Showing papers on "Equiaxed crystals published in 1995"

Compatibility of deformation in two-phase Ti-Al alloys : dependence on microstructure and orientation relationships

Abstract: A two-phase alloy of composition Ti-47.5Al-2.5Cr has been studied under two heat-treated conditions in order to obtain different microstructures. These consisted of lamellar and equiaxed distributions of y grains in which the α2 phase was distributed as long lamellae or smaller globules, respectively. The specific rotation relationships between γ/γ and γ/α2 grains have been measured, and these have been used to understand their effect on the compatibility of deformation across adjacent grains. For this, detailed analysis of active slip systems has been carried out by transmission electron microscopy (TEM) observations of deformed samples. A theoretical calculation of a geometric compatibility factor characterizing the best slip transfer across adjacent grains has been used in such a way that it has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.

Multiparticle interfacial drag in equiaxed solidification

Abstract: A physical model is proposed for the solid/liquid interfacial drag in both globular and dendritic equiaxed solidification. By accounting for the presence of multiple particles and the nonsphericity and porosity of the individual equiaxed crystals, a drag correlation is developed, which is valid over the full range of solid volume fractions. It is shown that neither the solid liquid interfacial area concentration nor the grain size alone is adequate to characterize the interfacial drag for equiaxed dendritic crystals in both the free particle and packed bed regimes; thus, the present model is based on a multiple length scale approach. The model predictions are compared to previous analytical and numerical results as well as to experimental data available in the literature, and favorable agreement is achieved.

Enhanced Machinability of Silicon Carbide via Microstructural Design

Abstract: The machinability of a heterogeneous silicon carbide with weak interphase boundaries, elongated grains, and high internal stresses is evaluated relative to a homogeneous control material with a well-bonded, equiaxed, and unstressed grain structure. Drilling and grinding rates for the silicon carbide are substantially enhanced by the microstructural heterogeneity—the weak boundaries enable easy grain-scale dislodgement in place of the more conventional macrofracture chipping mode of removal. At the same time, the residual machining damage in the machined surfaces is significantly less strength degrading in the heterogeneous material. Implications concerning the microstructural design of flaw-tolerant ceramics for enhanced machinability are considered.

Mechanisms of equiaxed grain formation in ferritic stainless steel gas tungsten arc welds

Abstract: Mechanisms of providing nuclei for equiaxed grains in gas tungsten arc welds in ferritic stainless steel were examined. Tin-quenching of the solid-liquid interface revealed that TiN particles in commercial steels could act as heterogeneous nucleation sites. Direct additions of TiN also promoted an equiaxed region. However, no evidence for equiaxed grain formation by dendrite fragmentation was observed, despite the fact that titanium additions did produce a more branched dendritic structure.

Fracture characteristics of Ti-6Al-4V and Ti-5Al-2.5Fe with refined microstructure using hydrogen

Abstract: The hydrogenation behavior of Ti-6Al-4V, with the starting microstructures of coarse equiaxed α and coarse Widmanstatten α, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5Al-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed α at 1023 K, just below hydrogen saturated β (denoted β″ (H)) transus temperature, produces a microstructure of a, orthohombic martensite (denoted α″ (H)) and β (H). Hydrogenation at 1123 K, above β (H) transus, results in a microstructure of α″ (H) and β (H). Microstructure refinement during TCP results mainly from decomposition of α″ (H) and ;β (H) into a fine mixture of α + β during dehydrogenation. An alternative TCP method is below β (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated β (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed α or with a little decrease in the tensile elongation in the starting microstructure of coarse Widmanstatten α, although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5Al-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe.

Creep deformation in near-γ TiAl: Part 1. the influence of microstructure on creep deformation in Ti-49Al-1V

Abstract: The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of γ/γ or γ/α2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed y microstructures.

Transformation strengthening by thermomechanical treatments in C-Mn-Ni-Nb steels

Abstract: The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation mechanism associated with the austenite → ferrite transformation caused by heavy deformation in the austenite range.

Microstructural examination of a superplastic yttria-stabilized zirconia: Implications for the superplasticity mechanism

Abstract: A detailed microstructural examination was conducted on specimens of high purity superplastically-deformed 3Y-TZP (tetragonal ZrO{sub 2} stabilized with {approximately}3 mol.% Y{sub 2}O{sub 3}). Several of the microstructural features were similar to conventional superplastic metallic alloys including the retention of an equiaxed grain configuration, evidence for only limited dislocation activity within the grains and the concurrent development of internal cavitation. There was no detectable amorphous phase at any of the grain boundaries or at the triple junctions but there was some segregation of yttria to the boundary regions. It is concluded that there are two types of superplastic Y-TZP materials depending upon whether there is an amorphous phase at the grain boundaries. When an amorphous phase is absent, the behavior is fairly similar to superplastic metals except that an additional mechanism operates at the lower stress levels to impede grain boundary sliding.

Redistribution of Particles during Solidification

Abstract: During solidification, particles (inclusions and equiaxed grains) get pushed or engulfed by the freezing front The redistribution of particles in the solid is controlled by the forces acting on the particles and the flow in the liquid melt The present paper describes recent work on determining some of the more important parameters which govern the redistribution of particles during solidification

Relation between grain size and coherency parameters in aluminium alloys

Abstract: During equiaxed dendritic solidification, dendritic grains form at the point where primary dendritic crystals become coherent (or impinge on each other). An experimental and theoretical approach to evaluate the grain size in the alloys is proposed, using thermal and constitutional parameters, and the parameters at the dendrite coherency point. For an alloy with a solute concentration below the solid solubility limit, the grain size is inversely proportional to (dT/dt)1/2 Σ miCoi (ki−1) and the coherency parameters (T is the temperature, t is the time, m is the liquid line slope, Co is the original solute concentration, k is the solute distribution, and subscript i is the ith alloying element). For an alloy with a solute concentration above the solid solubility limit, the grain size can be calculated using the dendritic growth interval Σ ki(TLi−Teui), where TL and Teuare the liquidus and eutectic temperatures, respectively, instead of the dendritic restriction factor Σ miCoi (ki−1). For commercial ...

Dynamic recrystallization and dynamic recovery in pure aluminum at 583K

Abstract: In situ observations using transmission Laue X-ray photographs taken during 583 K compression tests were used to compare the restorative process of 99.999 mass% aluminum to that of 99.99 mass% aluminum. A regular multipeak stress oscillation, typical of dynamic recrystallization, was generated in 99.999 mass% aluminum, while monotonous work hardening, typical of dynamic recovery, were present in 99.99 mass% aluminum. Spot patterns as well as coarse recrystallized grains containing subgrains confirmed dynamic recrystallization in the 99.999 mass% aluminum. The dynamic recovery of 99.99 mass% aluminum was confirmed by asterism patterns and equiaxed subgrains. Dynamic recrystallization was found to take place only in the high purity range above 99.999 mass%. This dynamic recrystallization was considered to be the results of discontinuous dynamic recrystallization which is well known in Cu, Ni and γ-Fe.

Beryllium-containing alloys of aluminum and semi-solid processing of such alloys

Abstract: Disclosed is a practical aluminum based alloy containing 1 to 99 weight percent beryllium, and improved methods of semi-solid processing of aluminum alloys containing beryllium. The present methods avoid molten beryllium, agitation of molten aluminum-beryllium alloys and the need for introducing shear forces by utilizing atomized or ground particles of beryllium mixed with solid, particulate or liquidus aluminum. Retention of the equiaxed beryllium morphology after modified semi-solid processing of an aluminum-beryllium alloy is demonstrated by the photomicrograph in the figure.

Influence of Columnar Microstructure on Ultrasonic Backscattering

Abstract: Most structural materials are polycrystalline, that is, they are composed of numerous discrete grains, each having a regular, crystalline atomic structure The elastic properties of the grains are anisotropic and their crystallographic axes are differently oriented When an ultrasonic wave propagates through such a polycrystalline aggregate, it is scattered at the grain boundaries The fraction of sound energy thus removed from the main beam is responsible for important phenomenons like attenuation and dispersion of the main beam, and background “noise” associated with a given ultrasonic inspection system The amount of sound energy removed from the main beam depends on the size, shape, and orientation distributions of the grains If the grains are equiaxed and randomly oriented, propagation direction of the ultrasonic wave has no effect upon the magnitude of the scattered energy Such is not the case when an acoustic wave travels through materials like centrifugally cast stainless steel and austenitic stainless steel welds, which are used extensively in nuclear power plants The microstructures of these stainless steels vary from randomly oriented, equiaxed grains to highly oriented, columnar grains1,2 Since the backscattered signals tend to mask the signals from small and subtle defects, the estimation of probability of detection of such defects requires quantitative description of these signals Consequently, an effort has been undertaken in this research to quantify the backscattered signals from microstructures with favored grain orientation and grain elongation

Stochastic Modelling of Solidification Grain Structures

Abstract: Stochastic models have been developed for the simulation of grain structure formation during the solidification of metallic alloys. Nucleation is assumed to occur randomly in space according to a prescribed distribution of nucleation sites. For dendritic alloys, the hypothesis of a random orientation of the crystallographic directions of the new nuclei is also made. A cellular automaton (CA) and an interface-tracking technique are used to follow the growth-impingement of dendritic and eutectic grains, respectively. The influence of the local thermal conditions, namely the thermal gradient and the velocity of the isotherm, and of the nucleation parameters on the resulting grain structures is assessed. In particular, it is shown that the asymmetry of the grains along the thermal gradient is an increasing function of the thermal gradient and nucleation undercooling and a decreasing function of the velocity and grain density. The presence of the outer equiaxed zone and the transition from columnar to equiaxed microstructures can also be explained using such models.

Study of Dendrite Coherency in Al-Si Alloys During Equiaxed Dendritic Solidification

Abstract: Dendrite coherency (or impingement) is important for the formation of solidification structures. The effects of silicon concentration and modification of eutectic silicon crystals on dendrite coherency in Al-Si alloys have been studied by continuous torque measurement in solidifying samples. The fraction solid at the dendritic coherency point in the alloys decreases with increasing silicon concentration, and varies from 0.09 to 0.39 when the silicon concentration in the alloy is in the range 0.5 to 11 mass%. Modification of eutectic silicon crystals may increase the coherency fraction solid somewhat, but not much. When a grain refiner and a modifier are added together the coherency fraction solid will increase greatly. A theoretical approach is proposed to describe how these factors affect the dendrite coherency in the alloy. (orig.)

Method of superplastic extrusion

Abstract: A method of superplastic extrusion is provided for fabricating large, complex-shaped, high strength metal alloy components, such as large, thin cross section, closed-box panels or integrally "T-stiffened" aircraft skin panels. Superplastic extrusion is similar to conventional extrusion except that strain rate and temperature are carefully controlled to keep an ultra-fine grain high strength metal alloy within the superplastic regime where deformation occurs through grain boundary sliding. A high strength, heat treatable metal alloy is first processed, such as by equal channel angular extrusion (ECAE), to have a uniform, equiaxed, ultra-fine grain size in thick section billet form. Temperature and strain rate are controlled during superplastic extrusion of the ultra-fine grained billet so that the stresses required for metal flow are much lower than those needed in conventional extrusion. The low stresses allow use of more fragile extrusion dies, including multi-hole dies for hollow core extrusions, thereby achieving thinner section details in larger extruded components for a given press loading capacity. After superplastic extrusion, components may be solution treated, stretch straightened, and creep-age formed in an autoclave, as required. The resulting large, compound curvature, thin section, integrally stiffened, high strength metal alloy components retain a uniform, equiaxed, fine grain size, which imparts superior strength, isotropy, ductility, toughness, and corrosion resistance compared with conventional grain sized metal alloys.

Investigation of the formation of an amorphous film at the ZrO2Y2O3/NiCoCrAlY interface of thermal barrier coatings produced by plasma spraying

Abstract: The interface between metallic bonding layers (NiCoCrAlY) and ceramic coatings of 8% yttria partially stabilized zirconia (YPSZ), produced by air plasma spraying (APS) at different deposition temperatures, was investigated by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM) on transverse thin foils. The presence of an Al-rich amorphous film was ascertained, probably originating during the deposition of the bond coat as a crystalline alumina surface layer. This layer successively remelts, grows and solidifies in the amorphous state due to the rapid heat extraction during plasma spraying. The thickness of this interlayer is a function of the deposition temperature: the higher the substrate temperature during spraying, the thinner the interfacial amorphous film. Columnar YPSZ grains, elongated in the direction perpendicular to the surface and having typical dimensions of 50 nm × 500 nm, are superimposed on the amorphous layer. Equiaxed grains of comparable size extend above the columnar structure. Both columnar and equiaxed grains consist of a metastable tetragonal phase t′, characterized by a higher toughness than the face-centred cubic equilibrium phase. The negative influence of this brittle interlayer, in particular on the adhesion strength of the thermal barrier coating (TBC), was also evaluated by means of four-point bending tests. Results confirm a low adhesion strength between the ceramic and the bond coat in the case of TBCs produced at low deposition temperatures.

Solidification condition of bulk glassy Zr60Al10Ni10Cu15Pd5 alloy by unidirectional arc melting

Abstract: The relation between the formation of a glassy phase and the solidification parameters of moving velocity of liquid/ solid interface (V), temperature gradient (G) and cooling rate (R) was examined for a Zr 60 Al 10 Ni 10 cu 15 Pd 5 alloy, with the aim of clarifying a solidification condition for formation of a bulk glassy alloy by a unidirectional arc-melting method. The glassy phase was obtained in the condition of V> 4 mm/s, G>4 K/mm and R>40K/s. The decrease in G causes the formation of equiaxed dendrites, oriented dendrites and cell structure. The supercooling for the present alloy was measured to be as large as 385 K at a low cooling rate of 40K/s. The large supercooling ability is presumably due to the formation of a highly dense random packed structure where the nucleation of a crystalline phase and the atomic rearrangement for growth reaction are difficult. The glass formation of the present multicomponent alloy in the unidirectional arc melting method seems to be dominated by the ease of the supercooling ability rather than the achievement of high cooling rate.

In situ metal-ceramic microstructures by partial reduction reactions in the NiAlO system and the role of ZrO2

Abstract: Partial reduction reactions in the NiAlO system, starting with the spinel compound NiAl 2 O 4 , are used to form metal-ceramic microstructures in situ. Two different morphologies of nearly pure Ni particles, equiaxed and rod-like, form within a ceramic matrix, which is either α-Al 2 O 3 or a metastable ‘defect spinel’ depending on the choice of processing parameters, such as reduction temperature, oxygen partial pressure and time. Electron microscopic studies were performed for microstructural characterization, phase identification and chemical analysis. The fracture toughness of the NiAl 2 O 3 mixture was significantly improved with respect to that of the original spinal phase. The important issues that must be addressed to form useful metal-ceramic microstructures by partial reduction reactions are discussed. It is shown that cracking at the original spinel grain boundaries, probably due to the large volume change associated with the reduction reaction, can be avoided by the addition of small amounts of ZrO 2 . Initial results are presented illustrating the effect of ZrO 2 on the microstructures.

Mechanical properties and microstructural characterization of a superplastic TiAl alloy

Abstract: Superplastic deformation in a γ-TiAl which had a duplex microstructure, equiaxed primary γ and lamellar α2 + γ, was investigated with respect to the effect of temperature, strain rate, and environment. A maximum elongation of 540% was obtained at 1280 °C and a strain rate of 8 × 10−5 s−1 in argon. The specimens tested in argon yield substantially higher ductility than those tested in vacuum. An argon environment is also more favorable than vacuum in terms of desirable microstructure because of the retardation of aluminum depletion at the testing temperature. Microstructural examination revealed significant grain growth and cavitation in a Ti-rich layer near the surface. The α phase interspersed between the γ phase accommodated grain boundary sliding and retarded grain growth because of the chemical dissimilarity between the α and γ phases. Possible high temperature deformation mechanisms of this alloy are also discussed.

Application of steady magnetic field for refining solidification structure and enhancing mechanical properties of 25Cr-20Ni-Fe-C alloy in centrifugal casting

Abstract: Electromagnetic centrifugal casting technique based on magnetohydrodynamic principles has been applied to improve the solidification structure and mechanical properties of a 25Cr-20Ni-Fe-C alloy. The experimental results show that the solidification structure can be controlled by electromagnetic centrifugal casting. As the magnetic flux density increases, the solidification macrostructure changes from coarse columnar grains to a mixed structure consisting of fine curved columnar and equiaxed grains, and finally, into fine equiaxed grains. The columnar-equiaxed transition is achieved through the fracture of growing dendrites, and the size of equiaxed grains is also affected by the magnetic flux density. The alloy with equiaxed grain structure produced by electromagnetic centrifugal casting shows both better tensile strength at room and higher temperatures, and longer stress-rupture life compared with the same alloy with columnar grain structure produced by conventional centrifugal casting under the same stress.

Morphology Evolution of Zinc‐Nickel Binary Alloys Electrodeposited with Pulse Current

Abstract: Zinc-nickel electrodeposits have been widely adopted for surface treatment of automobile body steel sheet for high corrosion resistance. This paper describes the morphology evolution of pulsed electrodeposited zinc-nickel binary alloys. With an increase in pulse current off-time, morphologies change from triangular-pyramidal-shaped crystals which are hexagonal columnar crystals to pyramidal-shaped crystals which are hexagonal columnar crystals with two independent c axes and then to pyramidal shaped crystals which are the hexagonal columnar crystals having granular crystals on their (10 {center_dot} 0){eta}.

In situ formation of metal-ceramic microstructures in the Ni-Al-O system by partial reduction reactions

Abstract: Metal-ceramic microstructures were obtained by partial reduction of the spinel compound NiAl2O4. Electron microscopy studies were performed for microstructural characterization, phase identification and chemical analysis. Depending on the reduction temperature, two different morphologies of nearly pure Ni particles, equiaxed and rod-like, form within a ceramic matrix. The equiaxed Ni particles (0.02–0.5 μm in diameter) are embedded in α-Al2O3, while the rod-like Ni particles (∼ 5 μm in length and ∼ 0.1 μm in diameter) are in a metastable “defect spinel” containing much less Ni than NiAl2O4. The fracture toughness of the NiAl2O3 mixture was significantly improved with respect to that of the original spinel phase.

Microstructure and properties of Ti-48Al-2Cr after thermomechanical treatment

Abstract: Ingots of Ti-48Al-2Cr exhibit a strong casting texture with chemical as well as structural inhomogeneities resulting in low ductility and anisotropic formability. To improve these properties, a thermomechanical treatment is required. A two-pass upset forging method to achieve the required fine-grained equiaxed microstructure is presented. The possibilities of influencing the grain size and the corresponding mechanical properties by variation of the time and temperature in the final heat treatment are shown. The yield stress at room temperature, as given by the Hall-Petch relationship, can be controlled in the range between 360 and 495 MPa by producing grain sizes between 30 μm and 2.7 μm respectively. Tensile and compression tests were performed as a function of temperature. Typical characteristics of this material are the negative strain rate sensitivity between 200 °C and 500 °C with corresponding strain aging effects.