Department of Metallurgical & Materials Engineering

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Al-Si/Al4C3 master alloy has been developed by reacting the SiC particles in Al melt. The extent of SiC conversion to Al 4C3 in the Al-Si/Al4C3 master alloy has been calculated using optical emission spectroscopy. Experimental results indicated that only 70 % of SiC particles have been converted into Al 4C3 after the reaction between Al and SiC in Al/5 wt% SiCp composites at 900 °C. The grain refining efficiency of Al-Si/Al4C3 master alloy has been assessed by adding this into the Mg-3Al alloy. Grain size of Mg-3Al alloy has been significantly refined from 480 to 220 μm by the addition of 0.07 wt% of Al4C 3 particles in the form of Al-Si/Al4C3 master alloy.

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Transactions of the Indian Institute of Metals

Influence of Al4C3 particles on grain refinement of Mg-3Al alloys

In order to understand the effect of grain size bimodality on the tensile properties, low-carbon steel samples having bimodal ferrite grain size distributions, with different coarse-grain sizes (15-54μm), fine-grain sizes (2.4-13.0μm) and coarse-grain fractions (0.10-0.45) have been tensile tested. Samples having bimodal grain structures showed lower strength (UTS: 395-744MPa), better tensile ductility (e u: 8.5-19.4%) and lower YS to UTS ratio (0.63-0.84) than the ultra-fined grained steel (UTS=900MPa, e u=5%, and YS:UTS=0.93). In case of mixed grain structures, effective grain size (D eff), numerical average grain size (D avg) and the average grain size in the fine-grain regions (D f) showed better correlation with strength (i.e. strength ∝ D -1/2) than the weighted average grain size (D wt). Coarse-grain sizes in the range of 15-35μm and fraction of the coarse-grain regions in the range of 15-35% can be recommended for achieving the best tensile ductility. Measurement of micro-hardness and the change in grain aspect ratios from the coarse- and fine-grain regions indicated that the bimodal grain structure can be considered as a composite structure, where stress and strain vary between the coarse- and fine-grain regions. Lower nucleation and higher growth of the voids in the carbide-depleted, coarse-grain regions (than carbide-rich, fine-grain regions) allow higher plastic deformation till fracture and improve the total-elongation of the steels. © 2012 Elsevier B.V.

Materials Science and Engineering: A

Effect of bimodal distribution in ferrite grain sizes on the tensile properties of low-carbon steels

Mechanical properties of a nanocrystalline Fe 20Cr 10Ni alloy prepared by in situ consolidation followed by severe deformation were investigated using microhardness and shear punch tests (SPT). The nanocrystalline alloy with a nominal Cr content of 20 wt% and Ni content of 10% was prepared by high-energy ball milling. The low strain hardening observed in nanocrystalline materials is shown to dramatically decrease ductility in both tension and compression. Crack nucleation or propagation instability requires an external source to supply enough stress at a crack tip to overcome or exceed the critical toughness value. Microhardness of the small discs was found to be 7.8 GPa which is at least 6 times greater than conventional grain size stainless steel of similar composition. Yield strength determined from the SPTs is found to be 2.6 GPa, which is consistent with the yield stress determined by microhardness tests.

Journal of Materials Science

Synthesis, characterization and mechanical behaviour of an in situ consolidated nanocrystalline FeCrNi alloy

The high temperature deformation behaviour of near alpha titanium alloy IMI834 with a bimodal microstructure has been evaluated by carrying out isothermal compression tests over a range of temperature and strain rate. The optimum thermomechanical processing (TMP) parameters i.e., temperature, strain rate that can be used to produce various aeroengine components were identified using dynamic materials modeling (DMM). Using kinetic analysis, a unified constitutive equation that describes the deformation behavior of the material in the selected temperature - strain rate regime has been established and the deformation mechanisms operating in the material were identified. © (2012) Trans Tech Publications.

Materials Science Forum

Modeling the high temperature deformation behaviour of a near alpha titanium alloy with bi-modal microstructure

Amongst the Metal Matrix Composites (MMCs), Al-Si-Mg alloy/ SiCp MMCs are very attractive for their mechanical properties. But, the formation of brittle interfacial reaction product, Al4C3 has been one of the major concerns when these materials are processed by liquid phase methods. The extent of Al4C3 formation depends on various processing parameters such as temperature, wt% of SiCp, particle size of SiCp and chemical composition of the matrix alloy especially Silicon (Si) content. According to recent studies, various difficulties in finding the process parameters to get desirable properties of Al alloy/ SiCp MMCs as desired by the industries. Thus, in the present study thermodynamic &amp; structural estimates in Al alloy/ SiCp MMCs under various process conditions, composition, microstructures were performed. The relatively low cost liquid stir casting technique is used in the production of Al alloy/ SiCp MMCs with varying Silicon content (0-7%) in the alloy matrix using process temperature 710oC. Al4C3 layer formation on SiCp surface, Critical Si content for Al4C3 separation from SiCp surface, eutectic Si formation, existence of near-dislocation segregation regions after formation of Al4C3 on SiCp surface and the equilibrium amount of Si to suppress Al4C3 formation were investigated using Transmission electron microscopy (TEM). The separation of Al4C3 from SiCp surface was observed after 3% Si. The equilibrium Si content of 7% was found to suppress the formation of Al4C3 with thermodynamic model and its significance has been assessed. © (2012) Trans Tech Publications, Switzerland.

Advanced Materials Research

Synthesis and characterization of particulate SiCp reinforced Al-Si-Mg Alloy composite with varying Si content

Commercial grade AA8090 Al-Li alloy sheet contained three layers of distinct microstructural features along thickness direction. The surface layer contained well recrystallized equiaxed grains with higher fraction of T {4 4 11}<11 11 8>, Cu {1 1 2}<111>, H {0 0 1}<110> and CH {0 0 1}<1 2 0> texture components whereas middle layer contained unrecrystallized elongated grains with higher fraction of brass {0 1 1}<211>, CG {0 2 1}<100> and goss{011}<100> texture components. Superplastic forming of the separated surface and middle layers was carried out at constant pressure. Microstructure and texture evolution was studied at different location of the formed bulge. The extent of texturing or anisotropy was calculated from standard deviation of texture estimated normal anisotropy (R) values, Rstdev. At all conditions, in both the surface and middle layers, the texture randomization was found to be more at the apex than that at the edge of the bulge. The overall reduction in texturing was predominantly accompanied by weakening in strong texture components and strengthening in the weaker ones. © (2012) Trans Tech Publications, Switzerland.

Microstructural evolution during superplastic forming of Al-Li 8090 alloy

Micro and Nano features of microstructural properties along with micro-cracks and voids, which severely affect the strength and toughness of the material, were studied in commercial EN9 steel round product by using Atomic Force Microscopy (AFM). This has overcome the limitations of optical and scanning microscopy. The aim of the present work is to characterize the pearlite matrix properties for nanoscale results as pearlitic morphology and lamellae play a significant role in influencing the mechanical properties. It is also shown that the atomic force microscope is a 3D characterizing tool which can facilitate visualizing the adjacent corners in alternate layers of ferritecementite lamellae. It is also noted that the micro-sized cracks exist at the weak interfaces between the ferrite and cementite lamellae, which would limit the work hardening property of pearlite and thus reduce the ultimate tensile strength significantly. In addition, phase transformation from γ- phase to pearlite was schematically predicted and discussed as evident from microstructural characterization. © (2012) Trans Tech Publications, Switzerland.

Characterization of pearlite morphology and associated micro-cracks in EN9 grade carbon steel by atomic force microscopy

krravi.psgias@gmail.com indunainar@gmail.com rsm@mtl.psgtech.ac.in murty@iitm.ac.in,In the present work, synthesis of bulk Fe amorphous alloy by mechanical alloying of 70Fe-15Cr- 4Mo-5P-4B-1C-1Si powders followed by consolidation of powders using Spark Plasma Sintering process is reported. The evolution of phase, microstructure, relative density and hardness of amorphous alloy as a function of spark plasma sintering temperature is discussed. Transmission electron microscopy investigation has shown partial divitrification of amorphous matrix into nanocrystalline phase during spark plasma sintering of amorphous Fe alloy. The hardness of amorphous alloy sintered at 525 - 575°C are in the range of 675 - 725 MPa. © (2012) Trans Tech Publications.

Spark plasma sintering of Fe-Cr-Mo-P-B-C-Si amorphous alloy

The formation and growth of intermetallic compounds by diffusion reaction of Cu and Ti were investigated in the temperature range 720-860 °C using bulk diffusion couples. Only four, out of the seven stable intermediate compounds of the Cu-Ti system, were formed in the diffusion reaction zone in the sequence CuTi, Cu 4Ti, Cu 4Ti 3 and CuTi 2. The activation energies required for the growth of these compounds were determined. The diffusion characteristics of Cu 4Ti, CuTi and Cu 4Ti 3 and Cu(Ti) solid solution were evaluated. The activation energies for diffusion in these compounds were 192.2, 187.7 and 209.2 kJ mol -1 respectively, while in Cu(Ti), the activation energy increased linearly from 201.0 kJ mol -1 to 247.5 kJ mol -1 with increasing concentration of Ti, in the range 0.5-4.0 at.%. The impurity diffusion coefficient of Ti in Cu and its temperature dependence were also estimated. A correlation between the impurity diffusion parameters for several elements in Cu matrix has been established. © 2012 Carl Hanser Verlag, Munich, Germany.

Fulltext

International Journal of Materials Research

Diffusion characteristics in the Cu-Ti system

Repetitive Upsetting Extrusion (RUE) is a relatively new severe plastic deformation (SPD) process in which two conventional metal forming operations such as upsetting and extrusion has been combined to impart large plastic strain. Though originally invented to process powder materials for bulk mechanical alloying, a more recent study on the process has revealed that by suitably modifying the die design, the process can be used to impart large deformations in bulk materials as well. Using this new die, oxygen free high conductivity (OFHC) copper has been subjected up to 10 cycles of RUE. Microstructures across the cross-section of the RUE processed samples were evaluated using optical, scanning and transmission electron microscope to ascertain the deformation behaviour and the mechanism of grain refinement. The results obtained are presented and discussed. © (2012) Trans Tech Publications.