Aluminide Coatings on Titanium-based Alloy IMI-834 for High Temperature Oxidation Protection

Microstructural aspects and cyclic oxidation behaviour of plain aluminide and Pt-aluminide coatings on the near-a Ti-alloy IMI-834 have been studied. Both the coatings provide good oxidation resistance to the above alloy at 650 °C, 750 °C, and 850 °C. However, significant cracking develops in the coatings during coating formation as well as during cyclic oxidation exposure. The extent of cracking during oxidation is found to increase with the exposure temperature. Presence of through-thickness cracks in the coatings leads to localised oxidation damage of the underlying substrate at all the three temperatures. At 850 °C, such localised oxidation generates enough TiO2 so that this oxide phase grows through the cracks and emerges at the sample surface forming a clearly identifiable mud-crack pattern. The extent of such oxidation damage is comparatively much lower at 750 °C and 650 °C.Defence Science Journal, 2011, 61(2), pp.180-190, DOI:http://dx.doi.org/10.14429/dsj.61.47

Underpinning and benchmarking multi-scale models with micro- and nanoscale experiments

Predictive models of materials behavior depend on: accurate databases of constitutive material properties, identification of underlying deformation mechanisms, and the availability of experimentally measured benchmarks with which to compare. Micro- and nano-scale experiments can be used to facilitate collection of salient mechanical properties of individual phases at appropriate temperatures, chemistries and microstructural states. Coupling with TEM observations allows one to identify underlying deformation mechanisms and to imbibe models with the requisite fundamental physics and materials science. Simulations must be benchmarked with experiments, conducted at scale with relevant material volumes and identifiable microstructures. This presentation will outline efforts to characterize the constitutive behavior of materials, to identify deformation mechanisms, and to benchmark crystal plasticity simulations at appropriate length scales. Micro-scale experiments designed and conducted to complement crystal plasticity modeling of two different microstructural variants of Ni-base superalloys, polycrystalline Rene 88 and directionally solidified GTE 444, will be presented. If size-scale effect can be avoided, constitutive (single-crystalline) data may be obtained with micro-tensile tests at various orientations and temperatures. Moreover, preparing and testing specimens with reduced volumes and a finite number of grains allows for direct comparison with crystal plasticity simulations of stress-strain behavior as well as strain localization. With regard to the latter, digital image correlation (DIC) of spatially resolved surface displacements produces strain maps that provide a much more rigorous benchmark for crystal plasticity predictions than stress-strain curves. Using directionally solidified specimens allows for 2.5D microstructures (grains that extend through the thickness of the specimen) and greatly simplifies such comparisons. Moving beyond uniaxial tension, micro-bending resonance fatigue experiments provide an opportunity to measure the number of cycles, location, and microstructural features associated with slip, intragranular crack formation, and eventual transgranular crack growth. These experimental measures can in turn be used to inform and benchmark multi-scale fatigue simulations. Similarly, strain-controlled fracture experiments involving 2.5D unidirectional polymer matrix composites (PMC) have been developed and are being used to identify the microstructural features and fracture paths that govern delamination and fracture. The availability of orientation mapping techniques (EBSD, PACOM, TKD) now allows for nano-scale characterization of underlying deformation mechanisms and their relation to crystallographic microstructures and surrounding neighborhoods. Studies investigating the role of grain growth, twinning and dislocation plasticity will be presented. Special attention will be placed on attempts to measure intragranular strains that can be related to the accumulation of geometrically necessary dislocations (GNDs) and compared with crystal plasticity simulations. Support for these projects has been provided through the AFOSR and AFRL funded Center of Excellence on Integrated Materials Modeling and the DOE office of Basic Energy Sciences

Footprinting studies of DNA‐sequence recognition by nogalamycin

We have studied the DNA sequence binding preference of the antitumour antibiotic nogalamycin by DNase‐I footprinting using a variety of DNA fragments. The DNA fragments were obtained by cloning synthetic oligonucleotides into longer DNA fragments and were designed to contain isolated ligand‐binding sites surrounded by repetitive sequences such as (A)n· (T)n and (AT)n. Within regions of (A)n· (T)n, clear footprints are observed with low concentrations of nogalamycin (&lt; 5 μM), with apparent binding affinities for tetranucleotide sequences which decrease in the order TGCA &gt; AGCT = ACGT &gt; TCGA. In contrast, within regions of (AT)n, the ligand binds best to AGCT; binding to TCGA and TGCA is no stronger than to alternating AT. Within (ATT)n, the preference is for ACGT &gt; TCGA. Although each of these binding sites contains all four base pairs, there is no apparent consensus sequence, suggesting that the selectivity is affected by local DNA dynamic and structural effects. At higher drug concentrations (&gt; 25 μM), nogalamycin prevents DNAse‐I cleavage of (AT)n but shows no interaction with regions of (AC)n· (GT)n. Regions of (A)n· (T)n, which are poorly cut by DNase I, show enhanced rates of cleavage in the presence of low concentrations of nogalamycin, but are protected from cleavage at higher concentrations. We suggest that this arises because drug binding to adjacent regions distorts the DNA to a structure which is more readily cut by the enzyme and which is better able to bind further ligand molecules.</p

Aluminide Coatings on Titanium-based Alloy IMI-834 for High Temperature Oxidation Protection

Microstructural aspects and cyclic oxidation behaviour of plain aluminide and Pt-aluminide coatings on the near-a Ti-alloy IMI-834 have been studied. Both the coatings provide good oxidation resistance to the above alloy at 650 °C, 750 °C, and 850 °C. However, significant cracking develops in the coatings during coating formation as well as during cyclic oxidation exposure. The extent of cracking during oxidation is found to increase with the exposure temperature. Presence of through-thickness cracks in the coatings leads to localised oxidation damage of the underlying substrate at all the three temperatures. At 850 °C, such localised oxidation generates enough TiO2 so that this oxide phase grows through the cracks and emerges at the sample surface forming a clearly identifiable mud-crack pattern. The extent of such oxidation damage is comparatively much lower at 750 °C and 650 °C.Defence Science Journal, 2011, 61(2), pp.180-190, DOI:http://dx.doi.org/10.14429/dsj.61.47

Small-scale mechanical testing of materials

Small-scale mechanical testing of materials has gained prominence in the last decade or so due to the continuous miniaturization of components and devices in everyday application. This review describes the various micro-fabrication processes associated with the preparation of miniaturized specimens, geometries of test specimens and the small scale testing techniques used to determine the mechanical behaviour of materials at the length scales of a few hundred micro-meters and below. This is followed by illustrative examples in a selected class of materials. The choice of the case studies is based on the relevance of the materials used in today's world: evaluation of mechanical properties of thermal barrier coatings (TBCs), applied for enhanced high temperature protection of advanced gas turbine engine components, is essential since its failure by fracture leads to the collapse of the engine system. Si-based substrates, though brittle, are indispensible for MEMS/NEMS applications. Biological specimens, whose response to mechanical loads is important to ascertain their role in diseases and to mimic their structure for attaining high fracture toughness and impact resistance. An insight into the mechanisms behind the observed size effects in metallic systems can be exploited to achieve excellent strength at the nano-scale. A future outlook of where all this is heading is also presented

Micromechanisms of fracture and strengthening in free-standing Pt-aluminide bond coats under tensile loading

Free-standing Pt-aluminide (PtAl) bond coats exhibit a linear stress strain response under tensile loading and undergo brittle cleavage fracture at temperatures below the brittle-to-ductile transition temperature (BDTT). Above the BDTT, these coatings show yielding and fail in a ductile manner. In this paper, the various micromechanisms affecting the tensile fracture stress (FS) below the BDTT and yield strength (YS) above the BDTT in a PtAl bond coat have been ascertained and quantified at various temperatures. The micromechanisms have been identified by carrying out microtensile testing of stand-alone PtAl coating specimens containing different levels of Pt at temperatures between room temperature and 1100 degrees C and correlation of the corresponding fracture mechanisms with the deformation substructure in the coating. An important aspect of the influence of Pt on the tensile behavior, slip characteristics, FS/YS and BDTT in the PtAl coating has also been examined. The addition of Pt enhances the FS of the coating by Pt solid solution strengthening and imparts a concomitant increase in fracture toughness and yet causes a significant increase in the BDTT of the coating. Published by Elsevier Ltd. on behalf of Acta Materialia Inc

High Temperature Resistant Coatings for Strategic Aero space Applications

The aerospace components operating in hot sections of aero-engines and combustors experience extreme environments. Typically, the components are subjected to high service temperatures exceeding 1100°C and oxidizing conditions. Protective coatings are essential for preventing oxidation-induced dimensional degradation of the components and enhancing their high temperature capability as well as durability. Defence Metallurgical Research Laboratory (DMRL) has developed a variety of metallic and ceramic thermal barrier coating (TBC) systems for Ni-base superalloys, and refractory Nb-alloys for strategic aerospace applications involving ultra-high temperatures and high flow velocities. These coatings have demonstrated significant effectiveness against thermal degradation at temperatures as high as 2000 °C during oxidation in static air as well as in dynamic conditions involving high flow velocities (Mach &gt; 2). The present article provides an overview of the advanced oxidation resistant and thermal barrier coatings developed in DMRL. The effectiveness of the TBCs in preventing dimensional degradation of the metallic and composite substrate materials has been evaluated at the laboratory scale. The developed TBCs have the potential for use in aero-engines and propulsion systems of hypervelocity vehicles

Microtensile testing of a free-standing Pt-aluminide bond coat

A finite element method (FEM)-based study has been carried out for the design of flat microtensile samples to evaluate tensile properties of Pt-aluminide (PtAl) bond coats. The critical dimensions of the sample have been determined using a two-dimensional elastic stress analysis. In the present testing scheme, the ratio of the dimensions of the holding length to the fillet radius of the sample was found important to achieve failure within the gage length. The effect of gage length and grip head length also has been examined. The simulation predictions have been experimentally verified by conducting microtensile test of an actual PtAl bond coat at room temperature. The sample design and testing scheme suggested in this study have also been found suitable for evaluation of tensile properties at high temperature. (C) 2010 Elsevier Ltd. All rights reserved

Study of Brittle-to-ductile-transition in Pt-aluminide bond coat using micro-tensile testing method

The Brittle-to-ductile-transition-temperature (BDTT) of free-standing Pt-aluminide (PtAl) coating specimens, i.e. stand-alone coating specimens without any substrate, was determined by micro-tensile testing technique. The effect of Pt content, expressed in terms of the thickness of initial electro-deposited Pt layer, on the BDTT of the coating has been evaluated and an empirical correlation drawn. Increase in the electrodeposited Pt layer thickness from nil to 10 mu m was found to cause an increase in the BDTT of the coating by about 100 degrees C