Showing papers on "Equiaxed crystals published in 2007"

The principles of grain refinement in equal-channel angular pressing

Abstract: Equal-channel angular pressing (ECAP) is a convenient processing tool for introducing very significant grain refinement, typically to the submicrometer level, in a wide range of metals. It is shown by experiment that processing by ECAP produces very similar microstructures in single crystals and in polycrystalline materials. Thus, after a single ECAP pass, aluminum single crystals and polycrystalline high-purity aluminum both exhibit microstructures consisting of bands of elongated subgrains and the experiments on single crystals have established unambiguously that these bands lie with their longer axes oriented parallel to the primary slip system. A model for grain refinement is developed incorporating the major experimental observations. Calculations of the shearing patterns for different processing routes lead to the conclusion that an equiaxed microstructure is achieved most rapidly in ECAP when slip occurs on three orthogonal planes over a wide range of angles: an example is route BC where the sample is rotated by 90° in the same sense about the longitudinal axis after every pass through the ECAP die.

Dilatant shear bands in solidifying metals

Abstract: Compacted granular materials expand in response to shear1, and can exhibit different behaviour from that of the solids, liquids and gases of which they are composed. Application of the physics of granular materials has increased the understanding of avalanches2, geological faults3,4, flow in hoppers and silos5, and soil mechanics6,7. During the equiaxed solidification of metallic alloys, there exists a range of solid fractions where the microstructure consists of a geometrically crowded disordered assembly of crystals saturated with liquid. It is therefore natural to ask if such a microstructure deforms as a granular material and what relevance this might have to solidification processing. Here we show that partially solidified alloys can exhibit the characteristics of a cohesionless granular material, including Reynolds’ dilatancy1 and strain localization in dilatant shear bands 7–18 mean crystals wide. We show that this behaviour is important in defect formation during high pressure die casting of Al and Mg alloys, a global industry that contributes over $7.3 billion to the USA’s economy alone8 and is used in the manufacture of products that include mobile-phone covers and steering wheels. More broadly, these findings highlight the potential to apply the principles and modelling approaches developed in granular mechanics to the field of solidification processing, and also indicate the possible benefits that might be gained from exploring and exploiting further synergies between these fields.

Microstructure and mechanical properties of commercial purity titanium severely deformed by ARB process

Abstract: Commercial purity titanium was deformed by accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. This is the first study on ultra-high straining of h.c.p. metals by the ARB process. The microstructure of the ARB-processed specimens showed two kinds of characteristic ultrafine microstructures. One was the lamellar boundary structure elongated along RD, which has been also reported in the ARB-processed cubic metals. The lamellar boundary spacing decreased with increasing ARB strain and reached about 80 nm after 5 ARB cycles. The other microstructure was the equiaxed grains having mean grain size of 80–100 nm. Such a fine and equiaxed grain structure has not yet been reported in the as-ARB-processed materials before. The fraction of the equiaxed grains increased as the ARB process proceeded, and 90% of the specimen was filled with the equiaxed grains after 8 ARB cycles. As the number of the ARB process increased, the tensile strength increased and the total elongation decreased gradually. After 6 ARB cycles, the specimen exhibited almost the same mechanical properties as that of commercial Ti-6Al-4V alloy.

Effect of phosphorus on microstructure and growth manner of primary Mg2Si crystal in Mg2Si/Al composite

Abstract: The effects of phosphorus on primary, eutectic Mg2Si phases and growth manner of Mg2Si crystal in Mg2Si/Al composite with steel die and sand mould cast are investigated. The results show that, with the addition of phosphorus, the morphologies of primary Mg2Si particulates changed from dendritic or equiaxed to polygonal shape, and their sizes decrease from ∼300 μm and 200 μm to ∼20 μm and ∼50 μm, respectively. Furthermore, the morphologies of eutectic Mg2Si phases change from fibriform to short fibriform and/or dot-like, while the eutectic Si phases do not change obviously. The exact morphologies of Mg2Si crystals are identified as tetrakaidecahedron and/or octahedron after modification with P, due to V[1 0 0] is depressed. Also, growth mechanism and the formation of hopper-like crystal are discussed.

Evolution of microstructure and microtexture in fcc metals during high-pressure torsion

Abstract: Pure nickel and commercially pure (CP) aluminium were selected as model fcc materials for a detailed investigation of the experimental parameters influencing grain refinement and evolution of microstructure and microtexture during processing by high-pressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of ∼170 nm in pure Ni and ∼1 μm in CP aluminium. It is shown that homogeneous and equiaxed microstructures can be attained throughout the samples of nickel when using applied pressures of at least ∼6 GPa after 5 whole revolution. In CP aluminium, a homogeneous and equiaxed microstructure was achieved after 2 whole revolutions under an applied pressure of 1 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the centre and at the periphery of the samples. It is shown that simple shear texture develops in fcc metals subjected to high-pressure torsion. Some grain growth was detected at the periphery of the Al disk after 8 revolutions. The factors influencing the development of homogeneous microstructures in processing by HPT are discussed.

Using a Three-Phase Deterministic Model for the Columnar-to-Equiaxed Transition

Abstract: In order to overcome the limitations of previous columnar-to-equiaxed (CET) models, which neglect melt convection and the movement of free equiaxed grains, this article presents a three-phase deterministic CET model. With appropriated multiphase volume-averaging approaches, it is possible to account for nucleation and growth of equiaxed grains ahead of a growing columnar front, the influence of melt convection, and grain sedimentation, and the occurrence of a CET in a casting of engineering scale. Special modeling assumptions ensure that both CET mechanisms, namely, “hard” and “soft” blocking, are tackled. It is highly recommended that both mechanisms should be considered, especially in the situation where grain sedimentation and melt convection are present. Although the current model incorporates almost all the physical variables relevant to a CET event, under special condition of a one-dimensional case, the model still reproduces the results of Hunt’s classical CET approach.

Influence of Mg17Al12 intermetallic compounds on the hot extruded microstructures and mechanical properties of Mg–9Al–1Zn alloy

Abstract: Mg–9Al–1Zn (AZ91) cast ingots annealed at 420 °C for 2 and 24 h were hot extruded at 270 °C, respectively. Microstructural observation showed that dynamic recrystallization (DRX) occurred in both cases resulting in fine equiaxed grains. It was revealed that the bar fabricated by extruding short-time annealed ingot had a much smaller average grain size (about 4 μm) than that extruded with a long-time annealed ingot (about 30 μm). It was found that in the former situation, large amounts of small spherical β (Mg17Al12) particles (about 0.3–0.8 μm) were broken directly from the unresolved β phases during the hot extrusion process, most of them finally distributing along the recrystallized grain boundaries. Tensile tests showed that the bars extruded from ingot with short annealing time had superior mechanical properties because of the refined microstructure and the small β particles.

Recrystallization mechanisms in 5251 H14 and 5251 O aluminum friction stir welds

Abstract: The combination of deformation and heat in the nugget of friction stir welding (FSW) tends to transform by dynamic recrystallization the large grains of the base metal into a microstructure where the grains are completely equiaxed and strongly disorientated. To follow the different stages of dynamic recrystallization in the nugget, the evolution of the microstructure of two aluminum samples, cold rolled or annealed, is observed from the parent metal to the nugget using electron back-scattered diffraction (EBSD). The dynamic recrystallization mechanisms in each region of the weld strongly depend on the initial microstructural state of the aluminum sheet, in particular on the deformation energy stored. So, a static recrystallization prior to a continuous dynamic recrystallization was evidenced for the initially cold rolled Al alloy, while a geometric dynamic recrystallization occurred for the initially annealed microstructure. Fractions of low angle boundaries, crystallographic texture and grain size were compared in both weld nuggets.

Increasing Magnetoplasticity in Polycrystalline Ni-Mn-Ga by Reducing Internal Constraints through Porosity

Abstract: Foams with 55% and 76% open porosity were produced from a Ni-Mn-Ga magnetic shape-memory alloy by replication casting. These polycrystalline martensitic foams display a fully reversible magnetic-field-induced strain of up to 0.115% without bias stress, which is about 50 times larger than nonporous, fine-grained Ni-Mn-Ga. This very large improvement is attributed to the bamboolike structure of grains in the foam struts which, due to reduced internal constraints, deform by magnetic-field-induced twinning more easily than equiaxed grains in nonporous Ni-Mn-Ga.

In-Situ and Real-Time Investigation of Columnar-to-Equiaxed Transition in Metallic Alloy

Abstract: In this article, we present a review of observations during Al-3.5 wt pct Ni alloy solidification experiments performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble. These experiments provide direct access to dynamical phenomena during columnar growth (initial transient and breakdown of a planar solid-liquid interface), and for the first time to the transition from columnar-to-equiaxed microstructure (nucleation ahead of a columnar front and blocking of a columnar front by an equiaxed microstructure) and fully equiaxed growth (propagation of an effective front). Based on these experimental observations, critical parameters such as columnar growth velocity variation during the transition or equiaxed-grain diameter are measured and discussed.

Laser Repair of Superalloy Single Crystals with Varying Substrate Orientations

Abstract: The casting and repair of single-crystal gas turbine blades require specific solidification conditions that prevent the formation of new grains, equiaxed or columnar, ahead of the epitaxial columnar dendrites. These conditions are best determined by microstructure modeling. Present day analytical models of the columnar-to-equiaxed transition (CET) relate the microstructure to local solidification conditions (temperature gradient and interface velocity) without taking into account the effects of (1) a preferred growth direction of the columnar dendrites and (2) a growth competition between columnar grains of different orientations. In this article, the infiuence of these effects on the grain structure of nickel-base superalloy single crystals, which have been resolidified after laser treatment or directionally cast, is determined by experiment and by analytical and numerical modeling. It is shown that two effects arise for the case of a nonzero angle between the local heat flux direction and the preferred dendrite growth axis: (1) the regime of equiaxed growth is extended and (2) a loss of the crystal orientation of the substrate often occurs by growth competition of columnar grains leading to an “oriented-to-misoriented transition” (OMT). The results are essential for the definition of the single-crystal processing window and are important for the service life extension of expensive components in land-based or aircraft gas turbines.

Anisotropic Microstructural Development During the Constrained Sintering of Dip‐Coated Alumina Thin Films

Abstract: Alumina thin films were manufactured from aqueous slurries by dip coating. Film thickness as function of substrate withdrawal velocity could be correctly modeled by Landau and Levich's theory. Samples were sintered on a rigid substrate at 1350°C for different isothermal times to achieve relative densities from 84% to 97%. Microstructural analysis of polished cross sections revealed a continuous development of pore alignment, as expected by theoretical considerations. With increasing density pores become more anisometric and orientate along the thickness direction. A further preferential orientation of pores was found in the normal plane, apparently due to the coating process. Constraining conditions had less influence on the size and shape of the grains; they tend to become more equiaxed in the constrained plane, presumably due to the biaxial tensile stress state.

Microstructural evolution and mechanical properties of friction stir welded Mg–Zn–Y–Zr alloy

Abstract: Six millimetre thick Mg-Zn-Y-Zr plate was friction stir welded (FSW). Under a tool rotation rate of 800 rpm and a traverse speed of 100 mm/min, the defect-free weld was obtained. After FSW, coarse strip-like microstructure in forged Mg-Zn-Y-Zr alloy changed into fine equiaxed recrystallized grains in the nugget zone. Furthermore, bulky I-phase particles were broken up and dispersed with some of them being transformed to W-phase, and most of MgZn precipitates were dissolved. Micro-hardness measurement indicated that the hardness of the nugget zone was higher than that of the parent material and the lowest value of hardness was found in the heat-affected zone (HAZ). Transverse tensile tests showed that the strengths and elongation of the weld was only slightly lower than those of parent material with ultimate tensile strength of the weld reaching 95% of the parent material. (C) 2007 Elsevier B.V. All rights reserved.

The real-time, high-resolution x-ray video microscopy of solidification in aluminum alloys

Abstract: The directional solidification of thin alloy sheets in a Bridgman furnace has been studied by x-radiography using high-brilliance synchrotron x-radiation in combination with a low-noise, fast-readout camera. Spatial resolutions down to 1.5 μm and a temporal resolution of about 0.15 s have permitted real-time video microscopy of microstructural evolution during columnar and equiaxed dendrite growth and eutectic and monotectic growth. The technique has also allowed for direct observations of important solidification phenomena such as dendrite fragmentation and porosity formation, primarily in aluminium alloys. As a result, insights have been gained into mechanisms of dendrite fragmentation, criteria for dendrite tip kinetics and interface stability during transient growth, and microstructure formation mechanisms during monotectic solidification. The results are expected to be important for validation of dendrite growth models. This paper presents a review of the technique as well as examples of images obtained during solidification of aluminum alloys.

Influence of Solidification Thermal Parameters on the Columnar-to-Equiaxed Transition of Aluminum-Zinc and Zinc-Aluminum Alloys

Abstract: Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between –0.338 °C/mm and 0.167 °C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys.

Sliding-Wear-Resistant Liquid-Phase-Sintered SiC Processed Using α-SiC Starting Powders

Abstract: Low-cost α-silicon carbide (SiC) starting powder, instead of the more expensive β-SiC starting powder, has been used to process liquid-phase-sintered (LPS) SiC ceramics with different microstructures: (i) elongated SiC grains (in situ toughened LPS SiC), (ii) fine equiaxed SiC grains, and (iii) coarse equiaxed SiC grains. The effects of microstructure on the sliding-wear properties of these LPS SiC ceramics have been studied. The sliding-wear resistance of the in situ toughened LPS SiC ceramic is found to be significantly better than that of two equiaxed-grain LPS SiC ceramics. This has been attributed to the existence of a hard, interlocking network of elongated SiC grains and the isolated nature of the yttrium aluminum garnet (YAG) second phase in the in situ toughened LPS SiC ceramic. This is in contrast to the equiaxed-grain LPS SiC ceramics, where the equiaxed grains are embedded within a continuous YAG phase matrix. The use of the α-SiC starting powder allows the processing of low-cost LPS SiC ceramics that are both sliding-wear resistant and tough.

Microstructural evolution in twin-roll strip cast Mg–Zn–Mn–Al alloy

Abstract: A study has been made on the microstructural evolution in strip cast Mg–6 wt.% Zn–1 wt.% Mn–1 wt.% Al (ZMA611) alloy. The as-cast ZMA611 alloy strip shows equiaxed α-Mg dendritic structure throughout the thickness of the strip. Quasicrystalline phase and primary Al8Mn5 particles are formed in interdendritic region. During solution treatment at 330 °C, the interdendritic quasicrystal phase is rapidly replaced by equilibrium Φ-phase (Mg21(Al,Zn)17), which gradually disappears within α-Mg. In addition, fine secondary Al8Mn5 dispersoids precipitate from supersaturated as-strip cast α-Mg and these dispersoid particles act as effective nucleation sites for precipitation of MgZn2 phase during aging treatment.

Boron-based refiners: Implications in conventional casting of Al–Si alloys

Abstract: The effect of sole boron addition as inoculant has been studied in conventional casting of A356 alloy. It is shown that small addition of boron shifts up the cooling curve and makes the recalescence to disappear entirely. The columnar dendritic structure transforms to equiaxed morphology. For master alloys containing mainly AlB12 particles, it is proposed the dissolved boron, the AlB2 particles present in the master alloy, and some transformation of AlB12 to AlB2 are responsible for the enhanced refining. A hypothesis is proposed to clarify the role of the trace Ti on the nucleation of the primary -Al particles. © 2006 Elsevier B.V. All rights reserved.