CONTRIBUTIONS TO BASSIC RESEARCHES IN MATERIALS SCIENCE AND ENGINEERING AND METALLURGY OF
PROF. K A PADMANABHAN
His model for grain boundary sliding and its application to superplastic flow in microcrystalline metals and ceramics questioned the “central dogma” in high-temperature deformation modeling till that point that Grain Boundary Sliding is an inherently fast process, which at no stage could be controlling the rate of high-temperature deformation. By developing a physical description that involves a basic shear at the atomistic scale (oblate spheroid of diameter ~ 5 atom diameters and height ~2.5 atom diameters), which is different from that of grain boundary diffusion (earlier picture of boundary sliding was that it was the same as diffusion parallel to the grain boundary at every point along its curvature, which (a) did not explain how mass transport leads to shear between grains, and (b) how such a process can be distinguished from grain boundary diffusion), and suggesting that due to a drive to minimize the free energy of the system and the principle of doing maximum work, such a sliding process could lead to mesoscopic boundary sliding/the formation of plane interfaces under conditions of superplasticity. With such an approach, this near-ubiquitous phenomenon, which is present in different classes of materials like pseudo-single phase and microduplex metals and alloys, ceramics, composites, dispersion strengthened materials, bulk metallic glasses, geological materials etc. could be explained on a common basis.
His model for inverse Hall-Petch effect in nanocrystalline and quasi-crystalline materials extends the ideas contained in the above model to explain why when the grain size in a nanocrystalline material goes below a value ~ 10-15 nm, there is grain size softening, rather than the usual increase in hardness with decreasing grain size. The softening is traced to grain boundary sliding control of the deformation process. When the grain size reaches ~ 10-15 nm, the dimensions of grain boundary obstacles become comparable to those of the grain size and by assuming the grain shape to be rhombic dodecahedron (literature survey reveals that real crystals have shapes which are close to this idealized shape), an inverse Hall-Petch effect is derived, in which below a critical grain size, the hardness decreases inversely as the square root of grain size. This prediction is validated in several systems. At the international conference on “Severe Plastic deformation” held in Ufa, Russia, in 2011 this paper of 1997 was named as the most significant publication in this area.
His model with H Gleiter (the inventor of engineered nanostructured materials and nanoglasses) extends the above ideas to cover superplastic deformation in metals and ceramics of microcrystalline, sub-micron and nanocrystalline grain sizes. Using Herring’s equation, which describes the most stable configuration of grain boundary misorientations at triple junctions, a physical justification is given to mesoscopic boundary sliding/plane interface formation using the minimization of the total free energy as the determining criterion. Expressions have been obtained for theoretically determining the long range threshold stress needed to be overcome for the onset of mesoscopic boundary sliding (when local boundary migration as well as localized diffusion is the faster accommodating process), the number of grain boundaries that will align to form a plane interface as a function of the experimental variables, the grain size below which plane interface formation will be aided only by diffusion and above which a combination of dislocation / partial dislocation emission from grain boundary and diffusion will be present. This paper was described by Nature Materials as a “break-through from India”.
His paper on the deformation of molecular crystals brings out the breadth of the nominee’s expertise in the area of deformation and fracture of materials. While synthesizing molecular crystals, chemists found that some were flat while others were either bent or sheared. As a result of this study involving SEM, AFM, nanoindentation and x-ray examination these differences could be attributed to the degree of cross-linking and the direction of the molecular forces of attraction between atoms (parallel to the covalently bonded sheet or perpendicular to them). In a recent communication, a former President of the International Crystallographic Union said, “This mechanical properties work is going on full swing in many labs worldwide now. We are organizing a mini-symposium on this topic in November. Many papers are appearing in top journals in this area. Your student Malla's PhD work was a sort of nucleation for much of this.”
Severe Plastic deformation is a highly localized, dynamic process, which apparently does not reach a steady state of deformation regardless of the number of repetitions in deformation the material is subjected to. Still, for a given set of experimental conditions the final grain size obtainable remains constant even when the strain is increased continuously. Classical theories of recovery and recrystallization teach us that so long as the stress dos not reach a steady state value, there should be continued grain refinement. His paper on this theme brings in the notion of plasticity-induced fracture after the material experiences significant deformation so that once the limit of plasticity in the material for the given experimental conditions is approached, thanks to the intervention of fracture processes, the limiting grain size develops. In addition, the plasticity-induced fracture process in BCC materials is traced to Cottrell’s ductile to brittle transition and in FCC materials to Lomer – Cottrell lock formation.
Grain/interphase boundaries / interfaces of varying misorientations, free volume fraction, curvatures and irregularities are present in materials of both 3D and 2D forms, regardless of whether these materials are crystalline or amorphous/glassy. It is suggested in a paper with H Gleiter that a generalized structural / basic unit (crystalline, non-crystalline or of any scale) model, which depends on the interatomic (including electronic) interactions, the spatial distribution of atoms / electrons, the number of atoms and free volume fraction present in the structural / basic unit and the experimental conditions should serve the purpose. As the development of a quantitative model, which reflects the effects of all these variables, is difficult, slightly defective material boundaries are modeled treating the entire boundary as planar and using the notions of crystallography. For highly disordered boundaries, a description in terms of a representative volume, made up of a non-crystalline basic unit or combination of such units, which depend on interatomic (including electronic) interactions/forces, is advocated. The size, shape, free volume fraction and number of atoms in the representative volume could differ with material composition and experimental conditions.
He developed a model for predicting the formability of steels under different stress states. This method was extended subsequently to Al-alloys also under stretch, draw, stretch-draw and plane strain conditions. The computations take into account diffuse necking during stretching, localized necking under drawing, plane strain at the minimum in the forming limit and an Avrami type of cavitation process in regions where a high tensile component exists in the stress state. This makes the analysis
close to the physical situation. The results are consistent with data generated in the USA based on numerous industrial stampings, the so called “Keeler-Goodwin” band.
TECHNOLOGICAL ACHIEVEMENTS OF PROF. K. A. PADMANABHAN IN THE FIELD OF METALLURGICAL AND MATERIALS SCIENCE AND ENGINEERING
Prof. Padmanabhan’s research contributions are known for their mathematical rigor and have encompassed both fundamental and applied aspects of metallurgical / materials engineering and materials science. His researches have directly benefited high technology areas in the Indian Atomic Energy, Defense and Space sectors. His comprehensive expert level book on superplasticity, published in 1980 by Springer Verlag of Germany, has been identified recently as one of about 40 books out of about 4500 books published by Springer Verlag in the period 1842 –2005 (163 years) to be brought out as eBooks and also in print form under their new series “Springer Book Archives”. They are being marketed with a slogan “Great Minds Never Go out of Print”.
His work for the Department of Atomic Energy was mainly concerned with the physical and mechanical metallurgy aspects (mainly deformation, fracture, stress corrosion cracking, fracture, low cycle fatigue) of AISI 304, 304 LN, 316 and 316 LN austenitic stainless steels and an improved grade D9, of vital interest as construction materials for the Fast Breeder Reactor program.
He developed the procedure for the superplastic forming of hemispherical domes of Ti-6Al-4V alloy of minimum wall thickness 5 mm for Defence Research and Development Laboratory in the early years of the Integrated Missiles Development Program. Today this technology is used in regular production. He also transferred two technologies to the Hindustan Aeronautics Limited in the area of superplastic forming. In all, he has 10 patents to his credit (2 more under process).
Prof. Padmanabhan was closely associated with the Liquid Propulsion Systems Centre, Indian Space Research Organization, when the decision to use an AFNOR 7020 alloy (a French alloy) equivalent for water tankages was taken. He demonstrated that it was possible to make this material to required quality in India. This is produced these days in a routine manner at an ordnance factory. He also set up the facility to superplastically form hemispheres of 415 mm diameter out of alloy Ti-6Al-4V at the Vikram Sarabhai Space Centre, Thiruvananthapuram - a work that received the Consultancy Development Centre - Department of Scientific and Industrial Research (CDC-DSIR) Certificate of Merit, Government of India. Several components made by the superplastic forming route by the Indian Space Research Organization are made using this facility.
He was a consultant to the R & D Centre for Iron and Steel, Steel Authority of India Limited (SAIL), when they produced for the first time in the country both Extra Deep Drawing (EDD) and Liquid Petroleum Gas (LPG) grades of steel. He developed a model for predicting the formability of steels under different stress states. This method has helped one to reduce the number of experiments needed to construct the entire forming limit diagram of steels (subsequently extended to Al-alloys also) under stretch, draw, stretch-draw, plane strain etc. conditions. In addition, he helped SAIL launch their Engineering Applications Centre to transfer the fruits of industrial research & development on sheet metal formability for forming complex components involving severe stretch and draw that are regularly used in industry. He assisted Tata Iron and Steel Company also with a similar type of work for the LPG grade material. Today both these grades are regularly produced in the country in large tonnages.
He was a consultant to Tata Engineering and Locomotive Company (now known as Tata Motors) when they introduced the microalloyed ferrite - pearlite steel (first generation), 49MnVS3, for the first time in India for the forging of crankshafts, just a year after it was introduced by Daimler Benz of Germany in their vehicles. Today such forged crankshafts are very regularly produced in the country.
Prof. Padmanabhan was a consultant to the Indian Stainless Steel Development Association (ISSDA) when some Indian steel companies decided to make very low nickel austenitic stainless steels for which no international standards existed. This has proved successful. Today more than 600,000 tons of this class of steels is manufactured in the country.
Most recently, he developed a multi-stage closed die forging technology for the production of undercarriage base plate fitting component for the air frame of a high-performance (supersonic) Aircraft for the Indian Aeronautical Development Agency. The technology has been qualified as “airworthy” by the Centre for Military Airworthiness. Till now, this component was made through machining. Now the ‘buy to fly’ ratio has been reduced from 18:1 to 4:1. Considerable savings in cost, improved properties and time of production are also involved.
In all, ten technologies developed by him and members of his research group are used in Indian industries.
Recently he has obtained an international patent (IPR protected in Europe as well as the USA) for a process for the manufacture of thin films of sub-stoichiometric TiNx, which have excellent mechanical, optical and opto-electronic properties. A Pune-based company has expressed a desire to commercialize this invention. Another patent application dealing with a CNT (Carbon Nanotube) added water-based coolant, which has cooling characteristics well above those of commercially available coolants has been submitted to Indian authorities. There are four more patent applications pending with the Indian authorities for the production of cutting tool inserts out of extremely hard ceramic, ceramic-composite materials and for the design of two new types of hybrid atomizers involving a gas as well as a water blanket.
As Director, IIT Kanpur, he was able to widen the scope of his activities and he served as a spokesperson for the metallurgical and other engineering professions at the highest levels of decision-making. This position he held was a logical culmination of his outstanding contributions as an academic administrator to engineering education in general and metallurgical/ materials science and engineering education in particular – as Head of Laboratory, Head of Department, Chairman, Centre for Continuing Education and Dean, Academic Research, all at IIT Madras and Director, IIT Kanpur. In September 2000 he visited the USA as a member of the Indian Prime Minister’s (Shri Atal Behari Vajpayee) Scientific Delegation and participated in the Indo-US Science and Technology Round Table meetings held in Washington, D.C. The details of the various positions held by him and the honors and recognitions received are available in his CV.