Discontinuous Grain Research Articles

A Generalized Finite Element Method for polycrystals with discontinuous grain boundaries

AbstractWe present a Generalized Finite Element Method for the analysis of polycrystals with explicit treatment of grain boundaries. Grain boundaries and junctions, understood as loci of possible displacement discontinuity, are inserted into finite elements by exploiting the partition of unity property of finite element shape functions. Consequently, the finite element mesh does not need to conform to the polycrystal topology. The formulation is outlined and a numerical example is presented to demonstrate the potential and accuracy of the approach. The proposed methodology can also be used for branched and intersecting cohesive cracks, and comparisons are made to a related approach (Int. J. Numer. Meth. Engng. 2000; 48:1741). Copyright © 2006 John Wiley & Sons, Ltd.

Recovery of deformation substructure and coarsening of particles on annealing severely plastically deformed Al–Mg–Si alloy and analysis of strengthening mechanisms

An Al–Mg–Si alloy was annealed to various solutionized and aged states and was then severely plastically deformed by equal channel angular pressing (ECAP). These materials were subsequently annealed for a range of times and temperatures to induce precipitation, dislocation recovery, and grain growth, with changes of mechanical behavior followed by tensile testing. Precipitation of excess solute was seen to occur in all cases, independent of the initial heat treated state, but the particles present appear to play only a small role in stabilizing the deformation substructure, at least until significant particle and grain coarsening has occurred, when discontinuous grain coarsening can be provoked. The strength of materials is examined, and the respective contributions of loosely arranged dislocations, many grain boundaries, and dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.

On Structural Mechanism of Continuous Recrystallization in Ferritic Stainless Steel after Large Strain Processing

The annealing behaviour of an Fe – 22%Cr – 3%Ni ferritic stainless steel processed by bar rolling/swaging to total strain of 4.4 at an ambient temperature was studied in the temperature range of 400 ~ 700oC. The annealing behaviour was characterised by the development of continuous recrystallization involving recovery processes followed by a normal grain growth. The large strain deformation caused the very fast recovery resulting in the development of almost equiaxed polygonized microstructure in place of the highly elongated deformation (sub)grains. The polygonization development was accompanied by some increase in the transverse (sub)grain size and the formation of many low-angle subboundaries. The latter ones could be composed from the dislocations, which were emitted by the strain-induced deformation (sub)boundaries. In spite of relatively large fraction of low-angle subboundaries, such polygonized microstructure was essentially stable against a discontinuous grain coarsening. Upon further annealing, therefore, the microstructure evolution was considered as a normal grain growth.

The temperature ranges for maximum effectiveness of grain growth inhibitors in WC–Co alloys

In order to more effectively control the WC grain growth during liquid phase sintering of submicron WC–Co alloys, the temperature ranges where common grain growth inhibitors are most active have been determined. In the study, small additions (0.5% by weight) of TiC, TaC, NbC, VC, and Cr 3C 2 were added to WC–6Co and WC–10Co alloys. In some cases, a small amount of coarse WC powder was added to simulate the effect of discontinuous grain growth. The powder compacts were vacuum sintered at temperatures ranging from 1300 to 1575 °C. The magnetic coercivity and microstructure results indicated that VC is equally effective in limiting continuous grain growth for all temperatures within the temperature range while the other inhibitors become more effective with temperature. VC was also found to be the most effective inhibitor of discontinuous grain growth. The results of the investigation are expressed in terms of the effectiveness of grain growth inhibition as determined by microstructural analysis, magnetic coercivity and hardness.

Failure analysis of GCr15SiMn steel bearing sleeve

This paper presents a failure analysis of a bearing sleeve made from GCr15 steel. The bearing sleeve fractured, when being interference fitted. The fracture surfaces were observed by visual and SEM. The crack initiated from the top end of the bearing sleeve and propagated towards the bottom. Intergranular brittle fracture is the main failure mechanism. Detailed investigation of the microstructure indicates needle-like bainite, with lamellar troostite occuring at grain boundaries as a discontinuous network. A banded structure was observed on the bearing sleeve material, but Charpy impact tests of samples taken from different orientations of the failed sleeve show that the failure of the bearing sleeve has nothing to do with the banded structure. The discontinuous network structure lowers the strength of the grain boundary. Insufficient quenching rate and low quenching temperature are responsible for the formation of the discontinuous grain boundary network.

The effect of homogenization on the recrystallization of an Al-Mn-Mg alloy sheet

The present study examines the effect of homogenization of the roll-cast material on the microstructure and ductility of an Al-Mn-Mg alloy sheet. The phenomenon ofrecrystallization and the effect of precipitation and second phase particles have beeninvestigated in cold rolled sheets by means of differential scanning calorimetry (DSC),electrical conductivity measurement and image analysis. It appears that recrystallizationcharacteristics in different homogenization conditions are important parameters thatcontrol the grain sizes and consequently the ductility of the homogenized sheets. It isbelieved that homogenization promotes normal discontinuous recrystallization andsubsequent grain coarsening mainly due to the elimination of precipitation interaction andthe coarsening of the second phase particles.

Continuous and discontinuous grain coarsening in a fine-grained particle-containing Al–Sc alloy

An Al–0.2 wt% Sc alloy was solution treated, deformed by equal channel angular pressing (ECAP) to an effective true strain of 9.2 then aged for 3 h at 350 °C to produce a fine-grained (0.8 μm diameter) microstructure containing a large fraction (∼0.7) of high angle grain boundaries (HAGBs). This ageing treatment also generated a relatively uniform dispersion of 5 nm diameter Al 3Sc particles. Grain stability was investigated at temperatures up to 550 °C using SEM, EBSD and TEM. It was found that the fine-grain structure was remarkably stable at temperatures up to 500 °C with grain coarsening occurring gradually with no marked change in the grain size distribution, texture and grain boundary character. This homogeneous coarsening behaviour is usually termed continuous recrystallization. In this regime, both the fine-grained microstructure and Al 3Sc particles exhibit third order coarsening kinetics with d R/d t ∝ d r/d t which indicates that grain coarsening is controlled by the rate of particle growth with the latter controlled by bulk diffusion of scandium in the Al matrix. During extended annealing at 500 °C and for short times at higher temperatures, there is a notable transition from continuous to discontinuous grain coarsening whereby a small number of grains grow rapidly to produce a coarse (>10 μm) grain size. An analytical mean field model of grain coarsening in particle-containing alloys was shown to adequately predict this transition in coarsening behaviour.

Dynamic restoration mechanisms in α-zirconium at elevated temperatures

The creep behavior of α-zirconium at high temperatures is not understood. Recently, steady-state stress exponents between 5 and 7 have been suggested over a range of elevated temperatures, indicating the predominance of dislocation climb (dynamic recovery) as the restoration mechanism. However, the activation energies are significantly higher than those of self-diffusion of pure Zr, as expected from climb-controlled mechanisms. This discrepancy and the observations of increased high-angle grain boundary area with straining have been attributed to the possible occurrence of discontinuous recrystallization and/or grain growth as additional restoration mechanisms. Tension, torsion and creep tests to small and large strains were performed at temperatures from 400 to 800 °C. The microstructure of the deformed samples was characterized by optical microscopy, transmission electron microscopy, as well as texture analysis using X-ray and electron backscatter diffraction. Dynamic recovery through dislocation climb appears to be the prevailing restoration mechanism. The increase in high angle boundary area with larger strains is a consequence of geometric dynamic recrystallization.

Mechanism for Developing the Boundary Barrier Layers of CaCu3Ti4O12

CaCu3Ti4O12 (CCTO) was prepared by solid‐state reaction and identified by X‐ray diffractometry. The evolution of the microstructure was observed by scanning electron microscopy (SEM). It was found that discontinuous grain growth developed during sintering, and large abnormal grains played a very significant role in the dielectric behavior. Cu ions segregated to the boundaries and CuO located at the triple‐point sites of the abnormal large grains were observed by electron energy loss and energy‐dispersive X‐ray spectroscopy, respectively. Moreover, two kinds of morphologies, i.e., terraces with ledges and bump domains, were discovered inside the grains under the thermal etching conditions at 960°C for 72 min, which solves the contradiction related to the interpretation of the giant dielectric response between ceramic and single‐crystal CCTO. Complex impedance spectroscopy was used to analyze the conductivity of ceramic CCTO, which suggests that it consists of conducting domains with two kinds of insulating barrier layers of domain and grain boundaries. The insulating domain and grain boundaries were attributed to orderly arranged dislocations and segregation of Cu ions, respectively. The conduction of CCTO was found to be related to the porosity, the grain size, and the thickness of the insulating boundary layers. For a sample sintered at 1065°C for 3 h, the anomalously low resistivity and temperature‐dependent dielectric constant were due to the fact that domain boundaries were not substantially formed. The possible reasons for the development of barrier layers and the variation of the dielectric constant with the sintering time are discussed. A barrier‐layer model with dielectric response based on the Maxwell–Wagner type of relaxation for ceramic CCTO is proposed. Two kinds of dielectric responses occur, depending on the microstructure: they are dominated by the domain and domain boundary and by the grain and grain boundary for large grains and fine grains, respectively.

Effect of Nano-Sized Oxides on Annealing Behaviour of Ultrafine Grained Steels

The annealing behaviour of ultrafine grained steels containing nano-scale dispersed oxides was studied in a temperature range of 600– 900� C by means of microstructural observations and hardness measurement. The starting materials with submicrocrystalline structures were developed by mechanical milling of Fe-Fe3O4 powders followed by consolidating bar rolling at 700 � C. Depending on the initial oxygen content and the mechanical milling time, the fraction of dispersed oxides varied from 0.3 to 3.0 vol%. During the heating up to 800� C( i.e. within the ferrite region), the initial ultrafine grained microstructures were essentially stable against any discontinuous grain growth. The grain coarsening and the softening can be roughly expressed by power-law functions of annealing time. The main mechanism of microstructure evolution that operated during annealing is considered as a normal grain growth accompanied by recovery. The grain coarsening is characterized by a rather high value of the grain-growth exponent of about 20. The grain growth kinetics correlates with the oxide coarsening. The effect of dispersed oxides on the annealing behaviour of submicrocrystalline oxide bearing steels is discussed in some detail.

Annealing behavior of submicrocrystalline oxide-bearing iron produced by mechanical alloying

The structural changes upon annealing of an ultrafine-grained iron containing dispersed oxides were studied. The starting material was subjected to mechanical milling followed by consolidating bar rolling. The fine-grained microstructures were essentially stable against discontinuous grain growth and/or primary recrystallization during annealing at temperatures up to 800 °C, where the specimens maintain a very fine ferrite grain size. The annealing behavior is mainly characterized by normal grain growth accompanied by recovery. The grain-growth kinetics correlates with the oxide coarsening. Both the grain growth and the oxide coarsening can be enhanced by an increase in the temperature. The static restoration/recrystallization processes operating in the ultrafine-grained matrices, as well as the effect of dispersed oxides particles, are discussed in some detail.

Effect of crystallographic texture on the isothermal beta grain-growth kinetics of Ti–6Al–4V

The effect of crystallographic texture on the kinetics of grain growth during isothermal beta annealing of Ti–6Al–4V was established. For this purpose, samples were subjected to a thermomechanical process to produce material with a uniform, fine equiaxed-alpha microstructure with either a strong or weak (110) beta-phase texture. Grain growth measurements revealed that the classical isothermal grain-growth relation was incapable of describing the observations. Specifically, the grain-growth exponent n was found to have a strong dependence on temperature. In addition, the phenomenon of discontinuous grain growth, or a stagnation in the growth behavior at short times, was documented and ascribed to changes in texture during the grain-growth process.

Effect of atmospheres on the sintering behavior and microstructural evolution of (Ba 0.87Ca 0.13) 1.000(Ti 0.85Zr 0.14Mn 0.01) 1.003O 3

The powder with a composition of (Ba 0.87Ca 0.13) 1.000(Ti 0.85Zr 0.14Mn 0.01) 1.003O 3 sintered in different atmospheres was investigated. It was found that the densification occurred at lower temperature and discontinuous grain growth was inhibited but pores significantly grew when sintered in a reducing atmosphere. It is suggested that the inhibition of the discontinuous grain growth be attributed to the segregation of Mn 2+ ions to the grain boundary and the absence of the Ti-rich liquid phase. It was also found that grain growth became significant at temperatures >1300 °C when sintered in the reducing atmosphere, which might be due to the increase of the oxygen vacancy concentration. Residual carbon played a significant role in pore growth.

Microstructural changes in sintered Al 2O 3 by acid treatment of compacts produced by slip casting in gypsum molds

Slip casting in gypsum molds is one of the important forming methods industrially. Sintering compacts which were produced by slip casting using gypsum molds showed abnormal grain growth. The grain size of the compacts sintered at 1350 °C was in the range of 0.2–2 μm. The grain size of the compacts sintered at 1450 and 1550 °C were in excess of 2 μm and the sintered compacts showed discontinuous grain growth. Calcined bodies produced by slip casting using gypsum molds were washed with acid to remove the contamination from the bodies. The grains of these sintered compacts grew normally. The mean grain sizes were about 0.8, 5 and 8 μm respectively after sintering at 1350, 1450, and 1550 °C. The microstructure became more homogeneous. The relative densities of the sintered compacts were higher than those of the sinteried compacts. Grain-growth activation energy was calculated from experimental data. The activation energy was 4.8×10 3 J/mol.

Conformal Platinum Thin Films Prepared by Chemical Vapor Deposition under High Oxygen Partial Pressure

Conformal deposition of platinum thin films by chemical vapor deposition using (methylcyclopentadienyl)trimethylplatinum as a liquid precursor, whose vapor was transferred by an argon carrier gas, is presented. Conformal coverages at thicknesses of less than 30 nm were obtained under the condition of high oxygen partial pressures and low temperatures. The surface morphology roughened and changed to a discontinuous grain growth when the film thickness and growth temperature were increased. The dense and smooth films with conformal coverage can be used in the storage nodes of capacitors in gigabit dynamic random access memories. © 2001 The Electrochemical Society. All rights reserved.

Densification behaviour, microstructure development and dielectric properties of pure BaTiO 3 prepared by thermal decomposition of (Ba,Ti)-citrate polyester resins

The thermal decomposition of (Ba,Ti)-citrate polyester resin method was used to prepare high purity BaTiO 3 powders. After milling, the green compacts’ uniformity was studied using both the SEM observations on surfaces of the fractured green compacts, and the pore size distribution results obtained by using Hg-porosimmetry. The densification behaviour was found to be closely related to the pore distribution in the powder compacts and, thus, the different maxima in the pore size distribution were assumed to be connected to the maxima in the densification rate. The sintering behaviour of BaTiO 3 compacts was evaluated using both non-isothermal and isothermal experiments. Microstructural development during sintering at different heating rates was in accordance with the nonuniformity of the green compacts microstructure. Thus, denser areas could favour the development of twinned grains and the discontinuous grain growth process in the BaTiO 3 ceramics. Dielectric constants below the Curie temperature were evaluated according to both densification level and grain size. Although a high density, (⩾99% theoretical) and fine grained, (∼1.01 μm) microstructure of BaTiO 3 could be produced by the used method and sintering in air, in which a high dielectric constant, (⩾5000 at room temperature and 1 kHz) and a dissipation factor as low as 3% was achieved, however, dielectric constant degradation up to about 2500 with increasing grain size, and some fluctuations in the Curie temperature were observed. It was assumed to be a consequence of the inhomogeneous microstructure and the stresses generated through the cubic-tetragonal transition on cooling.

A sintering study on the β-spodumene-based glass ceramics prepared from gel-derived precursor powders with LiF additive

Beta-spodumene (Li2O·Al2O3·4SiO2, LAS) powders were prepared by a sol-gel process using Si(OC2H5)4, Al(OC4H9)3, and LiNO3 as precursors and LiF as a sintering aid agent. Dilatometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED) were utilized to study the sintering, phase transformation, microstructure, and properties of the β-spodumene glass-ceramics prepared from the gel-derived precursor powders with and without LiF additives. For the LAS precursor powders containing no LiF, the only crystalline phase obtained was β-spodumene. For the pellets containing less than 4 wt pct LiF and sintered at 1050 °C for 5 hours the crystalline phases were β-spodumene and β-eucryptite (Li2O·Al2O3·2SiO2). When the LiF content was 5 wt pct and the sintering process was carried out at 1050 °C for 5 hours, the crystalline phases were β-spodumene, β-eucryptite (triclinic), and eucryptite (rhombohedral (hex.)) phases. With the LiF additive increased from 0.5 to 4 wt pct and sintering at 1050 °C for 5 hours, the open porosity of the sintered bodies decrease from 30 to 2.1 pct. The grains size is about to 4 to 5 µm when pellect LAS compact contains LiF 3 wt pct as sintered at 1050 °C for 5 hours. The grains size grew to 8 to 25 µm with a remarkable discontinuous grain growth for pellet LAS compact contain LiF 5 wt pct sintered at 1050 °C for 5 hours. Relative densities greater than 90 pct could be obtained for the LAS precursor powders with LiF > 2 wt pct when sintered at 1050 °C for 5 hours. The coefficient of thermal expansion of the sintered bodies decreased from 8.3 × 10−7 to 5.2 × 10−7/°C (25 °C to 900 °C) as the LiF addition increased from 0 to 5 wt pct.

WC grain growth and grain growth inhibition in nickel and iron binder hardmetals

WC grain growth and growth inhibition of an 0.6 μm FSSS WC powder (average SEM size: 0.35 μm) were studied in WC–10 wt% Ni alloys by adding 0–2 wt% of inhibitor carbides (VC, Cr 3C 2, TaC, TiC and ZrC). Alloy gross carbon content turned out to be a crucial factor for WC growth in Ni alloys, even with high inhibitor additions. Coarsening was more pronounced in high carbon alloys, compared with low carbon grades, resulting in a significantly lower hardness. VC proved to be by far the most effective grain growth inhibitor in WC–Ni hardmetals, followed by TaC, Cr 3C 2, TiC and ZrC. Hardness increased with increasing amount of additive but reached a maximum above which it remained about the same. Experiments on WC–Fe–(VC) alloys revealed that WC grain growth is strongly restricted in Fe-binder alloys, even without additions of growth inhibitors. Binder chemistry thus strongly influences both continuous and discontinuous WC grain growth. This chemistry is determined by the nature of the binder matrix (Fe, Co, Ni), the alloy gross carbon content (which determines the composition of the binder matrix) as well as the inhibitor additive.

Computational study on grain growth in cemented carbides

Micro-grained cemented carbides are widely applied to the tools such as solid endmills and micro-drills. The knowledge on grain growth of carbides is very important for manufacturing micro-grained cemented carbides. In the present study, continuous and discontinuous grain growth in such cemented carbides is investigated using the Monte Carlo computer simulation technique. The Ostwald ripening process (solution/re-precipitation) and the grain boundary migration process are assumed in the simulation as grain growth mechanism. The effects of liquid phase fraction, grain boundary energy and an implanted coarse grain are examined. The results of these simulations qualitatively agree with experimental ones and suggest that distribution of liquid phase and WC/WC grain boundary energy as well as contamination by coarse grain are important factors controlling discontinuous grain growth in cemented carbides.

Fine and ultrafine hardmetals at Plansee 2001

An attempt has been made to follow the scientific and technological steps associated with the production of fine grained hardmetal. This task has not been easy because of the restrictions placed by commercial—and in some cases research organisations—in not publicising relevant information until long after the date of the initial work. Thus, placing data in an historical, chronological order has been somewhat difficult. However, it has been possible to establish that as far back as 1929, approximately 6 years after the first patent was granted, Krupp/Osram workers had identified the positive aspects of tungsten carbide grain refinement. By 1939 they had also discovered the beneficial effects of adding a small amount of vanadium and tantalum carbide. This effectively controlled discontinuous grain growth.What was considered ‘fine’ in one decade was considered not so fine in the next. Thus a grain size in the range 0.5–3.0 μm was considered fine in the early years but now we have entered, in 1993, the era of the nano-crystalline material with a grain size of 20–50 nm. Have we now reached the end of the story?It is hoped that the chronological data given in this paper will be of interest to those engaged in the hardmetal industry, whether in the past or present.