Search

Microstructure of most technical alloys for an engineering application would involve multiple phases that could differ in their physical properties, configuration, morphology, size, volume fraction etc. While the choice of a material for a specific application could involve relevant functional property, often a secondary property (such as freezing range, toughness, corrosion or creep resistance, biological compatibility etc.) could play an overwhelming role in the successful usage of that material. Designing the microstructure, taking the material and processing constraints into account, is one of the major areas of materials research today. In the recent past, several novel processing techniques have emerged that have aimed at obtaining a specific microstructure of the material with an intended application. Severe plastic deformation, electromagnetic processing of melts, multi-step thermo-mechanical processing, rapid solidification are some examples of emerging techniques where research is focused on both the materials as well as the processing technique to obtain the designed microstructure. Kinetic constraints such as rate of deformation and rate of cooling imposed during the processing have an important role in the evolution of microstructure. Several new characterization techniques and tools such as orientation imaging, focused ion beam milling and analytical electron microscopy have aided in assessing the efficacy of the engineering approach taken in the design of a microstructure. Advances in the computational modeling of the processing techniques and simulation of microstructure have opened new avenues in the microstructure engineering. In this thematic volume, we present a collection of five papers that bring out the microstructure engineering in the current state of the art. Arvind Kumar, Miha Zaloznik and Herve Combeau have presented their work on prediction of equiaxed grain structure and macrosegregation in an industrial steel ingot and have compared their results with experiments. Sumantra Mandal, V. Subramanya Sarma and A. K. Bhadhuri have performed grain boundary engineering in alloy D9 through thermo-mechanical processing and have studied influence of process variables and delved on the aspects of micro-mechanisms in depth. Georg J. Schmitz, Bernd Bottger, Jain Eiken, Markus Apel, Antoine Carre, Alexandre Viardin and Gottfried Laschet have reviewed the current status of the phase-field based simulations of microstructure evolution in technical grade alloys. Bikramjit Basu and Garima Tripathi have presented their work on processing and biological evaluation of porous hydroxyapatite/poly methyl methacrylate hybrid composites. Seshacharyulu Tamirisakandala and Dan Miracle have reviewed the efforts to engineer the microstructure of titanium alloys using small additions of boron as alloying element. The materials covered in these studies are of current interest for the industry and the processes chosen are of high impact. This thematic issue has a fine balance of experimental and computational studies on the topic. Articles in this theme were contributed by researchers whose groups have contributed significantly to this area for last several years. We hope that readers will find this issue valuable and informative on the theme of microstructure engineering of materials. The ubiquitous presence of microstructure characterization tools in academic departments of varied engineering disciplines indicates that slowly and steadily, the mesoscale information of materials is being studied and engineered for desired performance. B. S. Murty G. Phanikumar (&) Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India e-mail: gphani@iitm.ac.in

Nanocrystalline Fe-12Cr-2W (wt.%) and oxide dispersion strengthened (ODS) Fe-12Cr-2W-0.25 Y2O3 (wt. %) alloy powders were produced through mechanical alloying in a high energy planetary ball mill. Microhardness studies on individual milled powder particles revealed a clear increase of hardness for the Yttria containing alloy, thus, proving the ODS effect to be operative in the powders. Consolidation was carried out in a uniaxial hot press at 900°C with a pressure of 200 MPa. TEM and XRD analyses consistently revealed the nanocrystalline grain size before (12 to 30 nm) and after consolidation (120 nm), respectively. Compression creep studies in a temperature range between 800 and 1000°C revealed that diffusional creep is obviously suppressed in these materials. However, the creep resistance of the ODS ferritic steel is only marginally higher than its base alloy and both alloys are by far less creep resistant than that of a ferritic steel of comparable baseline composition strengthened by complex and exceptionally thermally stable Ti-Y-O nanoclusters [1]. Both differences can be rationalized by the instability of the microstructure leading to significant particle and grain growth after creep testing.

The magnetoelectric (100-x) BaTiO3 - (x) NiFe1.98 O4 (x=20, 40, 60, and 80 wt %) particulate composites have been prepared and the effects of size and interface are studied through microscopy measurements. Large magnetoelectric voltage coefficient (?E) values accompanied by large piezoelastic coefficient, large magnetostrictive strain coefficient, and an adequate interface contact between the magnetic and electric phases were observed in these nanocomposites. The nanocomposite with x=40 has an ?E value of 252 mV cm-1 Oe-1, which is the largest value for any particulate magnetoelectric composite, based on the available open literature. � 2009 American Institute of Physics.

Nanocomposite Sm(2)Co(17)-5 wt% FeCo magnets were synthesized by high energy ball milling followed by consolidation into bulk shape by the spark plasma sintering technique. The evolution of magnetic properties was systematically investigated in milled powders as well as in spark plasma sintered samples. A high energy product of 10.2 MGOe and the other magnetic properties of M(s) = 107 emu g(-1), M(r) = 59 emu g(-1), M(r)/M(s) = 0.55 and H(c) = 6.4 kOe were achieved in a 5 h milled and spark plasma sintered Sm(2)Co(17)-5 wt% FeCo nanocomposite magnet. The spark plasma sintering was carried out at 700 °C for 5 min with a pressure of 70 MPa. The nanocomposite showed a higher Curie temperature of 955 °C for the Sm(2)Co(17) phase in comparison to its bulk Curie temperature for the Sm(2)Co(17) phase (920 °C). This higher Curie temperature can improve the performance of the magnet at higher temperatures.

The xNi0.83Co0.15Cu0.02Fe1.9O4δ (NCCF) + (1−x)PbZr0.52Ti0.48O3 (PZT) particulate magneto ferroelectric composites with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 mole fraction were prepared by conventional ceramic double sintering method. The presence of two phases (perovskite structure of ferroelectric phase and spinal structure of ferromagnetic phase) was confirmed by X-ray diffraction. The magnetoelectric (ME) property of the particulate composites was determined at room temperature as a function of intensity of magnetic field. The temperature variation of the longitudinal modulus (L) and the internal friction (Q −1 ) of these particulate composites at 104.3 kHz was studied in the wide temperature range 30-420 ℃. The temperature variation of the longitudinal modulus in each composition of these particulate composites showed two abrupt minima. One minimum coincided with the ferroelectric - paraelectric Curie transition temperature (θE) and the other with the ferromagnetic-paramagnetic Curie transition temperature (θM). The internal friction measurement also showed two sharp peaks in each composition corresponding to those temperatures where the minima were noticed in the temperature variation of the longitudinal modulus behaviour.

The grain boundary structure is usually described by the coincidence site lattice (CSL) model based on the misorientation of adjoining crystals. Therefore, the objective of the present investigation is to seek the correlation between CSL fraction and creep resistance of modified 9Cr–1Mo steel (P91) with and without boron addition. Results showed that CSL fraction increases with increase in heat treatment temperature and this increase is more prominent in boron containing modified 9Cr–1Mo steel. Creep test results show the increase in creep rupture life with increase in CSL fraction for both the base metals; but this increase is more in boron containing steel than the boron free steel. This improvement is attributed to the stability of CSL boundaries in the material. In spite of the increase in CSL boundaries with normalizing heat treatment temperature, boron free material shows less creep rupture life in its weldment than the boron containing steel weldment.

The grain refining response of Al and Al–7Si alloy has been studied with various Al–Ti, Al–B and Al–Ti–B master alloys at different addition levels. The results show that Al–B and B rich Al–Ti–B master alloys cannot grain refine Al, while they are efficient grain refiners to Al–7Si alloy. The level of grain refinement saturates after 0.03% of Ti or B for most of the master alloys studied both at short and long holding times. The grain refining efficiency of some elements other than Ti and B on Al–7Si alloy has also been studied. Interestingly, all the elements studied (B, Cr, Fe, Mg, Ni, Ti and Zr) have resulted in some grain refinement of Al–7Si alloy at short holding time and have shown fading/poisoning on long holding, which increased in the order of B (no poisoning), Ti, Cr, Ni, Fe, Mg, Zr. Sr (0.02%) has been found to provide complete modification of the eutectic in Al–7Si alloy within 2 min, which is not lost even after long holding up to 120 min. Significant improvements in the mechanical properties have been obtained by a combination of grain refinement and modification to an extent that was not possible by either of them alone.