Influence of grain size on high temperature fracture in a Mg AZ31 alloy

Tensile experiments at 673 K and grain sizes from similar to 8 to 17 mu m revealed large ductility at a low strain rate and a reduced ductility at a high strain rate, corresponding to a change from a high to a low value for the strain rate sensitivity. High strain rate deformation led to fracture by flow localization, whereas low strain rate deformation involved fracture by cavity nucleation and growth. Analysis revealed that grain boundary migration can assist significantly in reducing the stress concentrations caused by grain boundary sliding, thereby retarding cavity nucleation. Calculations demonstrate that the interlinkage of voids parallel and perpendicular to the tensile axis occurs significantly, so that it is not always possible to use the cavity shapes to distinguish between diffusion and plasticity controlled growth. Cavitation damage evolves slowly in materials with a coarser grain size because of reduced nucleation related to a reduction in the strain rate sensitivity and associated grain boundary sliding. (C) 2011 Elsevier B.V. All rights reserved.

Hot deformation characteristics of INCONEL alloy MA 754 and development of a processing map

The characteristics of hot deformation of INCONEL alloy MA 754 have been studied processing maps obtained on the basis of flow stress data generated in compression in the temperature range 700-degrees-C to 1150-degrees-C and strain rate range 0.001 to 100 s-1. The map exhibited three domains. (1) A domain of dynamic recovery occurs in the temperature range 800-degrees-C to 1075-degrees-C and strain rate range 0.02 to 2 s-1, with a peak efficiency of 18 pct occurring at 950-degrees-C and 0.1 s-1. Transmission electron microscope (TEM) micrographs revealed stable subgrain structure in this domain with the subgrain size increasing exponentially with an increase in temperature. (2) A domain exhibiting grain boundary cracking occurs at temperatures lower than 800-degrees-C and strain rates lower than 0.01 s-1. (3) A domain exhibiting intense grain boundary cavitation occurs at temperatures higher than 1075-degrees-C. The material did not exhibit a dynamic recrystallization (DRX) domain, unlike other superalloys. At strain rates higher than about 1 s-1, the material exhibits flow instabilities manifesting as kinking of the elongated grains and adiabatic shear bands. The material may be safely worked in the domain of dynamic recovery but can only be statically recrystallized.

Texture Evolution in Boron Modified Ti-6Al-4V Alloy

Owing to their high strength-to-weight ratio, excellent mechanical properties and corrosion resistance, titanium (Ti) and its alloys, especially (alpha+beta) alloys like Ti-6Al-4V is the backbone materials for aerospace, energy, and chemical industries. Trace boron addition (similar to 0.1 wt. %) to the alloy Ti-6Al-4V produces a reduction in as-cast grain size by roughly an order of magnitude resulting in enhanced ductility, higher stiffness, strength and good fracture resistance. Boron addition could also affect the evolution of texture and microstructure in the material. The solidification microstructures of Boron free as well as Boron containing Ti-6Al-4V are found to be almost homogeneous from periphery towards the center of as-cast ingot in terms of both alpha-colony size and distribution. Boron addition substantially reduces alpha-colony size (similar to 50-80 mu m). A gradual change in alpha texture from periphery towards the center has been observed with orientations close to specific texture components suggesting the formation of texture zones. The mechanism of texture evolution can be visualized as a result of variant selection during solidification through (alpha+beta) phase field.

Evolution of Transformation Texture in a Metastable B - Titanium Alloy

Texture evolution in h. c. p. (alpha) phase derived from aging of a differently processed metastable b.c.c. (beta) titanium alloy was investigated. The study was aimed at examining (i) the effect of different b. c. c. cold rolling textures and (ii) the effect of different defect structures on the h. c. p transformation texture. The alloy metastable beta alloy Ti-10V-4.5Fe-1.5Al was rolled at room temperature by unidirectional (UDR) and multi-step cross rolling (MSCR). A piece of the as-rolled materials were subjected to aging in order to derive the h. c. p. (alpha) phase. In the other route, the as-rolled materials were recrystallized and then aged. Textures were measured using X-ray as well as Electron Back Scatter Diffraction. Rolling texture of beta phase, as characterized by the presence of a strong gamma fibre, was found stronger in M S C R compared to UDR, although they were qualitatively similar. The stronger texture of MSCR sample could be attributed to the inhomogeneous deformation taking place in the sample that might contribute to weakening of texture. Upon recrystallization in beta phase field close to beta-transus. the textures qualitatively resembled the corresponding beta deformation textures; however, they got strengthed. The aging of differently beta rolled samples resulted in the product alpha-phase with different textures. The (UDR + Aged) sample had a stronger texture than (MSCR + Aged) sample, which could be due to continuation of defect accumulation in UDR sample, thus providing more potential sites for the nucleation of alpha phase. The trend was reversed in samples recrystallized prior to aging. The (MSCR + Recrystallized + Aged) sample showed stronger texture of alpha phase than the (UDR + Recrystallized + Aged) sample. This could be attributed to extensive defect annihilation in the UDR sample on recrystallization prior to aging. The (MSCR + Aged) sample exhibited more alpha variants when compared to (MSCR + Recrystallized + Aged) sample. This has been attributed to the availability of more potential sites for nucleation of alpha phase in the former. It could be concluded that alpha transformation texture depends mainly on the defect structure of the parent phase.

Processing map for hot working of Ni–16Cr–8Fe alloy (IN 600)

The hot deformation characteristics of IN 600 nickel alloy are studied using hot compression testing in the temperature range 850-1200-degrees-C and strain rate range 0.001-100 s-1. A processing map for hot working is developed on the basis of the data obtained, using the principles of dynamic materials modelling. The map exhibits a single domain with a peak efficiency of power dissipation of 48% occurring at 1200-degrees-C and 0.2 s-1, at which the material undergoes dynamic recrystallisation (DRX). These are the optimum conditions for hot working of IN 600. At strain rates higher than 1 s-1, the material exhibits flow localisation and its microstructure consists of localised bands of fine recrystallised grains. The presence of iron in the Ni-Cr alloy narrows the DRX domain owing to a higher temperature required for carbide dissolution, which is essential for the occurrence of DRX. The efficiency of DRX in Ni-Cr is, however, enhanced by iron addition.

Processing of an α-2 aluminide alloy, Ti---24Al---11Nb

Compressive stress-strain curves have been generated over a range of temperatures (900-1100-degrees-C and strain rates (0.001-100 s-1) for two starting structures consisting of lath alpha2 and equiaxed alpha2 in a Ti-24Al-11Nb alloy. The data from these tests have been analysed in terms of a dynamic model for processing. The results define domains of strain rate and temperature in which dynamic recrystallization of alpha2 occurs for both starting structures. The rate controlling process for dynamic recrystallization is suggested to be cross-slip in the alpha2 phase. A region of processing instability has also been defined within which shear bands form in the lath structure. Recrystallization of the beta phase is shown to occur for different combinations of strain rate and temperature from those in which the alpha2 phase recrystallizes dynamically

Analysis of texture evolution in pure magnesium and the magnesium alloy AM30 during rod and tube extrusion

The evolution of microstructure and texture during extrusion of pure magnesium and its single phase alloy AM30 has been studied experimentally as well as by crystal plasticity simulation. Microstructure and micro-texture were characterized by electron back scattered diffraction (EBSD), bulk-texture was measured using X-ray diffraction and deformation texture simulations were carried out using visco-plastic self consistent (VPSC) model. In spite of clear indications of the occurrence of dynamic recrystallization (DRX), simulations were able to reproduce the experimental textures successfully. This was attributed to the fact that the textures were c-type fibers with their axis of rotation parallel to the c-axis and DRX leads to simply rotate the texture around the c-axis. (C) 2011 Elsevier B.V. All rights reserved.

A comparative study of the interaction of CO with Cu and Cu-Zn alloy clusters

Interaction of CO with Cu clusters deposited on a ZnO(0001) crystal and on ZnO/Zn surfaces (prepared in the electron spectrometer) has been examined by UV and X-ray photoelectron spectroscopy. The interaction is stronger with the small Cu clusters deposited on ZnO/Zn surfaces. Interaction of CO is evert stronger with annealed Cu/ZnO/Zn surfaces where Cu-Zn alloy particles are present. Copyright (C) 1996 Published by Elsevier Science Ltd

A study of the influence of mischmetal additions to Al-7Si-0.3Mg (LM 25/356) alloy

This article deals with the effect of 0.25-1.5 wt pct mischmetal (MM) addition on the mechanical properties, microstructure, electrical conductivity, and fracture behavior of cast Al-7Si-0.3Mg (LM 25/356) alloy. Modification of eutectic silicon by MM is compared with strontium modification in terms of microstructure, mechanical properties, and fading behavior. Loss of magnesium encountered on holding the molten alloy and its resultant effect on mechanical properties of alloys modified with MM and Sr are compared with those in the unmodified alloy.

Correlation of resistivity and microstructure in dilute titaniulll–nitrogen alloy

The precipitation kinetics of the quenched dilute Ti-1.6 at.-%N alloy has been followed by resistivity measurements at 77 K using the four probe method. Resistivity behaviour has been studied for various durations for the alloys aged in the temperature range 273-373 K. The resistivity behaviour has been explained on the basis of the growth and decay of interfacial strain fields associated with the precipitation process. In addition, the resistivity changes have been correlated with transmission electron microscopy observations. (C) 1995 The Institute of Materials.

Reactivities of aluminium and aluminium-magnesium alloy powders in polymeric composites

Polymeric compositions containing Al-Mg alloys show higher reactivities, in comparison with similar compositions containing aluminium. This is observed irrespective of the amount of oxidizer, type of oxidizer used, type of polymeric binder, and over a range of the particle sizes of the metal additive. This is evident from the higher calorimetric values obtained for compositions containing the alloy, in comparison to samples containing aluminium. Analysis of the combustion residue shows the increase in calorimetric value to be due to the greater extent of oxidation of the alloy. The interaction between the polymeric binder and the alloy was studied by coating the metal particles with the polymer by a coacervation technique. On ageing in the presence of ammonium perchlorate, cracking of the polymer coating on the alloy was noticed. This was deduced from differential thermal analysis experiments, and confirmed by scanning electron microscopic observations. The increase in stiffness of the coating, leading to cracking, has been traced to the cross-linking of the polymer by magnesium.

Structure and properties of fibre reinforced zn-27% al alloy based cast MMCs

Metal matrix composites (MMCs) based on a zinc-27% aluminium alloy (ZA-27) were produced using a pressure infiltration technique. Preforms of alumina fibres and aluminosilicate fibres were used for reinforcement. Uniform distribution of fibres and satisfactory interfacial bonding were achieved. UTS, specific strength, hardness and wear resistance were improved significantly by the alumina fibre reinforcement, but UTS decreased when using aluminosilicate fibres for reinforcement mainly due to unavoidable clustering of particles in the fibre preforms. Structure-property relations have been analysed in all cases.

Microstructural stability and nanoindentation of an electrodeposited nanocrystalline Ni-1.5wt.%P alloy

Microstructural stability of nanocrystalline Ni-1.5wt.%P alloy with an initial grain size of 3 nm processed by pulsed electrodeposition was studied using differential scanning calorimetry (DSC) and annealing. Microstructural characterization suggests that the observed exothermic peak during heating in DSC is related to both concurrent grain growth and Ni3P formation. Nanoindentation on samples with grain sizes from 3 to 50 nm revealed a breakdown in Hall-Petch strengthening in nano Ni-P alloy at grain sizes <= 10 nm, consistent with some previous observations. It is concluded that there is a grain boundary weakening regime for grain sizes < 10 nm, based on analysis which show that the data cannot be rationalized in terms of microstrain relaxation, variation in elastic modulus, texture evolution and duplex structure formation.