packed metals zirconium, titanium and cadmium, I have extended the ideas to more complex alloy systems like austenitic stainless steels and nickel-base super alloys. I have some original contributions to my credit in the development of novel experimental techniques for studying dislocation dynamics. Combining Hall-Petch analysis with thermally activated strain rate analysis and designing novel experimental techniques to determine contributions of thermal and athermal components of flow stress to grain size dependent and grain size independent stress components, I have been able to develop a deeper understanding of polycrystal deformation behaviour. The development of the decremental unloading technique for studying stress relaxation and its combination with temperature cycling has been a hallmark in studies related to dislocation dynamics. I have also applied acoustic emission technique and the associated detailed signal analysis to the dynamics of dislocations, their immobilisation as well as cracking events during tensile deformation. This technique has also been applied to the study of dynamic strain ageing (DSA).

My contributions to the area of serrated yielding is the most well known internationally. In Cu-Au alloy, serrated yielding has been shown, to arise due to ordering around dislocations, by carefully planned experiments and theoretical evaluations of interaction between dislocation stress fields and strain due to tetragonal distortion accompanying ordering. My work on CuZn, is equally important as I have suggested a new mechanism, of inhomogeneous ordering with dislocations getting locked in an atmosphere of lower degree of order, for the yield points and serrated flow. DSA studies have been extended to type 316 stainless steel and Nimonic PE16 super alloy. DSA has been shown to depend on grain size and the grain boundaries have been found to be the preferred sites for DSA effects. The role of grain boundaries and precipitates has been further elucidated by conducting a detailed study of the influence of thermal ageing which determines the size and distribution of precipitates in 316 stainless steel, on serrated flow. Certain time‑temperature combinations of ageing are shown to eliminate serrated yielding thus further confirming the role of precipitates and grain boundaries in determining serrated flow.

The correlation of mechanical properties with microstructure has been an important aspect of my contributions. An outstanding example of such studies is the detailed characterization of microstructures produced, in Nimonic alloy PE16, through a variety of thermo‑mechanical treatments. By studying the conditions under which various type of carbides and form, it has been possible to arrive at an optimum heat treatment for best mechanical properties. Various deformation mechanisms and their dependence on the size and distribution of precipitates have been identified and correlations developed with flow and fracture behaviour. The characteristic AE signals from this alloy when deformation takes place by different mechanisms like Orowan looping/cutting, decohesion/fracture of carbides etc., have been deciphered.

An extensive investigation of the influence of various metallurgical parameters like grain size, prior cold work and alloy chemistry in type 316 stainless steels on creep and low cycle fatigue (LCF) has been carried out by my group to develop an understanding of mechanisms of deformation and fracture at elevated temperatures. Such an understanding is extremely important in the design of high temperature alloys. An optimum grain size range for best creep properties has been arrived.

My very early work (1968) pioneering work on the influence of oxygen environment on creep of 304 SS is noteworthy and has been invariably cited, by all the subsequent environment-creep interaction researchers. Another major area in which I have made valuable contributions is creep‑fatigue interaction in austenitic steels. The knowledge of how these two high temperature damage mechanisms interact to determine useful life of components operating at high temperatures is extremely important both for prediction of life as well as in designing alloys resistant to such damage. In addition to the damaging effects of creep and oxidation on low cycle fatigue, my studies have revealed that DSA can also lead to degradation in life. The synergism of the damaging effects of creep, environment and DSA on fatigue life have been shown to depend on test variables like strain range, temperature, frequency, hold periods as well as on micro-structural parameters like grain size and prior cold work. As in the case of tensile deformation, DSA effects in LCF were again found to be dependent on grain size. DSA also influenced fracture mode. Premature grain boundary cracking occurred as revealed by fractographic studies.

The knowledge of formation and growth of cavities is crucial in arriving at meaningful life prediction techniques. The formation of cavities at heterogeneous sites like matrix-precipitate interface has been known and accounted for in models predicting life. The first ever observation on the existence of uniform matrix cavitation during LCF has been made by my group. The conditions under which uniform cavitation occurs have been identified. Prior cold work or coarse grain size has been found to be essential for the development of uniform cavitation. The evolution of cavitation damage has been studied by interrupting LCF tests and conducting TEM studies.

Welding metallurgy is another area where very significant contributions have been made. Complex microstructures developed in multipass welds and the variation in properties across the weld thickness have been studied and correlations developed between microstructures and properties. The importance of amount, morphology and distribution of ferrite phase in austenitic welds in determining mechanical properties has been brought out by systematic studies. While it is known that the transformation of ferrite to sigma phase leads to deterioration in fracture toughness and creep rupture properties, recent studies by me and my colleagues have shown that there is no direct link between low ductility creep failures and the appearance of a particular phase during high temperature exposure. The salient controlling features are the amount and morphology of the precipitates on phase boundaries and their effect on interface properties.

A new ductile fracture toughness parameter shave been proposed which can be derived from the simple monotonic tensile test of smooth specimens of uniform circular cross section, by considering the energy absorbed in the post-necking regime of deformation. These parameters are empirical, but conceptually belong to the realm of damage mechanics approach to fracture. This approach has also given rise to proposing new ductility parameters which are meant to better reflect the strain localization in the specimen neck. A great advantage of the new toughness parameters known as RBR (Ray-Bhaduri-Rodriguez) parameters is that in composite specimens from weld joints, the toughness of the section least resistant to void growth is automatically determined. In contrast, in conventional fracture testing methods utilizing notched/cracked specimens, it is necessary to know a priori the region expected to have the lowest toughness, and also it may be difficult to accurately locate the notch/crack in this region. In addition, for dissimilar metal joints, it is necessary to contend with the fact that the location of the weakest toughness may vary with exposure duration, an effect which cannot be simulated even by weld thermal simulators. The efficacy of the new toughness parameters and the superiority of the new approach over the conventional one, has been demonstrated in a number of studies on weld joints.

It is well known that heat to heat variations in stress rupture properties of structural alloys is often very large. However, multiple heat stress rupture correlation has received only limited attention in the literature despite its potential benefits.In a recent comprehensive study, my group (Ray, Sasikala and Rodriguez) has proposed a novel generic method for extending any single heat rupture correlation to the corresponding multiple heat version using two heat-indexing constants. A novel single heat stress rupture correlation has been proposed that attempts to incorporate in a semi-empirical manner the broad features of the mechanisms of creep void growth; taking stress rupture data for 11 heats of a 9Cr-1Mo steel it has been shown that the efficacy of this new single-heat correlation compares favourably with the best of the more popular sress rupture parameters. The corresponding multiple heat version was successful in simultaneously correlating the data from all the eleven heats. The comprehensive study established that in this instance two linearly independent heat-indexing constants are necessary, but points out the situations in which one such constant could suffice. Empirical correlations between material chemisry and the heat-indexing constants are derived by a trial and error method which attempts to optimize between the large number of chemistry variables on the one hand and the small number of heats on the other. The strategy for characterizing a "new" heat using only three short duration rupture tests was evolved, where such predictive correlations, if available, can be advantageously utilized. The studies have been now extended to a number of stainless steels and different product forms (tubes, rolled plates, forgings etc.)

An understanding of the various mechanisms of materials ageing and degradation is a crucial component of any strategy for life assessment and extension. It is in this context that my four decades of R&D experience towards understanding damage caused by creep, fatigue, creep-fatigue interaction, irradiation, corrosion and other environmental effects, dynamic strain ageing and micro-structural degradation by thermal ageing, characterising the damage through micro-structural and non-destructive evaluation and developing advanced approaches for structural integrity assessment (structural mechanics, fracture mechanics and damage mechanics) has been unique. My expertise has been utilised not only by the unitss of the Department of Atomic Energy but also by defence and aerospace agencies and a variety of industries both in the public and private sectors in India.

(Placid Rodriguez)