Jerzy A. Szpunar

Experimental parameters influencing grain refinement and microstructural evolution during high-pressure torsion

Abstract Pure nickel was selected for a detailed investigation of the experimental parameters influencing grain refinement and microstructural evolution during processing by high-pressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of ~170 nm in pure Ni, and homogeneous and equiaxed microstructures are attained throughout the samples when they are subjected to at least ~5 whole revolutions under applied pressures of at least ~6 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the center and at the periphery of the samples. A simple model is proposed to explain the development of a homogeneous microstructure in HPT.

Orientation imaging microscopy of ultrafine-grained nickel

Orientation imagining microscopy was used to measure the distributions of grain boundary misorientations in ultrafine-grained nickel processed by high-pressure torsion and equal-channel angular pressing. Both procedures give high fractions of high-angle boundaries but also higher fractions of low-angle boundaries than anticipated from a random distribution.

Evolution of recrystallization texture in nonoriented electrical steels

Abstract The mechanism responsible for the formation of recrystallization texture is still disputed although recrystallization texture has long been a subject of research. This is mainly related to the complexity of recrystallization itself. The mechanism of recrystallization texture in a 2% Si steel was investigated. In addition, the origin of nuclei with specific orientations was studied. The formation of recrystallization texture is explained by oriented nucleation. Most nuclei have a high misorientation angle of 25∼55° with the surrounding deformed matrices. New Goss grains are mainly nucleated within shear bands in the deformed {111} , {111} and {112} grains, and the number of shear band decreases in the same order. The nucleation of new Cube grains also takes place within shear bands. New {111} grains are nucleated within deformed {111} grains and new {111} grains originated in the deformed {111} grains.

The role of texture and morphology in optimizing the corrosion resistance of zinc-based electrogalvanized coatings

Abstract Zinc-based electrogalvanized coatings have attracted increasing interest from the automobile industry and steelmakers because of their excellent corrosion resistance and good mechanical properties. The coating properties are closely related to the microstructure of electrodeposits. Texture is an important factor which influences the coating properties. This study demonstrates a relationship between the corrosion behavior of the electrogalvanized zinc, zinc-iron, and zinc-nickel coatings and their microstructure. Corrosion behavior is analyzed by the Tafel extrapolation method from the potentiodynamic polarization curves. In this study, it is found that the texture of coatings strongly influences corrosion resistance. The coatings with strong {0001} texture have better corrosion resistance than the coatings with weak {0001} texture or a {101X} fiber texture. The influence of alloy content and phase composition upon corrosion behavior is also discussed. Consequently, methods are proposed for optimizing the deposition process to obtain the desirable texture, morphology and alloy content of the deposit for better corrosion resistance.

The role of grain boundary character distribution in secondary recrystallization of electrical steels

Abstract Grain Boundary Character Distributions (GBCD) in grain-oriented electrical steels were calculated from the Orientation Distribution Functions of the Primary recrystallization texture. The probability of a grain boundary having a misorientation angle from 20° to 45° is highest around the Goss grain. The observed difference in GBCD implies that the Goss grain is surrounded by the highest number of high-energy boundaries. The high-energy boundary has more structural defects, and these are often linked to a high grain boundary migration rate and a high grain boundary diffusion rate. An explanation of Goss texture development in secondary recrystallization is proposed, based on the property of the high-energy boundary. The role of special Coincident Site Lattice boundaries is analysed, and we conclude that they do not play a significant role in the development of Goss texture.

Neutron diffraction method for stored energy measurement in interstitial free steel

Part of the energy acquired by polycrystalline metals during plastic deformation is stored in the form of dislocations and other crystal defects. A method is described for the measurement of the orientation dependence of the stored energy of cold work, using neutron diffraction line broadening. To illustrate the method, the measured stored energy values for 80% cold rolled interstitial free (IF) steel are compared with the stored energy in a stress relieved specimen. The correlation between the Taylor factor and the stored energy for an 80% cold rolled specimen is discussed.

A new model of Goss texture development during secondary recrystallization of electrical steel

A new model of Goss texture development during the secondary recrystallization of electrical steel is proposed and tested in this paper. This model is based on the assumption that high-energy boundaries have high grain boundary diffusion coefficients. High diffusion coefficients facilitate precipitate coarsening along the high-energy boundaries, which in turn makes them mobile before other boundaries. Recent results obtained by the authors show that Goss grains are surrounded by the highest number of high-energy boundaries. The assumption that these boundaries are highly mobile allows the Goss grains to grow during secondary recrystallization. The new model is tested using a Monte-Carlo simulation of the grain growth in a 3-D polycrystalline computer specimen which describes both the microtexture and crystallographic texture of the electrical steel. The results of the simulation agree with experiment. Specifically, the pinning force and incubation time of secondary recrystallization as well as the orientation of the secondary recrystallized grains and the texture changes during grain growth are explained.

The changes of grain boundary character distribution during the secondary recrystallization of electrical steel

Abstract A new model of development of the Goss ({110}〈001〉) texture during the process of the secondary recrystallization of electrical steels, was proposed based on the assumption that the high-energy boundary has a high mobility. This assumption is tested by a direct orientation measurement using Electron Back Scattering Patterns (EBSP). It is demonstrated that the frequency of the high-energy boundary decreases during the normal grain growth of the matrix grains. This result suggests that high-energy boundaries move quickly and disappear during the grain growth process. Therefore, this result may be used to support the assumption of a high mobility for the high-energy boundary. We conclude that the role of the high-energy boundary is important in the late stage of secondary recrystallization when large Goss grains consume matrix grains, and that the CSL boundaries do not play a positive role in the secondary recrystallization.

The nanocrystalline ceria sol-Gel coatings for high temperature applications

Sol-gel technology was utilized to produce nanocrystalline thin films of CeO2 on substrates with different oxidation resistance at high temperatures. After deposition, the coatings were composed of CeO2 crystallites with a size of 5 nm, randomly oriented on all the substrates. The annealing of gel deposited on Pt and Si substrates was accompanied by evaporation of water and other species and followed by growth of crystallites, without formation of any evident texture. Conversely, the annealing of coatings deposited on easy oxidizing substrates of Nr, Cr, or Ni−Cr alloys resulted in incorporation of CeO2 crystallites into growing native oxide, and in slowing the oxidation rate. Examples of the application of CeO2 ceramics as coatings deposited on selected metals and alloys for improving their oxidation performance are presented.

A role of fractions of mobile grain boundaries in secondary recrystallization of Fe-Si steels

Abstract Grain growth during annealing of polycrystalline materials is influenced by the type of grain boundaries a grain encounters as it grows. Simple computer experiments have been performed to ascertain which type of boundaries are responsible for the abnormal grain growth (AGG) in Fe–3% Si steels. In modelling abnormal grain growth, two different assumptions are used, the first one is that the coincidence site lattice (CSL) boundaries have high mobility and the second is that high energy boundaries are more mobile than other boundaries. The results of the computer experiments support the latter model for abnormal grain growth in Fe–3% Si steels. Finally, the importance of fractions of mobile grain boundaries on the development of Goss texture is discussed.