Evolution of texture and its influence on the failure of components in some aluminium alloys

This paper describes the evolution of crystallographic texture in three of the most important high strength aluminium alloys, viz., AA2219, AA7075 and AFNOR7020 in the cold rolled and artificially aged condition. Bulk texture results were obtained by plotting pole figures from X-ray diffraction results followed by Orientation Distribution Function (ODF) analysis and micro-textures were measured using EBSD. The results indicate that the deformation texture components Cu, Bs and S, which were also present in the starting materials, strengthen with increase in amount of deformation. On the other hand, recrystallization texture components Goss and Cube weaken. The Bs component is stronger in the deformation texture. This is attributed to the shear banding. In-service applications indicate that the as-processed AFNOR7020 alloy fails more frequently compared to the other high strength Al alloys used in the aerospace industry. Detailed study of deformation texture revealed that strong Brass (Bs) component could be associated to shear banding, which in turn could explain the frequent failures in AFNOR7020 alloy. The alloying elements in this alloy that could possibly influence the stacking fault energy of the material could be accounted for the strong Bs component in the texture.

Low thermal conductivity of endogenous manganese silicide/Si composites for thermoelectricity

Higher manganese silicide (HMS) based alloys with eutectic composition (Si-33.3 at% Mn) were prepared by arc-melting, melt-spinning and ball milling in order to evaluate the effect of microstructure on the thermal conductivity. Powder X-ray diffraction, SEM, EPMA and TEM analysis confirmed the presence of Si as a secondary phase distributed in the HMS matrix phase. Thermal properties of the samples were studied in the temperature range of 300-800 K. The microstructure refinement resulting from ball milling leads to a decrease of the thermal conductivity from 4.4 W/mK to 1.9 W/mK, whereas meltspinning is inefficient to this respect. The results show an opportunity to produce bulk higher manganese silicide alloys with reduced thermal conductivity in order to enhance its thermoelectric performance. (C) 2015 Elsevier B.V. All rights reserved.

Fracture behavior of magnesium alloys - Role of tensile twinning

In this work, Mode-I fracture experiments are conducted using notched compact tension specimens machined from a rolled AZ31 Mg alloy plate having near-basal texture with load applied along rolling direction (RD) and transverse direction (TD). Moderately high notched fracture toughness of J(C) similar to 46 N/mm is obtained in both RD and TD specimens. Fracture surface shows crack tunneling at specimen mid-thickness and extensive shear lips near the free surface. Dimples are observed from SEM fractographs suggesting ductile fracture. EBSD analysis shows profuse tensile twinning in the ligament ahead of the notch. It is shown that tensile twinning plays a dual role in enhancing the toughness in the notched fracture specimens with reduced triaxiality. It provides significant dissipation in the background plastic zone and imparts hardening to the material surrounding the fracture process zone via operation of several mechanisms which retards micro-void growth and coalescence. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A new class of high strength high temperature Cobalt based gamma-gamma ` Co-Mo-Al alloys stabilized with Ta addition

The present paper reports a new class of Co based superalloys that has gamma-gamma' microstructure and exhibits much lower density compared to other commercially available Co superalloys including Co-Al-W based alloys. The basic composition is Co-10Al-5Mo (at%) with addition of 2 at% Ta for stabilization of gamma' phase. The gamma-gamma' microstructure evolves through solutionising and aging treatment. Using first principles calculations, we observe that Ta plays a crucial role in stabilizing gamma' phase. By addition of Ta in the basic stoichiometric composition Co-3(Al, Mo), the enthalpy of formation (Delta H-f) of L1(2) structure (gamma' phase) becomes more negative in comparison to DO19 structure. The All of the L12 structure becomes further more negative by the occupancy of Ni and Ti atoms in the lattice suggesting an increase in the stability of the gamma' precipitates. Among large number of alloys studied experimentally, the paper presents results of detailed investigations on Co-10Al-5Mo-2Ta, Co-30Ni-10Al-5Mo-2Ta and Co-30Ni-10Al-5Mo-2Ta-2Ti. To evaluate the role alloying elements, atom probe tomography investigations were carried out to obtain partition coefficients for the constituent elements. The results show strong partitioning of Ni, Al, Ta and Ti in ordered gamma' precipitates. 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Organic alloys of room temperature liquids thiophenol and selenophenol

The first examples of organic alloys of two room temperature liquids, obtained and characterized via in situ cryo-crystallography, are presented. Thiophenol and selenophenol, which exhibit isostructurality and similar modes of S center dot center dot center dot S and Se center dot center dot center dot Se homo-chalcogen interactions along with weak and rare S-H center dot center dot center dot S and Se-H center dot center dot center dot Se hydrogen bonds, are shown to form solid solutions exhibiting Veggard's law-like trends.

A method for coupling the phase-field model based on a grand-potential formalism to thermodynamic databases

In this paper we derive an approach for the effective utilization of thermodynamic data in phase-field simulations. While the most widely used methodology for multi-component alloys is following the work by Eiken et al. (2006), wherein, an extrapolative scheme is utilized in conjunction with the TQ interface for deriving the driving force for phase transformation, a corresponding simplistic method based on the formulation of a parabolic free-energy model incorporating all the thermodynamics has been laid out for binary alloys in the work by Folch and Plapp (2005). In the following, we extend this latter approach for multi-component alloys in the framework of the grand-potential formalism. The coupling is applied for the case of the binary eutectic solidification in the Cr-Ni alloy and two-phase solidification in the ternary eutectic alloy (Al-Cr-Ni). A thermodynamic justification entails the basis of the formulation and places it in context of the bigger picture of Integrated Computational Materials Engineering. (C) 2015 Elsevier Ltd. All rights reserved.

Role of Si modification on the compressive flow behavior of Al-Si based alloy

The flow characteristics of a near-eutectic heat-treated Al-Si based cast alloy have been examined in compression at strain rates varying from 3 x 10(-4) to 10(2) s(-1) and at three different temperatures, i.e., room temperature (RT), 100 degrees C and 200 degrees C. The dependence of flow behavior on modification is examined by testing the alloy in both the unmodified and modified conditions. Modification has strong influence on strain rate sensitivity (SRS), strength and work hardening behavior of the alloy. The strength of the alloy is found to increase with increase in strain rate for both the conditions. The increase is more rapid above the strain rate of 10(-1) s(-1) for the unmodified alloy at all the temperatures. This rapid increase is observed at 1 s(-1) at RT and 100 degrees C, and at 10(-2) s(-1) at 200 degrees C for the modified alloy. The thermally dependent process of the Al matrix is rate controlling in the unmodified alloy. On the other hand, the thermally dependent process of both Al matrix and Si particles are rate controlling, which is responsible for the higher strain rate sensitivity (SRS) in the modified alloy. The unmodified alloy exhibits a larger work hardening rate than the modified alloy during the initial stages of straining due to fiber loading of unmodified Si particles. However, the hardening rate decreases sharply at higher strains for the unmodified alloy due to a higher rate of Si particle fracture. Thermal softening is observed for both alloys at 200 degrees C due to precipitate coarsening, which leads to a decrease in SRS at higher temperatures. Stress simulations by microstructure based finite element method support the experimentally observed particle and matrix fracture behavior. Negative SRS and serrated flow are observed at lower strain rate regime (3 x 10(-4) to 10(-2) s(-1)) at RT and 100 degrees C, in both alloys. The critical onset strain is found to be lower and the magnitude of serration is found to be higher for the modified alloy, which suggests that, in addition to dynamic strain aging, Si particle size and morphology also play a role in serrated flow. (C) 2015 Elsevier Inc All rights reserved.

On the Structural Stability of Melt Spun Ribbons of Fe95-x Zr (x) B4Cu1 (x=7 and 9) Alloys and Correlation with Their Magnetic Properties

Melt spun ribbons of Fe95-x Zr (x) B4Cu1 with x = 7 (Z7B4) and 9 (Z9B4) alloys have been prepared, and their structure and magnetic properties have been evaluated using XRD, DSC, TEM, VSM, and Mossbauer spectroscopy. The glass forming ability (GFA) of both alloys has been calculated theoretically using thermodynamical parameters, and Z9B4 alloy is found to possess higher GFA than that of Z7B4 alloy which is validated by XRD results. On annealing, the amorphous Z7B4 ribbon crystallizes into nanocrystalline alpha-Fe, whereas amorphous Z9B4 ribbon shows two-stage crystallization process, first partially to bcc solid solution which is then transformed to nanocrystalline alpha-Fe and Fe2Zr phases exhibiting bimodal distribution. A detailed phase analysis using Mossbauer spectroscopy through hyperfine field distribution of phases has been carried out to understand the crystallization behavior of Z7B4 and Z9B4 alloy ribbons. In order to understand the phase transformation behavior of Z7B4 and Z9B4 ribbons, molar Gibbs free energies of amorphous, alpha-Fe, and Fe2Zr phases have been evaluated. It is found that in case of Z7B4, alpha-Fe is always a stable phase, whereas Fe2Zr is stable at higher temperature for Z9B4. (C) The Minerals, Metals & Materials Society and ASM International 2015

The effect of alloy composition on instabilities in the beta phase of titanium alloys

A metastable nano-scale disordered precipitate with orthorhombic symmetry has been identified using high resolution scanning transmission electron microscopy. The phase, termed O', is metastable, formed by a shuffle mechanism involving a {110}<1<(1)over bar>0> transverse phonon wave in samples of Ti-26Nb-2Zr (at.%) quenched from the beta phase. The addition of 2% Zr to Ti-26Nb appears to suppress significantly the stability of both the {11 (2) over bar}<111> shear and 2/3 <111> longitudinal phonon wave but promotes the {110}<1<(1)over bar>0> transverse shuffle. This results in the nano-size O' phase being homogeneously formed in the parent beta phase matrix rather than the massive alpha `' phase. (C) 2016 Elsevier B.V. All rights reserved.

A nano-scale instability in the beta phase of dilute Ti-Mo alloys

A nano-scale instability in the beta phase resulting in the formation of the disordered orthorhombic O' phase has been discovered in a fairly dilute binary Ti-Mo alloy, using selected area electron diffraction and high resolution scanning transmission electron microscopy. The O' phase informed in the alloy when the Mo content exceeds a critical value. The instability occurs in beta-solutionized samples that have been quenched to room temperature and is found to co-exist with athermal omega to phase. Interestingly, this nano-scale instability, involving the {110}<1<(1)over bar>0> soft-phonon shuffle, occurs in the beta phase without deliberate additions of either interstitial or substitutional solutes. (C) 2016 Elsevier Ltd. All rights reserved.

Tailored specular reflectance properties of bulk Cu based novel intermetallic alloys

Significant research has been pursued to develop solar selective metallic coatings using a variety of coating deposition techniques, with limited attempts to assess the properties of bulk metallic materials for solar energy applications. In developing bulk solar reflectors with good reflectance in the entire solar range, we report a new class of reflector materials based on Cu-Sn intermetallics with tailored substitution of aluminium or zinc. Our experimental results suggest that the arc melted-suction cast Cu (78.8 at%)-Al (21.2 at%) alloy with nanoscale surface roughness can exhibit a combination of 89% bulk specular reflectance and 83% bulk solar reflectance, together with a hardness of 2 GPa. We show that the present alloy design approach paves the way for further opportunities of tuning the spectral properties of this new class of solar reflector material. (C) 2016 Elsevier B.V. All rights reserved.

On the issues of resolving a low melting combination as a definite eutectic or an elusive cocrystal: A critical evaluation

Cocrystals and eutectics are different yet related crystalline multi-component adducts with diverse applications in pharmaceutical and materials fields. Recently, they were shown to be alternate products of cocrystallization experiments. Whereas a cocrystal shows distinct diffraction, spectroscopic and thermal signatures as compared to parent components, the hallmark of a eutectic is its low melting nature. However, in certain cases, there can be a problem when one resorts to design a cocrystal and assess its formation vis-A -vis a eutectic. In the absence of a gold standard method to make a cocrystal, it is often difficult to judge how exhaustive should the cocrystallization trials be to ensure the accomplishment of a desired/putative cocrystal. Further, a cocrystal can manifest with intermolecular interactions and/or crystal structure similar to that of its parent compounds such that the conventional diffraction and spectroscopic techniques will be of little help to conclusively infer the formation of cocrystal in the lack of single crystals. Such situations combined with low melting behavior of a combination brings the complication of resolving the combination as a cocrystal or eutectic since now both the adducts share common features. Based on the curious case of Caffeine-Benzoic acid combination, this study aims to unfold the intricate issues related to the design, formation and characterization of cocrystals and eutectics for a way forward. The utility of heteronuclear seeding methodology in establishing a given combination as a cocrystal-forming one or a eutectic-forming one in four known systems is appraised.