Distribution Of Second Phase Particles Research Articles

Directional cavity stringer formation in a superplastic 7075 Al alloy

Abstract In the present investigation, directional cavity stringer formation in the fine-grained superplastic 7075 Al alloy, which has the different grain sizes and second phase particle distributions, were examined. Rolling reduction ratios 70 and 90%, were applied in the course of the thermomechanical treatment to produce different initial microstructures. Tensile tests were conducted at 789 K at strain rates from 2×10−4 s−1 to 1.33×10−2 s−1. The alloy, with a rolling ratio of 70%, exhibited a relatively large grain size of 12 μm and the stringer type of distribution of second phase particles along the rolling direction, while that with the rolling ratio of 90% showed a relatively small grain size of 8 μm and a uniform distribution of second phase particles. In the former, cavity stringers were always parallel to the rolling direction, irrespective of the testing direction. In contrast, in the latter, the cavity stringers were always aligned along the tensile axis regardless of the rolling direction and the cavity size was smaller than that observed in the former. Based on the experimental observation, the mechanisms of the cavity stringer formation in the alloy were explored in terms of the microstructural evolution during the thermomechanical treatment and the superplastic deformation.

Effect of alloy preheating on the mechanical properties of as-cast Co-Cr-Mo-C alloys

The effect of various alloy preheatings followed by full solid solution treatments on the resultant strength and ductility of as-cast Co-Cr-Mo-C alloys was investigated. Three preheating temperatures were evaluated: 815 °C, 950 °C, and 1100 °C for 4 hours and then solid solution treated at 1225 °C for 4 hours. Tensile and compressive tests were carried out on the heat-treated alloys. It was found that the strength and ductility of the heat-treated alloys exhibited significant improvements over the as-cast condition. In particular, optimum ductility of the heat-treated alloys and alloy strength were promoted by preheating at 815 °C. A relatively fine grained structure coupled with a uniform distribution of second-phase particles promoted homogeneous plastic deformation in the bulk. Fractographic observations indicated that the exhibited ductility was associated with the development of numerous plastic bands combined with band interlockings. Alloy preheats at 950 °C and 1100 °C prior to solutionizing lead to inferior strength and ductility. Although preheating at 1100 °C led to slight improvements, in both cases, the fracture path was dominated by the presence of continuous carbide films surrounding the dendritic grains. Hence, less than optimum combinations of strength and ductility were achieved by the heat treatments at the higher temperatures.

A model for the initiation of dynamic recrystallization in two-phase materials

Abstract A material model is presented for the prediction of the initiation of dynamic recrystallization in two-phase materials in which the second phase does not deform. Initiation is based on the criterion of Poliak and Jonas in which both energy and kinetic conditions are considered. The criterion is generalized for multiaxial stress states, and coupled with elastic-viscoplastic constitutive equations for high-temperature rate-sensitive materials undergoing strain hardening, dynamic recovery and strain softening due to dynamic recrystallization. The model is embodied within a finite-deformation finiteelement code. Micromechanical finite-element cells are then developed to model two-phase materials, and the dependence of the initiation of dynamic recrystallization on the second-phase volume fraction, shape and distribution is investigated. Stress-state dependence is also examined. Increasing volume fraction of the second phase is found to lead to considerably enhanced hardening at low strains but. at higher strains, dynamic softening reduces the steady-state flow stress approximately to that of the corresponding single-phase material. The steady-state flow stress is predicted to be largely independent of second-phase volume fraction. This is in agreement with experiments. The model correctly predicts the experimental trend of earlier initiation of dynamic recrystallization with increasing second-phase volume fraction, together with the sites of initiation in two-phase materials. Particle-matrix interfaces can act as preferential sites for initiation but, in a material with a randomly distributed second phase, the initiation is favoured in areas of particle clustering and can occur close to, but not necessarily at, particle-matrix interfaces. It is shown that the model predictions are entirely consistent with Avrami kinetics. For a uniform distribution of second-phase particles in a two-phase material, an Avrami exponent of 3.29 is predicted. For a random distribution, the Avrami exponent is predicted to be 3.01. Particle shape was found to influence the initiation of dynamic recrystallization. In addition, initiation was found to be delayed as the macroscopic-level stress state changes from one of pure shear to uniaxial stress and to pure hydrostatic pressure.

WidmanstÄtten Type Solidification in Squeeze Casting of Mg-Li-Al Alloys

The addition of 4--16 wt% Li to Mg lowers the density from 1.7 g/cm{sup 3} to 1.3--1.6 g/cm{sup 3}. Further this results in the h.c.p. {alpha} phase being replaced by the b.c.c. {beta} phase, leading to considerable low temperature formability. Detailed examination of the Mg-Li binary phase diagram shows that the {alpha} phase can precipitate in the {beta} phase matrix due to the decrease of Mg solubility in the {beta} phase as the temperature decreases. The addition of Al to Mg-Li alloys can improve strength by forming a uniform distribution of second phase particles. From the results of the previous study, Mg-8--10 wt% Li-3 {approximately} 5 wt% Al alloys consist of a mixture of {alpha} and {beta} grains, and fine second phase particles, showing the optimum combination of mechanical properties. Main emphasis in the present study was placed on investigating the effects of Widmanstaetten type solidification on the strengthening behavior in squeeze cast Mg-8 wt% Li-3 wt% Al and Mg-10 wt%-3 wt% Al alloys.

Study of the diffusion of Al–Li alloys subjected to an electric field

An experimental investigation of the homogenization treatment of 2091 Al–Li alloy in the presence of an electric field, has revealed the phenomena of reduced volume fraction, small size, spherical shape and random distribution of second-phase particles, which bring about an increase in ductility. The results show that the dissolution of second-phase particles is promoted by means of the vacancy mechanism, because of the larger diffusion coefficient of solute atoms than the vacancy–solute complexes at the beginning of the homogenization treatment. By increasing the homogenization time and applying an electric field, the diffusion coefficient of vacancy-solute complexes is raised, whereas that of solute atoms is reduced, because of the decreased potential energy of the second phase at grain boundaries. Therefore, the non-equilibrium segregation of magnesium and copper elements is generated near the surface of the ingot by a complex mechanism. An experimental study of the solution treatment under an electric field, revealed that the lithium non-equilibrium segregation is induced at grain boundaries responsible for the δ precipitates. © 1998 Chapman & Hall

Crack propagation behavior of A356 aluminum alloy under resonant vibration

During the application of a structural component, failure by resonant vibration may occur under some abnormal situations. The study of this failure is necessary in order to ensure the reliability of the component. In the current investigation, A356 aluminum alloy (Al-7wt% Si-0.3wt% Mg) was chosen to explore the crack propagation behavior under resonant vibration. This alloy, which contains eutectic silicon particles, has been used as many of the structural components in automobiles and aircrafts. Since the shape and distribution of second phase particles can affect the fatigue crack propagation of a two-phase material, they should also affect the crack propagation under resonant vibration. To study the effect of eutectic silicon morphology and distribution, the A356 alloy was tested under the as-cast condition and two solution-treated conditions.

Crack propagation under mixed mode (I+III) loading : Yates, J.R. and Mohammed, R.A. Prof. 4th Int. Conf. on Biaxial/Multiaxial Fatigue II, St. Germain en Laye, France, 31 May–3 June 1994, pp. 99–106

The blunt notch fracture toughness of four types of carbon-manganese steel (ASTM A516 grade 70) has been determined by J-integral tests on Charpy-V type samples with different values of notch root radius, ρ. J-ρ plots, determined using specimens with a notch depth to width ratio, a/w, equal to 0.5, have shown the existence of a limiting ρ value (ρeff) below which applied J-intergral values at fracture initiation are constant. These ρeff values have been seen to depend only on second-phase particle distribution and not on their volume fraction or on the steel ferritic grain size. The procedure for deriving J-integral values at the onset of stable crack growth from J resistance curves in the case of notches has also been discussed. Experiments with Charpy specimens with a/w = 0.2 do not allow the derivation of meaningful J-ρ plots. In all cases, a ductile fracture criterion based on the constancy of the notch tip strain at rupture initiation has been proved when ρ >ρeff.

Modeling Ductile Fracture of Al Alloys in Relation with Statistical Distribution of Second Phase Particles

Fracture toughness tests have been carried out on two Al alloys (7075 and 7475) containing different amounts of Fe and Si. These materials were tested in T3, T6 and T7 conditions, along six orientations (LT, LS, TL, TS, SL and ST). Large variations in the fracture toughness (20-60 MPa√m) were measured. Mechanical tests on notched specimens were also performed to determine the influence of stress triaxiality ratio on ductility. These specimens were calculated by finite element method. Quantitative metallography was largely used to measure the volume fraction of intermetallic particles (Mg 2 Si and Fe-rich precipitates) and the statistical distribution of these particles. The Gurson potential is used to interpret these test resulls carried out on volume elements and on cracked specimens. It is shown that the ductility of notched specimens and the measured variations in fracture toughness can be interpreted with the Gurson potential provided that the non homogeneous distribution of intermetallic particles is taken into account. A model has been developed to relate the toughness of the alloys to the field strength, the critical void growth rate calculated on notched specimen from Rice and Tracey equation and to the mean distance between the clusters of second phase particles.

Effects of alloy modification and thermomechanical processing on recrystallization of Al-Mg-Mn alloys

The 5083 Al alloy (Al-4.75Mg-0.8Mn) holds potential for superplastic forming (SPF), but slow rates of forming limit its use for many applications. Higher strain rates are believed possible through the development of finer grained microstructures or stabilized subgrain structures. Grain sizes after recrystallization and recrystallization characteristics are known to be dependent on the amount and distribution of second-phase particles in the matrix. In this study, the concentration and sizes of such particles were varied by additions of particle-forming elements of Mn and Zr and by modifications of the rolling and aging schedules (thermomechanical processing (TMP)). The investigation involved studying recrystallization kinetics at different temperatures and correlating the grain sizes with particle sizes and volume fractions. The addition of Mn and Zr, for the composition ranges and TMP methods studied, resulted in a substantial reduction of the recrystallization kinetics, but complete suppression of static recrystallization (or subgrain stabilization) was not observed. However, statically recrystallized grain sizes as small as 6 μm were achieved.

Mixed-mode fracture initiation in a ductile material with a dual population of second-phase particles

In this work, finite element simulations of mixed-mode (Modes I and II) fracture initiation in a ductile alloy with a dual distribution of second-phase particles are carried out under plane-strain, small-scale yielding conditions. The background material is represented by the Gurson constitutive model and strain controlled micro-void nucleation at uniformly distributed small-size particles is taken into account. A rectangular distribution of large synthetic inclusions around the notch tip is considered and a stress controlled void nucleation law is used within the framework of Gurson model to trigger void initiation at these particles. Attention is focused on the formation of porosity at these synthetic particles located around the notch tip, their growth and interaction with the notch tip. It is found that when the inclusions have low nucleation stress, the critical value of J at fracture initiation decreases for deviation from pure Mode I. However, when the particles have high nucleation stress, the critical value of J shows an increase for deviation from pure Mode I.

Influence of homogenization treatment in an electric field on the workability of 1420 Al-Li alloy during hot rolling

In A1-Li alloys after casting, a significant amount of dendritic segregation is present, which leads to collapse during bot rolling. Antes [1] shows that a dramatic improvement in ductility is achieved by eliminating dendritic segregation. The effect of undissolved second phase on ductility has been investigated by Singh and Flemings [2]. Their results show that the ductility during hot rolling depends on not only the volume fraction but also the shape and the distribution of second phase particles. For a given amount of second phase, spherical shape and random distribution of second phase particles are beneficial for the improvement of ductility. Purdy and Kirkaldy [3] have drawn attention to the advantages to be gained by homogenization at temperamre above the melting point of segregated alloys. However, for A1-Li alloys, high temperature homogenization treatment results in local overheating or local underheating and a heavy loss of Li. The homogenization process invented by David [4] is to heat slowly at rates not exceeding 50°C/h to temperature of at least 530°C up to a final homogenization. But, due to the difficulties in controlling temperature, the total content of Li and Mg is not permitted to exceed 6%. Deformation of segregated alloys promotes the homogenization of alloys [5]. However, for the as-cast A1-Li alloys, great risk of collapse exists during deformation. The heat treatment of 2091 AI-Li alloy with an electric field has been investigated [6, 7]. The results show a significant effect of the electric field on the diffusion and phase transformation of 2091 A1-Li alloy. In this letter, the effect of an electric field on the workability of 1420 A1-Li alloy is reported. 1420 A1-Li alloys were melted under an argon atmosphere. Table I gives the chemical compositions of the alloy. Ingots were treated either with or without an electric field at the same homogenization time (32 h) and temperature (470 °C). The effect of homogenization time (16 h, 24 h, 32 h) in an electric field on the volume fraction, shape and distribution of second-

Nonuniform distribution of second phase particles in melt-textured Y–Ba–Cu–O oxide with metal oxide (CeO2, SnO2, and ZrO2) addition

Segregation of second-phase particles within Y1Ba2Cu3O7−y domain was investigated in melt-textured Y-Ba-Cu-O with metal oxide (CeO2, SnO2, and ZrO2) addition. It is found that coarse particles (Y2Ba1Cu1O5) are trapped with a special pattern in the interior of Y1Ba2Cu3O7−y domain, while fine BaCeO3 and BaSnO3 particles are present within the remnant liquid-phase region. During the growth of Y1Ba2Cu3O7−y domain, fine particles appear to be pushed out of the advancing Y1Ba2Cu3O7−y /liquid interface toward the liquid phase. The particle segregation that occurred during peritectic growth of the Y1Ba2Cu3O7−y domain was explained in terms of the Uhlmann-Chalmers-Jackson theory based on the particle interaction at solid/liquid interface.

Characterization of Ni-25 wt.% Cr alloys containing reactive elements and the oxide scales grown on them at high temperatures using transmission electron microscopy

Nickel-base alloys of nominal composition Ni-25 Wt.% Cr-(0-0.6 Wt.%) RE (RE=Y, La, and Ce) were prepared by conventional arc-melting Ni, Cr, and Y metal. The microstructure of the alloys was characterized by using electron diffraction and X-ray energy dispersive spectroscopy (XEDS) to determine the structure, morphology, and distribution of second-phase particles. Selected alloys were oxidized at 900°C and 1000°C in 1-atm air, and the resultant oxide scales were characterized using the same analytical techniques. The experimentally determined electron-diffraction data were compared with a JCPDF-EDD database, and several compounds were matched. The observed phases were RE oxysulfide, cerium orthochromite-CeCrO3, yttria-Y2O3, yttrium orthochromite-YCrO3, and Ni5Y.

A geometrical description of particle distributions in materials

Describes a geometrical technique, based on the Dirichlet tessellation, for quantitative characterization of inhomogeneities in the spatial distribution of second-phase particles. The Dirichlet tessellation representation allows a detailed, statistical description of particle distribution properties, including local particle density and vector nearest-neighbor distances. Extension of the tessellation technique by specifying a distance over which second-phase particles are assumed to interact mechanically (particle interaction distance) facilitates quantitative characterization of particle clustering by allowing evaluation of characteristics such as the fraction of particles clustered, and the number, size, and spatial distribution of clusters. The tessellation and clustering characteristics of several types of computer-generated particle arrays (random, clustered, and hexagonal cellular) are presented. The effects of plane-strain deformation on the tessellation and clustering characteristics of the particle arrays have been examined. Tessellation and clustering characteristics of the random particle arrays are insensitive to plane-strain deformation.

高速変形特性と材料のミクロ組織

Mechanical properties at high strain rates were investigated by considering the specific strengthening mechanisms involving the effects of grain size, solid solution and second phase particles. Aluminum and its alloys were tested in tension or compression at strain-rates between 10-5 and 2000s-1 at room temperature. The conclusions were as follows:(1) Aluminum tested at high strain rates could not exhibit strengthening due to grain size refinement according to the well known Hall-Petch equation.(2) Solutes such as magnesium were able to increase the yield stress of aluminum at high strain rates. The yield stresses at high strain rates for Al-Mg alloys increased with magnesium addition, but the variation in strengthening rate with an additional content of magnesium was almost similar to that at low strain rates.(3) The treatment of strengthening by the second phase particles was clearly valid to improve the yield stress of Al-Mg-Si alloys at high strain rates. Basically, a decrease in the mean spacing between the second phase particles led the yield stress at high strain rates to be high. It was suggested that an optimum control in distribution of second phase particles was the most valid way for an improvement of the yield stress at high strain rates comparing with other strengthening methods.

Influence of Microstructural Characteristics on the Dynamic Properties of Aluminum Alloys. Influence of Distribution of Second-Phase Particles in Aluminum Alloys.

There are certain kinds of microstructural characteristics such as second-phase particles, solute atoms, grain size and grain orientation, which affect the impact strength of aluminum alloys. In this paper we discuss the influence of distribution of second-phase particles in various commericial Al-Mg-Si system alloys. Test alloys were prepared with five kinds of heat treatment : T4, under aging, T6, over aging and O temper. The deformation behavior of these alloys was investigated using a compression test at room temperature. Dynamic tests (4×102∼2×103s-1) were performed using a Hopkinson pressure bar system, and static tests (1×10-3∼1×10-ls-1) were performed with an Instron machine for reference. It has been revealed that strain rate sensitivity of flow stress varies with the interaction between second-phase particles and dislocations.

Model of ductile fracture initiation in spheroidized steel

Statistical analysis of the microstructure of spheroidized steel has been harnessed to evolve a new method for assessing the final stage of the ductile fracture process, the coalescence of voids. For a known particle size distribution and a known spatial distribution of second phase particles in the steel matrix, the Garrison and Thompson coalescence criterion is applied to determine the probability density of local fracture deformations, and the dependence of the probability of ductile fracture initiation upon the parameter expressing the distance ahead of the crack tip, x/S. This newly devised model of ductile fracture has been applied to a low-carbon niobium-microalloyed steel, containing 0.043% by weight of niobium; the results confirm that the probability of ductile fracture initiation ahead of a crack diminishes as the distance from the crack tip grows. As the coalescence of voids ahead of a crack tip governs the onset of stable crack propagation, the new model is expected to prove useful chiefly in the statistical prediction of upper bound of fracture toughness in engineering steels.

Microvoid formation during shear deformation of ultrahigh strength steels

Shear tests were performed on ultrahigh strength steels under both quasistatic and dynamic conditions, aimed at elucidating the fundamental mechanisms of shear localization underlying both adiabatic shear localization and fracture processes. Experiments were also devised to study the effect of hydrostatic pressure and austenitizing temperature on the critical strain to localization. Experimental evidence strongly suggests that strain localization in the steels investigated is driven by microvoid softening controlled by nucleation at 100 nm scale particles. This is supported by the observed pressure dependence of the instability strain, enhanced resistance to shear instability with particle dissolution, and direct observation of microvoids at these particles in deformed material. For the steels investigated with approximately equivalent strength levels, a direct correlation between the crack extension force and shear instability is demonstrated. Consequently, both fracture toughness and shear localization are dependent on the size, type, and distribution of second phase particles.

The microstructure of YBa2Cu3O7-x thin films on SrTiO3 substrate

Abstract The microstructure of high-temperature superconducting thin films (YBa2Cu3O7-x) deposited on SrTiO3 (100) substrates has been investigated. These films show a critical current density several orders of magnitude higher than previously reported for this class of materials. This was found by aligning the superconducting phase epitaxially with the substrate so that the direction of highest current density is parallel to the substrate (the c-axis of the orthorhombic structure perpendicular to the substrate) and by pinning the vortices with a fine distribution of second-phase particles. It is shown that, near the substrate/superconductor interface, the film is indeed mostly epitaxial. Lattice images of this epitaxial interface are presented. A small part of the film near the interface is misoriented so that its c-axis lies parallel to the substrate, with either a or b perpendicular to it. Numerous second-phase particles (Y2O3 and CuO) are distributed throughout the superconducting phase. Epitaxy deter...

The intracellular phase in rapidly solidified lloys based on the AlFe system

The nature of the second phase formed in rapidly solidified AlFe alloys has been determined. For those alloys studied exhibiting micro-cellular morphologies, the second phase is T′. The alloy Al-6.8Fe-2Mo-1.1V, in the melt-spun condition, exhibits a different microstructure, consisting of a uniform and extremely fine distribution of second phase particles. These particles possess the icosahedral structure, identical to that observed in rapidly solidified AlMn alloys. The mechanism of formation of these second phase particles is thought to involve the formation of an amorphous material following the nucleation of primary α-Al; this amorphous material is thought to be crystallized as a result of recalescence.