Showing papers on "Equiaxed crystals published in 1999"

Epitaxial laser metal forming: analysis of microstructure formation

Abstract: Epitaxial laser metal forming (E-LMF) is presented as a new cladding technique which combines the advantage of near-net-shape manufacturing with a close control of the solidification microstructure. E-LMF is a process where metal powder is injected into a molten pool formed by controlled laser heating. Laser surface treatment has the advantage that heat input is very localised, thus leading to large temperature gradients. This is used, in unison with closely controlled solidification velocities, to stabilise the columnar dendritic growth, thereby avoiding nucleation and growth of equiaxed grains in the laser clad. It is possible with this technique to deposit a single crystal clad by epitaxial growth onto a single crystal substrate. In this paper, the microstructure obtained by E-LMF is analysed by scanning electron microscopy (SEM), optical microscopy (OM) and indexing electron backscattered diffraction (EBSD) patterns. In particular, the grain structure formation in the deposit during the process and the influence of a subsequent heat treatment on precipitation and recrystallisation is characterised.

Flow visualization and residual microstructures associated with the friction-stir welding of 2024 aluminum to 6061 aluminum

Abstract: The friction-stir welding (FSW) of 0.6 cm plates of 2024 Al (140 HV) to 6061 Al (100 HV) is characterized by residual, equiaxed grains within the weld zone having average sizes ranging from 1 to 15 μm, exhibiting grain growth from dynamically recrystallized grains which provide a mechanism for superplastic flow; producing intercalated, lamellar-like flow patterns. These flow patterns are visualized by differential etching of the 2024 Al producing contrast relative to 6061 Al. The flow patterns are observed to be complex spirals and vortex-like, among others, and to change somewhat systematically with tool rotation (stirring) speed between 400 and 1200 rpm; depending on tool orientation. The equiaxed grain and sub-grain microstructures are observed to vary according to estimated temperature profiles (varying from 0.6 to 0.8 TM, where TM is the absolute melting temperature) referenced to the rotating tool axis. Dislocation spirals and loops are also observed in the 2024 Al intercalation regions within the weld zones at higher speeds (>800 rpm) corresponding to slightly elevated temperatures introducing dislocation climb, and residual microhardness profiles follow microstructural variations which result in a 40% reduction in the 6061 Al workpiece microhardness and a 50% reduction in the 2024 Al workpiece microhardness just outside the FSW zone.

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

Abstract: The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from β to α-SiC reveals a dependency on the initial β-content and the sintering atmosphere. The transformation rate decreases with an increasing β-content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure β-SiC starting powders when the additive system consists of 10.34 wt % Y2O3 and 2.95 wt % AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial α/β-ratio (> 1 : 9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.

Microstructure and mechanical behavior of arc-evaporated Cr–N coatings

Abstract: Cr–N coatings were grown by arc evaporation onto high speed steel substrates. The coatings were each grown using a different negative substrate bias voltage, VS, between 20 and 400 V. X-ray diffraction showed a microstructure containing primarily the NaCl structured CrN phase along with the BCC-Cr and HCP-Cr2N. Auger electron spectroscopy and transmission electron microscopy indicate a substoichiometric (N/Cr=0.85±0.08) composition and dense columnar microstructure, respectively. At VS=400 V, equiaxed grains and microcracks parallel the substrate–coating interface were observed. The fiber textured coatings are in a compressive residual stress state that increases from 2.9 (VS=20 V) to 8.8 GPa (VS=100 V). At higher bias voltages, a decrease of the compressive residual stress is seen, which is discussed in terms of lattice defect annihilation in the collision cascade and lattice defect diffusion during deposition. Nanoindentation showed a maximum hardness at VS=100 V of 29 GPa. The critical loads for cohesive failure in a scratch test decreased monotonically with increasing negative substrate bias. The scratch results suggest a transition in deformation mechanism from plastic deformation to cracking, which occurs at lower applied loads when VS is increased. Similar behavior was also seen in a crater grinding wear test where a shift in wear mechanism from plastic deformation to chipping occurred at VS=200 V. The influence of the microstructure on the deformation transition is discussed in terms of lattice defect density and the presence of equiaxed grains.

Effect of welding parameters on the solidification microstructure of autogenous TIG welds in an Al-Cu-Mg-Mn alloy

Abstract: The weld metal microstructures of autogenous TIG welds have been investigated for a range of welding conditions using an Al–Cu–Mg–Mn alloy. It was found that a combination of high welding speeds and low power densities provide the thermal conditions required for the nucleation and growth of equiaxed grains in the weld pool, providing heterogeneous nucleation sites are available. The most likely origin of the nucleants is from a combination of dendrite fragments and TiB2 particles that survive in the weld pool. The finest microstructure was observed in the centre of the weld and is attributed to the higher cooling rates which operate along the weld centreline. Composition profiles across the dendrite side arms were measured in the TEM and were found to follow a Scheil type segregation behaviour where there is negligible back diffusion in the solid. The measured core concentration of the dendrite side arms was found to rise with increasing welding speed and was attributed to the formation of significant undercoolings ahead of the primary dendrite tip, which enriched the liquid surrounding the dendrite side arms.

Laser surface engineering of steel for hard refractory ceramic composite coating

Abstract: Laser surface modification technique is applied to deposit ultrahard ceramic (TiB2) coating on 1010 steel. A uniform, continuous and crack free coating with a metallurgically sound interface is obtained. Coating is ‘‘composite’’ in nature comprising TiB2 particles and Fe in the laser melt zone. Polygonal and needle shaped boride particles are uniformly distributed in the laser melted zone. The interfacial microstructure consists of cellular dendrites and fine equiaxed dendrites. Metastable phase(s) such as FexBy and TimBn are also observed which is a characteristic feature of non-equilibrium synthesis by laser energy. Laser melt zone has a high hardness. However, hardness is not uniform in the layer. ” 1999 Elsevier Science Ltd. All rights reserved.

Experimental determination of mushy zone permeability in aluminum-copper alloys with equiaxed microstructures

Abstract: An experimental apparatus for measuring the mushy zone permeability of aluminum-copper alloys with equiaxed microstructures has been constructed. Permeabilities have been measured for high solid fractions (0.68 to 0.91) and different dendrite morphologies. Microstructure characterizations on both the interdendritic and extradendritic length scales have been performed on the samples. The results are in fairly good agreement with the predictions of the Kozeny-Carman relation and with more recent theory that takes flow partitioning between interdendritic and extradendritic regions into account.

Hot working of Ti-6Al-4V via equal channel angular extrusion

Abstract: The deformation behavior of Ti-6Al-4V during high-temperature equal channel angular extrusion (ECAE) with or without an initial increment of upset deformation was determined for billets with either a lamellar or an equiaxed alpha preform microstructure. For conventional ECAE (i.e., deformation by simple shear alone), flow localization and fracture occurred at temperatures between 900 °C and 985 °C. In contrast, billets deformed at temperatures between 845 °C and 985 °C using an initial increment of upset deformation immediately followed by the simple shear deformation of ECAE exhibited uniform flow with no significant cracking or fracture. A simple flow-localization criterion was used to explain the influence of preupsetting on the suppression of localization in billets with the lamellar microstructure. The suppression of flow localization for the equiaxed microstructure and the elimination of edge cracking for both types of microstructures were explained in terms of heat transfer (die chill) and workpiece geometry. Further evidence of the relative importance of microstructural and thermal effects was extracted from the results of two-pass extrusions, the first with upsetting and the second without upsetting.

Effect of La2O3 particles on the oxidation of electrodeposited nickel films

Abstract: Electrodeposited Ni-La2O3 composite films with nanometer-size La2O3 oxide inclusions were fabricated by the codeposition of nickel with La2O3 particles. The comparative oxidation behavior in air at 900 and 1000 degrees C of nickel coated with the Ni-La2O3 composite and films with and without nickel-plating was studied by TGA, AE, SEM/EDX, EPMA, and TEM/EDX. In general, the Ni-La2O3 composite-coated nickel had the slowest rate and the best resistance to thermal cycling. AE tests revealed that cracking events in NiO scales on Ni-La2O3 composite-coated nickel was significantly reduced in comparison to that of the scale on nickel-coated nickel during thermal cycling at 900 degrees C. SEM investigation showed that the La2O3-free NiO scale was composed of outer coarse columnar grains and inner equiaxed ones. By contrast, the scale on the Ni-La2O3 composite-coated nickel consisted of only fine equiaxed NiO grains. The scale on the La2O3-free samples was characterized by cracks that originated at the scale-substrate interface and spanned the scale thickness. By contrast, no scale cracks formed at the La2O3-doped NiO scale-substrate interface, but small voids were created at the triple points of the grain boundaries of NiO. In the La2O3-doped NiO scale, segregation of La ions to the NiO grain boundaries near the scale-surface was observed by EDX microanalyses in the TEM. It is believed that the La ions segregated at the grain boundaries of NiO led to an increase in the cohesion between nickel oxides and in a reduction of the scaling rate and the formation of scale with fine equiaxed crystal structure by blocking the outward and lateral growth of scale. The latter was due to the predominant outward diffusion of nickel along NiO grain boundaries being inhibited effectively by the segregated La ions. The mechanism of the effect of the added La2O3 particles on the nickel electrodeposits is discussed in detail.

Equiaxed solidification of Al–Si alloys

Abstract: An experimental programme has been undertaken to determine which of the grain formation mechanisms of equiaxed crystals are dominant in the solidification of Al-Si foundry alloys. Small ingots were cast from alloys of varying silicon concentration with and without gauze barriers, using different types of mould materials and different mould preheats. The results show that two mechanisms of grain nucleation are operating. The first is a wall mechanism where crystals are nucleated either on or near the mould wall owing to thermal undercooling. The second is a constitutional supercooling mechanism where nucleants are activated in the constitutionally undercooled zone ahead of the advancing interface. As a consequence, the grain size decreases with increasing silicon content. However a transition in the growth mode occurs once a critical degree of constitutional undercooling is exceeded. This change in growth is accompanied by an increase in grain size. The transition point can be shifted with respect to solute content by changing the casting conditions, and a mechanism is proposed to explain this effect. MST/4109

Silicon carbide ceramics prepared by pulse electric current sintering of β–SiC and α–SiC powders with oxide and nonoxide additives

Abstract: Using a pulse electric current sintering (PECS) method, β–SiC and α–SiC powders doped with a few weight percent of Al2O3–Y2O3 oxide or Al4C3–B4C–C nonoxide additives were rapidly densified to high densities (95.2–99.7%) within less than 30 min of total processing time. When Al2O3–Y2O3 additive was used, both ceramics resulting from β–SiC and α–SiC had fine, equiaxed microstructures. In contrast, when Al4C3–B4C–C additive was used, the ceramic resulting from α–SiC had a coarse, equiaxed microstructure, whereas the ceramic resulting from β–SiC was composed of large elongated grains whose formation was accompanied by the β →?α phase transformation of SiC. Compared with the Al2O3–Y2O3-doped SiC ceramics, the Al4C3–B4C–C-doped SiC ceramics had higher densities, lower fracture toughness, and higher hardness. The fracture mode of the oxide-doped SiC was mainly intergranular, whereas the nonoxide-doped SiC exhibited almost complete intragranular fracture that was attributed to the higher interfacial bonding strength.

Experimental Study of Free Growth of Equiaxed NH4Cl Crystals Settling in Undercooled NH4Cl-H2O Melts

Abstract: The present study deals with experiments carried out to assess the influence of the movement of equiaxed crystals on their growth kinetics The principle of the experiments is based on the settling of crystals growing in melts of transparent alloy undercooled at constant temperature Ammonium chloride-water mixtures have been chosen for their convenient properties The experimental set-up is composed of a settling tube and a video-device which allowed us to record continuously the images of dendritic NH4Cl equiaxed crystals all along their fall On these images two kinds of morphology have been observed of which the difference seems to be related to the spinning movements of the crystals Furthermore time evolutions of the vertical position and apparent sizes of the crystals have been measured A careful examination of these measurements was necessary because of the uncertainties of the undercooling and of the actual shapes and sizes of the crystals which are inherent in the set-up and in the procedure Average growth velocities were determined so as to identify some general trends concerning the effect of the settling on the growth of the equiaxed crystals Finally, a short comparison between these average growth velocities and the corresponding theoretical ones without any convection has shown a strong influence of the convection on the equiaxed growth kinetics

Dispersion of fine primary inclusions of MgO and ZrO2 in Fe-10 mass pct Ni alloy and the solidification structure

Abstract: The homogeneous dispersion of primary inclusions of MgO and ZrO2 was studied in an Fe-10 mass pct Ni alloy as a function of the holding time at 1873 K and the cooling rate. The spatial size distribution was estimated from the planar size distribution obtained in a cross section by applying the Schwartz-Saltykov transformation. It was found that the content of insoluble Mg or Zr estimated from the size distribution agreed with that obtained from chemical analysis. The influence on the solidification macrostructure such as columnar dendrite, equiaxed dendrite, and globular crystal of dissolved Mg or Zr and inclusion particles having the mean diameter of roughly 1 µm was investigated. The area fraction of globular crystals in the Mg deoxidation decreased with increasing dissolved Mg content (>30 mass ppm) in the presence of MgO particles. In the Zr deoxidation, however, globular crystals were only observed in the presence of ZrO2 particles without respect to the presence of dissolved Zr.

Mechanism of the formation of lamellar M23C6 at and near twin boundaries in austenitic stainless steels

Abstract: Precipitation characteristics of M23C6 during aging at 800 °C were studied by transmission electron microscopes in two austenitic stainless steels, A (type AISI 316L) and B (type DIN 4981), which had been quenched in water after their solution treatment at high temperatures (1150 °C and 1275 °C, respectively). After precipitation at grain boundaries, M23C6 precipitated at incoherent and coherent boundaries of twins and inside austenite grains. Close to an incoherent twin boundary on either side of the boundary, M23C6 mostly grew as elongated plates, although elsewhere in austenite matrix these grew usually as equiaxed particles. The plates lied on planes parallel to the twinning plane and were aligned unidirectionally along the axis of intersection of the twinning plane and a {110} plane perpendicular to the twinning plane. On coherent twin boundaries, similar plates of M23C6 formed along with some equiaxed particles. Existing models for the mechanism of formation of these lamellar carbides fail to explain the observed features of these carbides formed around different twins. It is suggested that the residual stress localized in the vicinity of twin boundaries in quenched specimens influences the nucleation and growth of these carbides during aging, resulting in their specific morphology.

Effect of annealing on mechanical properties of self-reinforced alpha-silicon carbide

Abstract: Alpha-SIC powder containing 7.2 wt % Y3Al5O12 (YAG, yttrium aluminum garnet) and 4.8 wt % SiO2 as sintering aids were hot-pressed (SC0) at 1820°C for 1 h and subsequently annealed at 1920°C for 2 h (SC2), 4 h (SC4) and 8 h (SC8). When the annealing time was increased, the microstructure changed from equiaxed to elongated grains and resulted in self-reinforced microstructure consisted of large elongated grains and small equiaxed grains. Development of self-reinforced microstructure, consisted of mostly 6H phase, resulted in significant improvements in toughness. However, the improved toughness was offset by a significant reduction in strength as in the materials consisted of 4H originated from β-SiC. The fracture toughness and strength of the 8-h annealed materials were 5.5MPa · m1/2 and 490 MPa, respectively.

Influence of grain orientations on the initiation of fatigue damage in an Al-Li alloy.

Abstract: The variation in microstructure and texture in a rectangular bar extruded from a billet of spray-cast 8090 Al-Li alloy has been examined. The fine grain size of the as sprayed billet and the moderate extrusion ratio ( approximately 25 : 1) were seen to cause geometric dynamic recrystallization (GDR) in regions of higher strain towards the edge of the bar. The grain morphology varied from the expected elongated grains at the centre of the bar to equiaxed grains where GDR occurred at the bar edges. A + double fibre texture, significantly distorted towards rolling components and varying through the bar thickness, was found using electron backscatter diffraction. Fatigue resulted in a high density of short secondary cracks, many of which had arrested at grain boundaries. The cracks preferentially nucleated in grains from the fibre texture corresponding to high Schmid factors.

Mathematical modeling of microstructural development in hypoeutectic cast iron

Abstract: A mathematical heat-transfer/microstructural model has been developed to predict the evolution of proeutectic austenite, white iron eutectic, and gray iron eutectic during solidification of hypoeutectic cast iron, based on the commercial finite-element code ABAQUS. Specialized routines which employ relationships describing nucleation and growth of equiaxed primary austenite, gray iron eutectic, and white iron eutectic have been formulated and incorporated into ABAQUS through user-specified subroutines. The relationships used in the model to describe microstructural evolution have been adapted from relationships describing equiaxed growth in the literature. The model has been validated/fine tuned against temperature data collected from a QuiK-Cup sample, which contained a thermocouple embedded approximately in the center of the casting. The phase distribution predicted with the model has been compared to the measured phase distribution inferred from the variation in hardness within the QuiK-Cup sample and from image analysis of photomicrographs of the polished and etched microstructure. Overall, the model results were found to agree well with the measured distribution of the microstructure.

Processing and Deformation Behavior of Gamma TiAl Alloys with Fine-Grained Equiaxed Microstructures

Abstract: This paper describes how a submicrocrystalline structure can be produced by isothermal deformation of cast and powder metallurgy γ-TiAl-based alloys at temperatures 1000°C and below using relatively inexpensive nickel-based superalloy die tooling. A detailed analysis of the effects of initial microstructure, chemical compositions, conditions of heat treatment and hot working on the formation of a homogeneous fine-grained microstructure in γ-TiAl-based alloys is presented.

An Experimental Investigation Into the Effects of Grain Transport on Columnar to Equiaxed Transition During Dendritic Alloy Solidification

Abstract: An experimental study has been conducted to investigate the effects of grain transport on the columnar to equiaxed transition (CET) in dendritic alloy solidification Using the aqueous ammonium chloride solution as a transparent model alloy, experiments were performed in a vertical test cell with cooling from the top, resulting in unidirectional columnar crystals growing downwards Ahead of the columnar front, equiaxed nuclei were observed to originate mostly by fragmentation of the columnar dendrites in the presence of a thermally driven flow in the melt beneath the columnar mushy zone Being heavier than the liquid, these fragments fall into the bulk melt where they may grow or remelt The survival equiaxed crystals finally settle towards the floor and pile up to form an equiaxed bed The CET occurs when the bottom equiaxed packed bed rises and eventually obstructs the columnar mushy zone growing from the upper surface Therefore, the CET in the present configuration was predominantly controlled by the sedimentation of equiaxed crystals A parametric study by varying initial concentration, cooling rate, and superheat was performed

On the occurrence of dynamic strain aging in near-alpha alloy Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si

Abstract: Alloy Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si (IMI 834) is the most recently developed near-alpha type titanium alloy for high-temperature application up to 873 K, as discs and blades in the high pressure part of compressors, in advanced jet engines. It possesses a good combination of creep and fatigue resistance at elevated temperature, in properly heat-treated condition, and has a fine bimodal microstructure, consisting of a small volume fraction of equiaxed alpha in a fine-grained matrix of transformed beta. Some investigations have already been done on the tensile behavior of this alloy, in different heat-treated conditions, over a wide range of temperature from 293 to 923 K. However, no report has been made on the occurrence of dynamic strain aging (DSA) in this alloy. The purpose of this article is to present the observation on the occurrence of DSA in the titanium alloy IMI 834, in the as-received (hot-rolled and mill-annealed) condition, over the temperature range 623 to 823 K.

Microstructural Evolution for Superplasticity Using Equal-Channel Angular Pressing

Abstract: Equal-channel angular (ECA) pressing is capable of introducing an ultrafine grain size into bulk materials and the precise nature of the microstructure is dependent upon the number of pressings and the combinations of the planes for shear deformation. When the strain is sufficiently high, it is possible to achieve equiaxed fine grains with high angle grain boundaries. The number of pressings required to establish the ultimate equiaxed grains increases with an increase in the alloy content. Tensile experiments at elevated temperatures under different initial strain rates have demonstrated that ECA pressed Al-5.5%Mg-2.2%Li-0.12%Zr (Al-1420) and Al-6%Cu-0.5%Zr (Supral) show excellent superplasticity, thereby providing the potential for substantially increasing the viability of superplastic forming in the metal forming industry.

Recrystallization in cast 45-2-2 XD titanium aluminide during hot isostatic pressing

Abstract: XD ™ TiAl alloys are well known to have the advantage of a homogeneous microstructure when compared with monolithic TiAl-based alloys. In this research, however, an inhomogeneous microstructure consisting of a fully lamellar structure in the edge region and a duplex structure of equiaxed grains and lamellar colonies in the central area was discovered in as-HIPed Ti–45Al–2Mn–2Nb+0.8 vol.% TiB 2 bar. Electron probe microanalysis and optical/transmission electron microscopy of HIPed material, optical microscopy of cast material indicated the formation mechanism of the duplex structure in the central area was recrystallization due to the strain energy stored by the collapse of cast porosity during HIP. Crack initiation at the interface between regions of a duplex and a fully lamellar structure in high cycle fatigue testing suggested that the duplex structure might have a detrimental effect on the mechanical properties of the material.

Nanostructural characteristics and sintering behavior of W-Cu composite powder prepared by mechanical alloying

Abstract: W-Cu alloy is one of the promising materials for heat sinks and packaging in the microelectronic devices due to its good thermal properties. In the present study, nanostructured (NS) W-Cu composite powder was prepared by the mechanical alloying (MA), and their nanostructural evolutions with milling times were analyzed by SEM, XRD and TEM. MA W-Cu powder milled for 50 hours was characterized by equiaxed shape with mean particle size of 2-3aem, and crystalline size was aboutt 20-30nm The MANS W 20wt%Cu and W 30wt%Cu powders compacts had shown the high sinterability, resulting in a nearly full density by sintering at 1100oC. The enhanced sinterability of MA W-Cu composite powder was attributed to the nanostructural characteristics of W and Cu phases as well as the activated sintering of Fe-impurity.