Department of Metallurgical & Materials Engineering

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The in-situ Fe based nanocomposite containing Fe-Al2O 3 particle is synthesized by reactive milling of Fe-Al 2O3-Al-Fe powder mixture in toluene medium followed by consolidation of powders using Spark Plasma Sintering process. Transmission electron microscopy investigation of consolidated Fe-Fe-Al2O 3 nanocomposites has shown heterogenous grain structure of Fe consisting of nano, submicron and micron size grains together with nanometer Fe-Al2O3 particles. The hardness of Fe-Al2O3 nanocomposites consolidated at 800°C is 795 MPa. © (2012) Trans Tech Publications.

World University root node

Materials Science Forum

Microstructure and mechanical Property of Fe-Al2O3 nanocomposites synthesized by reactive milling followed by spark plasma sintering

Interstitial-free (IF) steels were rolled upto 98% of reduction in area at room temperature and even upto 97% at sub-zero temperatures of-50 to-90 C. Ultrafine-grained structure can be achieved with hardness double to that of coarse grained material by suitable flash annealing of deformed IF steel. Extent of refinement increases with lowering of rolling temperature and with increase in degree of deformation. At the initial stage of deformation, the microstructure contains bands. Their width decreases and dislocation density increases with amount of deformation. The band consists of cells/sub-grains. Cell sizes decrease with the decrease in rolling temperature as well as with the increase in amount of deformation. Below certain lower temperature ($\sim$-50 0 C), temperature has less pronounced effect on refinement. Flash annealing induces recrystallisation in cold rolled IF steel at 650 C but recrystallisation process could be advanced to 550 C due to higher defect density in cryorolled samples.

Proc. Int. Conf. on Rolling and Finishing Technol. Steel

Development of ultrafine-grained interstitial-free steels produced by cryorolling followed by flash annealing

The effects of ferrite-pearlite and ferrite-martensite starting microstructures on ferrite grain sizes and carbide particle sizes after cold rolling and prolonged annealing treatment have been investigated. Ferrite-martensite starting microstructures showed finer grain and particle sizes and improved tensile properties after cold rolling annealing cycle compared to ferrite-pearlite starting microstructures. A 'fibrous' martensite morphology developed by intermediate quenching treatment is more beneficial in that respect compared to the 'blocky' martensite morphology obtained from the step quenching treatment. © 2012 Institute of Materials, Minerals and Mining.

Materials Science and Technology

Microstructure and mechanical property of cold rolled low carbon steel after prolonged annealing treatment

NiAl-10 vol.% Al2O3 in situ nanocomposite has been synthesized by reactive milling and subsequent spark plasma sintering. The synthesized nanocomposites have ∼96% of theoretical density after sintering at 1000 °C for 5 min. Microstructural analysis of consolidated samples using TEM has revealed the presence of α-Al2O3 particles of 10-12 nm size in NiAl matrix of submicron grain size. Consolidated NiAl-10 vol.% Al2O3 nanocomposite has shown very high hardness of 772 HV0.3 and compressive strength of 2456 MPa with ∼14% plastic strain. The high hardness and compressive yield strength are attributed to the presence of nanocrystalline α-Al2O3 particles and the appreciable plastic strain is attributed to the submicron grains of NiAl. © 2010 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Development of in situ NiAl-Al2O3 nanocomposite by reactive milling and spark plasma sintering

The stability of the microstructure of alumina-Inconel 600 brazed joints was investigated under simulated in-service conditions by subjecting them to prolonged heat treatment at 400 and 560 °C. The evolution of the microstructure and microchemistry of the brazing zone was examined using extensive microanalysis of the constituent phases. The layered structure of the brazing zone transformed to homogeneous, near-equilibrium, two-phase microstructure after heat treatment at 560 °C. Solid-state interdiffusion was identified as a primary factor responsible for such copious modification of the microstructure. Intermediate temperature heat treatment at 400 °C revealed that the mechanism of transformation of the microstructure was globulization of the Ni overlayer and dissolution of Mo from the metallization layer into the Ni-rich phase. The migration behavior of each of the elements, in response to heat treatment, was analyzed. Cr was found to diffuse out of Inconel and form a layer of Cr2O3 at the alumina-brazing alloy interface. The bond strength of the interface was high enough to cause cohesive failure in the alumina side of the joints. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Acta Materialia

Stability of microstructure and its evolution during solid-state annealing of Al2O3-Inconel 600 brazed couples

In the present study, Ni-30 vol pct Al2O3 in-situ nanocomposite was developed by reactive milling of NiO-Al-Ni powder mixture followed by spark plasma sintering (SPS). During milling, fcc to hcp transformation was observed in Ni(Al) phase and it transformed back to fcc phase around 773 K (500 °C). The hardness and yield strength of Ni-30 vol pct Al2O3 nanocomposite are approximately two times higher than that of pure Ni of similar grain size. The improved mechanical properties of nanocomposite are attributed to the presence of alumina particles of nanometer size. © 2011 The Minerals, Metals &amp; Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Development of Ni-Al2O3 in-situ nanocomposite by reactive milling and spark plasma sintering

Although the grain-refinement practice is well established for wrought Al alloys, in the case of foundry alloys such as near eutectic Al-Si alloys, the underlying mechanisms and the use of grain refiners need better understanding. Conventional grain refiners such as Al-5Ti-1B are not effective in grain refining the Al-Si alloys due to the poisoning effect of Si. In this work, we report the results of a newly developed grain refiner, which can effectively grain refine as well as modify eutectic and primary Si in near eutectic Al-Si alloys. Among the material choices, the grain refining response with Al-1Ti-3B master alloy is found to be superior compared to the conventional Al-5Ti-1B master alloy. It was also found that magnesium additions of 0.2 wt pct along with the Al-1Ti-3B master alloy further enhance the near eutectic Al-Si alloy's grain refining efficiency, thus leading to improved bulk mechanical properties. We have found that magnesium essentially scavenges the oxygen present on the surface of nucleant particles, improves wettability, and reduces the agglomeration tendency of boride particles, thereby enhancing grain refining efficiency. It allows the nucleant particles to act as potent and active nucleation sites even at levels as low as 0.2 pct in the Al-1Ti-3B master alloy. © 2011 The Minerals, Metals & Materials Society and ASM International.

Influence of Mg on grain refinement of near eutectic Al-Si alloys

Significant grain refinement in Mg-3Al alloy is achieved with the addition of charcoal due to the formation of Al4C3 particles, which act as effective nuclei for magnesium grains. Addition of 0.5wt% charcoal has lead to reduced grain size of Mg-3Al alloy from 500 to 80μm and no substantial grain refinement is obtained on further addition of charcoal. The results further reveal that the prolonged holding of the melt after the addition of charcoal has not affected the grain refining efficiency of Al4C3. Steady increase in tensile properties observed with increasing amount of charcoal addition has been attributed to the grain refinement and the presence of fine Al4C3 particles. The strengthening mechanisms due to charcoal addition are discussed in terms of Hall-Petch relation and dispersion strengthening. The predicted values are in good agreement with experimental results. © 2010 Elsevier B.V.

Materials Science and Engineering A

Microstructural refinement and tensile properties enhancement of Mg-3Al alloy using charcoal additions

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70:30 α-brass over the test temperature range of 298-973 K and strain rate range of 10-5-5×10-3 s -1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553-673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol-1 below 623 K and 196·4 kJ mol-1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol -1, with the time exponent of 0·37. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.

Flow behaviour and microstructural evolution in cold worked 70:30 α-brass

Spray formed billets of Al-4Li-0•14Zr (wt-%) were processed by a combination of hot isostatic pressing (HIPing), homogenisation, and solution treatment and aging. Extensive oxidation damage occurred after homogenisation in air, but could be alleviated by using controlled atmosphere or by prior HIPing. The microstructure and tensile properties were studied at various stages of processing. The volume fraction of AlLi (δ) decreased successively after HIPing, homogenisation and T6 temper, redistributing itself towards grain boundaries. The Al3Zr (β') phase was absent in the as formed condition, but precipitated after HIPing at 838 K/4 h and/or stepped homogenisation finishing at 838 K/4 h. Room temperature tensile properties were poor in the as spray formed condition, but improved after HIPing, homogenisation and T6 treatment successively. The tensile properties of the alloy were also evaluated between 648 and 848 K, for preliminary identification of optimum hot working parameters. HIPing did not affect high temperature yield strength significantly, but increased the ductility. © 2010 Maney Publishing.