Formation Of Grain Structure Research Articles

Mechanisms of Formation of Submicron Grain Structures by Severe Deformation

Keynote Presentation in Int. Symposium on Fundamentals of Deformation and Annealing, Manchester, September, 2006.The grain refinement mechanisms operating during severe deformation processes, like ECAE, are discussed using data obtained from model Al-alloys. Refinement occurs predominantly by orientation splitting, micro and macroshear banding, and the geometric requirement for high angle boundary area to increase with strain. The deformation structure evolution is affected strongly by the processing route, die geometry, and material parameters. Both cell bands and microshear bands show strong alignment with the dies shear plane. Shear bands are promoted by changes in the shear plane orientation each extrusion cycle. At high strains a steady state is approached, where the grain size converges with the subgrain size, controlled by dynamic boundary migration.

Toughening of a 1500 MPa class steel through formation of an ultrafine fibrous grain structure

A deformation of tempered martensite, namely tempforming at an elevated temperature, was applied to a medium-carbon low-alloy steel. This thermomechanical treatment led to an evolution of an ultrafine fibrous grain structure with a strong 〈1 1 0〉∥RD fiber deformation texture and to a remarkable improvement in Charpy impact strength (uE = 165 J) at a tensile strength of 1.5 GPa.

About computation of homogenization regimes for aluminum alloys

The basic experimentally determined laws of the appearance of dendritic segregation and formation of grain structure in an ingot, the mechanism of homogenizing annealing that promotes dissolution of inclusions of nonequilibrium phases, various approaches to computing homogenization regimes, the conditions for choosing the homogenization temperature and reasons for using stage homogenization are briefly considered.

Effect of ultrasonic surface treatment of steel 40Kh13 on the microstructure of nitrided layer formed by high-intensity low-energy implantation with nitrogen ions

X-ray diffraction and transmission electron microscopy were used to study the microstructure and phase composition of surface layers of steel 40Kh13 subjected to irradiation with high-intensity low-energy ion beams, ultrasonic surface modification, and combined treatment including ultrasonic surface modification and ion implantation. It was found that the ultrasonic modification of steel surface leads to changes in the structure of tempered martensite and the formation of grain structure with a grain size of 0.3 μm and nanosized special carbides Cr23C6 in the martensite lamellae. The ion implantation into this steel results in the formation of a nitrided layer consisting of a nitride region, which represents a mixture of several phases (α-, γ′-, ɛ, and ultrafine chromium nitrides), and a zone of internal nitriding (α″ and nitrogen-containing martensite αN). The preliminary ultrasonic modification causes an increase in the nanohardness and in the thickness of the nitrided layer, which is due to the more intense penetration of nitrogen atoms into the surface layer and an increase in the volume fraction of iron nitrides and density of ultrafine chromium nitrides in this layer.

Depth profiling using C 60+ SIMS—Deposition and topography development during bombardment of silicon

A C 60 + primary ion source has been coupled to an ion microscope secondary ion mass spectrometry (SIMS) instrument to examine sputtering of silicon with an emphasis on possible application of C 60 + depth profiling for high depth resolution SIMS analysis of silicon semiconductor materials. Unexpectedly, C 60 + SIMS depth profiling of silicon was found to be complicated by the deposition of an amorphous carbon layer which buries the silicon substrate. Sputtering of the silicon was observed only at the highest accessible beam energies (14.5 keV impact) or by using oxygen backfilling. C 60 + SIMS depth profiling of As delta-doped test samples at 14.5 keV demonstrated a substantial (factor of 5) degradation in depth resolution compared to Cs + SIMS depth profiling. This degradation is thought to result from the formation of an unusual platelet-like grain structure on the SIMS crater bottoms. Other unusual topographical features were also observed on silicon substrates after high primary ion dose C 60 + bombardment.

Fine-Grained Structure Formation in 7475 Al Alloy during Hot Multidirectional Forging

Effect of strain rate on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490oC under strain rates of 3 × 10-4 s-1 and 3 × 10-2 s-1. At a strain rate of 3 × 10-4 s-1, the stress – strain ( σ - ε) behavior shows significant work softening just after yielding and a steady-state flow at higher strains. The structural changes are characterized by development of deformation bands at early stages of deformation, followed by formation of a fine grain structure in high strain in the whole material. The volume fraction of new grains increases with strain and approaches a value of about 0.85 over a strain of 3. At a higher strain rate of 3 × 10-2 s-1, in contrast, a steady-state flow following small flow softening appears at a relatively low strain. New grains are formed during steady state flow along original grain boundaries and the volume fraction reaches below 0.2 even in high strain. The occurrence conditions and the mechanisms of grain refinement are discussed in detail.

On the origin of the lattice constant anomaly in nanocrystalline ceria

The lattice parameter of nanocrystalline ceria films prepared by sputtering was monitored as a function of annealing temperature. Within the temperature range of 150-420 degrees C, an equilibrium with atmospheric oxygen is established within a few hours, whereas grain growth does not occur. On the basis of the experimental results and analysis of literature data, we present a model that posits the formation of a non-uniform grain structure with stoichiometric interiors and oxygen deficient boundaries. This model, based on defect thermodynamics, correctly describes the dependence of the lattice parameter of nanocrystalline ceria on annealing temperature and grain size and can be extended to other materials as well.

Effect of Initial Grain Size, Die Angle and Pass Sequence on the Formation of Ultrafine Grain Structure in Cu by ECAP

Effect of Initial Grain Size, Die Angle and Pass Sequence on the Formation of Ultrafine Grain Structure in Cu by ECAP

Microstructural Evolution during Friction Stir Welding of Ultrafine Grained Al Alloys

Recently, several metallic materials with ultrafine-grained structures and characterized by high strength and toughness have been developed. When these ultrafine-grained materials are practically used, welding and joining processes are required. However, conventional fusion welding processes result in deterioration of the good mechanical properties of these ultrafine-grained materials due to the drastic grain growth of the ultrafine grains. On the other hand, friction stir welding (FSW) is a solid-state joining process having lower heat-input than fusion welding processes, enabling formation of a fine grain structure in the stir zone. Thus, this process would effectively alleviate deterioration of mechanical properties of the ultrafine-grained materials. The authors applied FSW to ultrafine-grained Al alloys and then examined the microstructural features associated with hardness in the friction stir welds. The present paper reviews microstructural evolution of ultrafine-grained Al alloys, produced by equal channel angular pressing (ECAP) and accumulative roll-bonding (ARB), during FSW.

Formation of Grain Structures of Thermal Sprayed Nickel Coatings and the Coating Properties

Pure nickel powder was low pressure plasma sprayed onto a steel substrate held at different temperatures during spraying. The as-sprayed coatings consist of columnar grains whose axes are nearly perpendicular to the lamellae composing the coatings. As the coating temperature becomes higher, the length of the columnar grains increases and is longer than the thickness of the lamellae, indicating the growth of the grains across the lamellar interfaces during spraying. On the other hand, the coatings that were heat treated after spraying consist of coarse equiaxed grains. The coatings that experienced high temperatures during spraying or the heat treated coatings have large porosity and contain large globular pores. The hardness, apparent density and the tensile strength of the coating itself were the highest for the coating prepared at a low temperature and became low on heat treatment. The thermal conductivity in the direction perpendicular to the coating was the largest for the coating that consisted of long columnar grains.

Evolution of particle morphology in semisolid processing

Experimental results are summarized of the formation of spheroidal grain structures in an Al-4.5 wt pct Cu alloy by (1) isothermally holding very fine equiaxed dendrites in the liquid-solid region and (2) direct spheroidal solidification from the melt. The fine-grained dendritic structure was obtained by very rapid solidification in a thin-section (0.8 mm) permanent-mold plate casting under conditions such that solidification took place during the filling process and, hence, during rapid fluid flow. Grains of approximately 30 µm in diameter (representing a grain density of about 3×104 mm−3) were obtained; these ripened to a spheroidal morphology in the liquid-solid region in less than 5 seconds. A similarly high grain count was then obtained in the bulk melt by vigorous agitation and rapid cooling near the liquidus temperature; the melt was thereafter cooled more slowly. Under these conditions, a spherical morphology formed early in solidification and grew in that form. The evolution of particle size with time in the liquid-solid region is shown to be essentially identical in the initially dendritic and spherical-growth experiments.

Mechanical and microstructural performance of convex dies in axisymmetric extrusion—Theory and experimental verification

An analysis of the convex die performance in the process of direct extrusion of 6063 and 2014 aluminum alloys is presented. Theoretical analysis of metal flow through convex dies and an optimization procedure using a proposed numerical model are complemented by the results of experimental verification. The proposed model is based on the upper bound theory and allows process parameters optimization. The optimized process parameters were then verified experimentally. Additionally, microstructure results for the profiles extruded under different processing conditions are presented and analyzed for the conditions responsible for formation of the peripheral coarse grain structure.

Evolution of surface recrystallization during indirect extrusion of 6xxx aluminum alloys

The fundamentals of coarse grain surface recrystallized structure formation in extrusion of 6xxx aluminum alloys are not yet completely understood. The objective of this article is to understand the metallurgical origins and mechanisms of the formation of the peripheral coarse grain (PCG) structure as the first step to understanding surface behavior of extruded aluminum alloys. Small-scale indirect extrusion tests were performed in which deformation parameters of strain, strain rate, and temperature were closely controlled. The deformed material was characterized via traditional metallography and orientation imaging microscopy (OIM) in order to understand the influence of processing conditions and alloy chemistry on surface grain formation. It was found that decreasing recrystallization-inhibiting elements such as Cr as well as increasing the starting extrusion temperature, extrusion ratio, and ram speed all increased the depth of the PCG. Additionally, a mechanism for favorable coarse grain formation at the surface of the extrudate is proposed based on microstructure development during extrusion.

Incidence Angle Dependences of Columnar Grain Structure and Texture in Obliquely Deposited Iron Films

From the viewpoint of the crystallographic contribution to the formation of columnar grain structure we investigated the incidence angle dependences of the columnar grain structure and the texture for iron films prepared at various deposition rates R. The films were vapor-deposited at incidence angles of 15°, 30°, 45° and 65° on glass substrates heated at 473 K. The X-ray analysis using the Schulz method revealed that there exist three kinds of texture: [100], type I [111] and type II [111] textures. From the observation using replica electron microscopy and the measurement of electrical resistivity, it was found that the two characteristics of the columnar grain structure (inclination and bundling of columnar grains) depend on the kind of texture irrespective of the incidence angle. For the films with type I [111] texture, the inclination angle and the degree of bundling of columnar grains take constant values with R at a given incidence angle. For the films with [100] or type II [111] texture, on the other hand, the inclination angle and the degree of bundling change markedly with R even if the incidence angle is constant. These changes in columnar grain structure with R depending on the kind of texture can be understood by considering the appearance of the {100} crystal habit.

Effect of a heterogeneous distribution of particles on the formation of banded grain structure in wrought Alloy 718

Alloy 718 is known to be sensitive to interdendritic segregation formed during ingot solidification. The occurrence of banded grain structures under heat treating conditions close to 1000 ° C related to interdendritic segregation is often reported. In order to have a better understanding of this microstructural evolution, an extensive experimental program has been carried out. Consequently, a model taking into account the selective dissolution of δ -phase (Ni 3Nb) is proposed. A grain growth simulation by Monte-Carlo method is then used to illustrate the grain structure evolution in a banded particle distribution. By comparing experimental data and computer simulation, the relationship between the Monte-Carlo step and the real time is assessed and the range of parameters when heterogeneous microstructures appear is specified.

STM and STS characterization of ZnO nanorods

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques were used for the morphological and electronic characterization of zinc oxide nanorods. Well crystalline ZnO nanorods of about 200 nm mean diameter and up to 5.0 μm in length were synthesized by a modified hydrothermal technique at low temperature, using ethylenediamine as a soft surfactant. By means of STM we could detect the formation of prismatic hexagonal and non-prismatic rounded shape ZnO nanorods. Formation of aligned grain structures induced by strain relaxation in the nanorods were also observed. Using STS we could determine the band gap and deep level states related with the formation of oxygen vacancies in the nanostructures by a strain relaxation process.

Grain Boundary Microstructural Control in Nanocrystalline Ni and Ni-20mass%Fe Alloy by Magnetic Annealing

The effect of a magnetic field on grain growth was studied in nanocrystalline samples of nickel and nickel-20mass% iron alloy produced by electrodeposition. It was found the application of a magnetic field restricts abnormal grain growth resulting in the formation of more homogeneous grain structure, while abnormal grain growth took place during ordinary annealing without a magnetic field. Abrupt grain growth took place at later stage of magnetic annealing but the grain structure with much larger grain size was still uniform. The DSC measurents during magnetic annealing revealed the appearance of a peak associated with grain growth in nanocrystalline pure nickel and nickel-20mass%iron alloy. It was found that the peak associated with grain growth was shifted to high temperature side by about 10K during magnetic annealing. This was ascribed to a decrease of the grain boundary energy under a magnetic field.

The combined role of nickel and boron on the nucleation and growth of Fe-based non-crystalline alloys

Amorphous ribbons of composition Fe 75− x Ni x Si 9B 16 ( x = 1 , 2, 3, 4) were in situ crystallized upon heat treatment and electron beam irradiation. Depending on the Ni concentration an eutectic or primary crystallization took place. The sequence of phases formation was followed by transmission electron microscopy observations and thermal analysis. Other studies (Scripta Metall. 12 (1978) 75, Appl. Phys. Lett. 33 (1978) 233, RQ 111, 1 (1978) 44428) led to the conclusion that the presence of a supersaturated solid solution of boron in Fe-based amorphous alloys strongly influences the crystallization process. In our case, the onset crystallization is determined by the boron solid solution in iron in the case of Ni content less than 3 wt%, while Ni admixtures more than 4 wt% alter the process of the nucleation and the subsequent crystallization. The last one results in the formation of an ultrafine grain structure composed mainly of α-Fe(Si) crystallites. It seems that the modification of the crystallization is a result of the inhibition of the formation of a supersaturated solid solution of boron in iron. An observed large scattering of the values of the lattice parameter of α-Fe(Si) crystallites was ascribed to a different degree of occupation of metalloid atoms in the α-Fe grains.

The effect of dispersoids on the grain refinement mechanisms during deformation of aluminium alloys to ultra-high strains

The effect of fine dispersoids on the mechanisms and rate of grain refinement has been investigated during the severe deformation of a model aluminium alloy. A binary Al–0.2Sc alloy, containing coherent Al 3Sc dispersoids, of ∼20 nm in diameter and ∼100 nm spacing, has been deformed by equal channel angular extrusion to an effective strain of ten. The resulting deformation structures were quantitatively analysed using high-resolution electron backscattered diffraction orientation mapping, and the results have been compared to those obtained from a single-phase Al–0.13Mg alloy, deformed under identical conditions. The presence of fine, non-shearable, dispersoids has been found to homogenise slip, retard the formation of a cellular substructure and inhibit the formation of microshear bands during deformation. These factors combine to reduce the rate of high-angle grain boundary generation at low to medium strains and, hence, retard the formation of a submicron grain structure to higher strains during severe deformation.

Grain boundary statistics in nano-structured iron produced by high pressure torsion

Abstract The microstructure and the spectrum of grain boundary misorientations were studied in Armco iron, following high pressure torsion (HPT) deformation, by means of transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). It was found that HPT deformation results in the formation of an equiaxed grain structure with a mean grain size of 270 and 130 nm using a shear strain of γ = 210 and 420, respectively. The misorientation spectra in HPT iron have a bimodal character with maxima in low (at 1–2°) as well as in high misorientation angle ranges. A marked increase in the fraction of special boundaries ( Σ 3– Σ 45) was revealed as a result of HPT. The microstructural changes due to HPT are discussed and compared with those obtained during conventional deformation modes.