Process Of Grain Refinement Research Articles

Strain-induced grain refinement of cobalt during surface mechanical attrition treatment

The microstructural evolution during surface mechanical attrition treatment of cobalt (a mixture of hexagonal close packed (hcp) and face-centered cubic (fcc) phases) was investigated, in order to reveal the mechanism of grain refinement and strain accommodation. The microstructure was systematically characterized by both cross-sectional and planar-view transmission electron microscopy. In the hcp phase, the process of grain refinement, accompanied by an increase in strain imposed in the surface layer, involved: (1) the onset of {101¯1} deformation twinning, (2) the operation of 〈112¯0〉{101¯0} prismatic and 〈112¯0〉 (0001) basal slip, leading to the formation of low-angle dislocation boundaries, and (3) the successive subdivision of grains to a finer and finer scale, resulting in the formation of highly misoriented nanocrystalline grains. Moreover, the formation of nanocrystallites at the grain boundary and triple junction was also observed to occur concurrently with straining. By contrast, the fcc phase accommodated strain in a sequence as follows: (1) slip of dislocations by forming intersecting planar arrays of dislocations, (2) {111} deformation twinning, and (3) the γ(fcc)→ε(hcp) martensitic phase transformation. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization occurring in the hcp phase and the γ→ε martensitic transformation in the fcc phase as well.

Development of Severe Torsion Straining Process for Rapid Continuous Grain Refinement

This study presents a rapid continuous process for grain refinement in metallic materials through severe plastic deformation (SPD). The principle is described and the process is applied to an Al-5056 alloy and an S45C carbon steel. The new process, designated in this study the severe torsion straining process (STSP), consists of producing a local heated zone in a rod and introducing torsion strain into the zone by rotating one end with the other. The process is continuous because the straining is achieved while the rod is moved so that the heated zone is shifted along the rod. The STSP does not require the use of any die and can be applicable to pipes or wires. Fine-grained structures produced with the STSP are confirmed using optical microscopy and transmission electron microscopy. Tensile properties are measured and compared with the unstrained fully annealed samples. The feasibility of the STSP is discussed with respect to the rotation speed and moving speed.

Phase Mixture Models for Metallic Materials with Submicrometer Grain Size

ABSTRACTPhase mixture models describing the mechanical properties of submicrometer grained metals are presented. In this approach, grain boundaries or cell walls are treated as a separate phase. Two cases are considered: the mechanical response of an ultrafine grained material and the process of grain refinement by equal channel angular pressing. Model predictions with regard to the evolution of the microstructure, strength and texture are verified for Cu.

Effect of Manganese Segregation on Fine-grained Ferrite Structure in Low-carbon Steel Slabs

Using a 100 mm thick continuously cast slab with a chemical composition of 0.10C-0.16Si-0.58Mn-0.01P-0.003S (mass%), we have clarified the effect of the manganese segregation on the transformed ferrite structure when the slab was reheated to make austenite grains equi-axed and smaller and followed by the Grain Refinement Process (GRP) of heavy compression and subsequent controlled cooling. Samples from which the Mn segregation was eliminated were also prepared for comparison. The resultant microstructure was examined for compressive strains between 0.7 and 2.1. When the austenite grain size is 160 or 120μm prior to GRP, the ferrite grain size of the samples with the Mn segregation is smaller at the given strain than that of the samples from which the Mn segregation was eliminated. The Mn band formed by the heavy compression is thought to act as a barrier in the form of a relatively stable austenite against the ferrite growth. Moreover, we found a unique equation to predict the transformed ferrite grain size in the samples with the Mn segregation. Namely, the ferrite grain size can be accounted for according to a parameter, ES (effective spacing): a mean spacing that considers both the pancake austenite grain boundary and the Mn band. We also suggest that the smaller Mn band spacing may make the ferrite structure finer even when the austenite grain size is large.

Quantities of grains of aluminum and those of TiB2 and Al3Ti particles added in the grain-refining processes

Al–Ti–B alloys are the most commonly used grain-refiners in the casting of aluminum alloys; however, the grain-refining mechanisms are still not conclusive. Quantities of the two kinds of phase particles, TiB2 and Al3Ti, introduced into the molten aluminum by the addition of Al–Ti–B grain refiners are determined experimentally. It is found that the quantities of Al3Ti particles introduced are much lower than those of grains in the as-cast aluminum alloys, while those of introduced TiB2 are higher. Thus it is concluded the Al3Ti particles in the Al–Ti–B grain-refiners cannot be the primary nucleation sites during solidification of aluminum.

New grain refinement method for aluminum cast alloys without adding grain refiner.

Grain refiner has conventionally been added during the casting process of grain refinement, but it has recently become a recycling problem, which calls for the efforts for development of a new grain refinement method. In this paper, a technological breakthrough has been achieved, where grain refinement was successfully completed by applying ultrasonic vibration during the solidification process of aluminum cast alloys. The refinement mechanism was clarified to determine the temperature range to apply the optimum vibration.

The process of grain refinement in equal-channel angular pressing

Experiments were conducted on high purity aluminum to investigate the process of grain refinement during equal-channel angular (ECA) pressing. Samples were subjected to 1 to 4 pressings and then sectioned for microstructural examination in three mutually perpendicular directions. The results show that a single pressing leads to the development of subgrain bands. The subsequent microstructural development in additional pressings depends upon whether these repetitive pressings are conducted with no rotation of the sample (route A), with a rotation of 90° between each pressing (route B) or with a rotation of 180° between each pressing (route C). The subgrains evolve more rapidly into arrays of high angle boundaries using route B, less rapidly using route C and the evolution is slowest using route A. The results are explained by considering the shearing patterns developed in the samples during each processing route.

Mechanical alloying of immiscible elements: Experimental results on Ag‒Cu and Co‒Cu

Abstract Equimolar solid solutions of Ag[sbnd]Cu and Co[sbnd]Cu were prepared by mechanical milling of the elements. The structure and the thermodynamic properties of the resulting phases were analysed by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Large mixing enthalpies (ΔH = 6–8kJg-atom−1 and ΔH = 4–8kJg-atom−1 for Ag-Cu and Co-Cu respectively) were measured in the alloyed phases. It is clear that the solid solution forms through a process of grain refinement down to the nanometre scale and is assisted by an atomic-scale dissolution. The possible role played by coherent interfaces in substantially increasing the energy of the solid solutions is discussed.

Solidification from undercooled melts

Techniques of containerless processing are applied to undercool and solidify metals and alloys. These techniques allow direct measurements of both the undercooling and the crystal growth velocity. Experimental results are presented for studies of nucleation of quasicrystalline and metastable crystalline phases. Measurements of dendrite growth velocity as a function of undercooling are presented for dilute Ni-B and CoSi alloys. The results are analysed within current theories of rapid crystal growth. The consequences of solute trapping on the formation of metastable supersaturated solutions are demonstrated. In analogy to the process of solute trapping, disorder trapping is investigated occurring during rapid solidification of deeply undercooled alloys of intermetallics. In addition, studies of crystal growth in undercooled semiconductors are presented. Finally, undercooling and its importance in the formation of grain-refined microstructures is highlighted. A new model is presented to explain the grain-refinement process in undercooled melts by the fragmentation of primarily formed dendrites. Recent investigations on magnetic ordering in undercooled melts of Co-Pd alloys are shortly introduced.

Simulation of normal grain growth by cellular automata

Prediction of microstructure evolution and properties of ultrafine-grained materials is one of the most significant problems in materials science. Cellular automata (CA) method is able for proper simulation of the microstructure evolution and can be used for evaluation of mechanical properties. The paper presents a new original application of cellular automata for modeling of grain refinement. CA take into account deformation, evolution of dislocation density and dislocation structures; and simulate microstructure evolution as grain refinement. Deformation of grains in polycrystalline materials depends on their crystallographic orientation, which is unique for every grain. It causes anisotropy of deformation in micro-scale. Joining CA with other methods (finite element, discrete element, crystal plasticity, etc.) improves accuracy of coupled phenomena modeling in micro- and macro-scale. Finite element method and crystal plasticity theory provide information for CA calculations. Grain refinement occurs in two stages: a creation of low-angle boundaries and their evolution into high-angle boundaries. Three CA models of low-angle boundaries creation were developed. They are described and discussed in the paper. Developed models can be used for study of processes of grain refinement, as well as can be a part of the system for modeling of microstructure evolution in the processes of solidification, hot and cold deformation and phase transformation. Examples presented in the paper are model application for processes with severe plastic deformation (SPD). Results of simulations of grain refinement during the accumulative roll-bonding (ARB) process and MAXStrain® technology are presented in the paper.

A new grain-refinement process for superplasticity of high-strength 7075 aluminium alloy

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10−4s−1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.

On the description of the process of recording TiB 2 particles on solid state nuclear track detectors by the use of neutron-induced autoradiography

Neutron-induced autoradiography (NIAR) allows grain-refining processes for which boron-containing master alloys are used for analysis through the nuclear reaction 10B(n, α) 7Li. The TiB 2 particles as potential nucleation centres play an important role in the mechanism of grain refinement. In this paper theoretical and practical aspects of the process of recording borides on the solid state nuclear track detector (SSNTD) are discussed. The influence of boride distribution in material depth on the image on the detector is modelled and calculated. A simple method of quantifying the relationships between size of TiB 2 particles and their image on the detector, which is based on quantitative image analysis, is described. Concluding calculations and results show that borides with a diameter ( d TiB 2 ) greater than 0.2-0.3 μm are completely detected by the use of NIAR. This gives new possibilities of assessing the role of TiB 2 particles in grain refinement. The exact knowledge of the crystallographic characteristics of borides might be important to obtain information about the behaviour of the SSNTD.

Bestimmung der Verteilung und des Gehaltes von Bor in Al-Gießband Teil I: Bestimmung des Borgehaltes mittels NIAR

Neutron-induced autoradiography was applied to investigate and optimize grain refining in the production of cast aluminium strip. The behaviour of the alloy AlTi5B1 used for grain refining was studied by analysis of both content and distribution of boron. This paper is concerned with the quantitative determination of track density by means of the image analyzer type A 6471 and the results thus obtained in comparison with the amount of tracks visually counted are discussed. Boron contents as calculated from track densities are compared with calibration functions relating track density to the boron content determined by chemical analysis. The content of boron depends upon the position of the autoradiographed plane in the sample, allowing, with its distribution taken into consideration, grain-refining processes to be studied.

Bestimmung der Verteilung und des Gehaltes von Bor in Al-Gießband Teil II: Untersuchung des Borverteilungszustandes mittels NIAR und seine bildanalytische Quantifizierung

The distribution of boron in aluminium, the grain of which had been refined by the addition of the alloy AlTi5B1, was investigated by means of neutron-induced autoradiography and quantitative image analysis. Some aspects and problems of metallegraphic methods for the determination of boron particles in aluminium are discussed. Besides the practice classification on the detector by image analysis the results thus obtained were converted into structure parameters by rising model assumptions and by measurements. In addition, particle density and average particle distance were determined. Experimental results confirm the possibility of using the method described to study grain-refining processes initialed by boron-containing master alloys.

Influence of structure factors on the hardening of stable and metastable austenitic-martensitic steel in the linear and plane stressed state

1. For the investigated austenites and austenitic-martensitic steels is it shown that an increase in strain and phase hardening (caused, respectively, by the transition from the linear to the plane stressed state and by the increase in the volume content of the hardening phase) is revealed in activation of the processes of grain refinement and of an increase in the dislocation density in the austenite. 2. A peculiarity of the substructure of the two-phase steel with stable austenite is relaxation of the effects of phase hardening in plastic deformation. 3. Under conditions of development of the γ→α transformation the more stressed substructure of the austenite is inherited by the deformation martensite, which leads to an increase in its strength. As the result of this there is an increase in the level of flow stresses of the austenitic steel in biaxial tension and a decrease in the sensitivity of the tensile strength of the austenitic-martensitic steel to the quantity of residual austenite. The latter is responsible for the independence of the tensile strength of the metastable austenitic-martensitic steel from the quantitative relationship of the phases.

Grain refinement in 7075 aluminum by thermomechanical processing

A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.