The effect of grain size and strain rate on the

The substructural developments taking place in nickel 200 with grain diameters of 47, 108, 141, and 274 μm have been studied at four different strain rates of 0.01, 0.25, 2.5, and 5/min during tensile testing at room temperature. The percent strain necessary to develop well-defined cell boundaries increases with an increase in grain size at a given strain rate. The cell size refinement takes place throughout the entire range of percent strains (up to 30 pct) in tension for the nickel samples with grain diameters of 47, 108, and 141 μm at all four strain rates used in this article. However, nickel, with the largest grain diameter of 274 μm, shows refinement and then sat- uration for tensile strains greater than 25 pct. The cell size strengthening described by the mod- ified Hall-Petch (MHP) equation at the selected four strain rates of this article indicates that the flow stress is higher for smaller grain size samples at a given cell size. The effect of strain rate on the slope from the MHP plots is such that even though it does not change with an increase in strain rate up to 0.25/min for the four grain sizes, the actual value of the slope decreases with an increase in grain size at a given strain rate. Beyond this strain rate, even though an increase in the slope value as a function of strain rate has been observed for all four grain diameter samples, the influence of grain size on the slope of the MHP plots is so small that it can be assumed that they may become grain size independent at extremely high strain rates. JYOTHI G. RAO, formerly Graduate Student, JYOTHI G. RAO, formerly Graduate Student, JYOTHI G. RAO, formerly Graduate Student,     

Effect of grain size on high strain rate deformation of copper

Experiments were performed to observe the deformation characteristics of oxygen-free high-conductivity (OFHC) copper at high strain rates (up to 40,000 s⁻¹) and to relate differenc in grain size with differences in deformation behavior. The rod impact and torsional Hopkinson bar test methods were used in these experiments. Results show that grain size reductions substantially reduce surface irregularities that develop during deformation. The effect of grain size on the yield stress and on the strain-hardening behavior of copper is small and is similar to the effect of grain size in copper at quasistatic strain rates. The observation that grain size has a substantial effect on surface irregularities may have important implications for applications in which stable deformation of thin sections is of concern.     

On the analysis of grain size in bulk nanocrystalline materials via x-ray diffraction

The Warren-Averbach (WA) analysis and other simplified methods that are commonly used to determine the grain size of nanocrystalline materials are discussed in terms of accuracy and applicabilities. The nanocrystalline materials used in the present study are prepared by cryomilling of A1 powders and subsequent consolidation (hot isostatic pressing and extrusion). Transmission electron microscopy observations of the as-extruded nanocrystalline A1 reveal a bimodal distribution of grain sizes centered around 50 to 100 nm and 250 to 300 nm. It is shown that the grain size determined by the WA analysis agrees with the lower bound grain size (e.g., 50 to 100 nm) observed experimentally. In the case of the integral method, it is useful to use a parabolic (Cauchy-Gaussian (CG)) relationship to approximate instrumental broadening and separate the intrinsic broadening. Compared to the Cauchy-Cauchy (CC) and Gaussian-Gaussian (GG) approximations, this is shown to give the best results. In addition, the reliability of the Scherrer equation is also discussed.     

The role of grain size and strain in work hardening and texture development

Polycrystalline copper (99.999 pct) having four different grain sizes (from 4 to 220 μm) was strained in tension at room temperature to true plastic strains of 0.05, 0.10, 0.20, and 0.30. The initial texture of the materials was determined by neutron diffraction, as were the deformation textures. Both inverse pole figures and calculated TaylorM factors were then derived from the data. In general, it was observed that the texture strengthens at increasing strain and that the effect of grain size on this development is not very pronounced. The measured change in the volume concentration of the (111) texture component was compared to that obtained from a model simulation, and in general, the experiments and the simulations agreed well. The effect on the flow stress of the initial texture, and on the texture which develops during straining, could be accounted for on the basis of TaylorM factors calculated from the experimental results, and it was found that there is an effect of texture on the flow stress. The flow stress at strains above yield, expressed as a modified Hall-Petch relationship, was not greatly affected by corrections toM induced by strain and grain size.     

Mechanical properties, ductility, and grain size of nanocrystalline iron produced by mechanical attrition

The main goal of this investigation is to determine the influence of grain size on the mechanical properties and, specifically, the intrinsic ductility of nanocrystalline (nc) Fe. Ball-milled nc Fe was consolidated into compacts of near theoretical density by uniaxial warm pressing. Compaction parameters and annealing treatments resulted in a range of grain sizes for subsequent mechanical testing. The miniaturized disk bend test, hardness, and the automated ball indentation (ABI) method were used to test nanocrystal (nc) iron in compression and tension. The deformation and fracture morphologies of the tested samples were characterized by light and scanning electron microscopy. The hardness, as a function of the grain size, was described with a Hall-Petch slope, which was smaller than that in coarse-grained Fe. In tension, the material failed in a macroscopically brittle manner, while local ductility in very concentrated shear bands was observed. The compressive characteristics of the nc Fe were similar to those of a perfectly plastic material. The results are discussed in the context of the mechanical behavior of coarse-grained polycrystalline metals and alloys. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

The effect of grain size and plastic strain on slip length in 70-30 brass

Polycrystalline 70−30 brass of varying grain size has been studied. Measurements were made of slip line lengths on the polished surface of specimens which had undergone different plastic strains. A relation between the slip line length and the grain size and plastic strain was found based on experimental data using a multiple regression technique. Qualitative agreement was found between the observed slip lengths and slip lengths calculated from a published work-hardening model. The effect of the boundary on slip behavior was observed, and both passive and active types of obstruction to slip by boundaries are suggested. H. DONG, formerly with Carnegie-Mellon University, is now with Jilin Institute of Engineering, Changchun, China. A. W. THOMPSON     

The combined effect of grain size and strain rate on the dislocation substructures and mechanical properties in pure aluminum

The effect of grain size on the development of dislocation substructures has been studied as a function of strain rate. Pure aluminum rods with grain diameters of 70, 278, and 400 μm were deformed in tension at room temperature to various percent strains at strain rates of 0.01, 0.25, 2.5, and 5/min. It has been confirmed that the smaller grain size results in higher flow stress in this strain-rate range. The cell size strengthening described by the modified Hall-Petch (MHP) equation is applicable to samples with 70 and 278 μm grain sizes at all four strain rates used in this study, while 400 μm grain sizes show deviation from this because of inhomogeneities developed in the microstructure. The influence of strain rate on the slope of the MHP plots, for a grain size of 70 μm, is such that at lower strain rates, the slope does not change much, but at higher strain rates, there is an increase in the slope value. At all strain rates, the values of slopes from the MHP plots of the smaller grains are higher than for the larger grains.     

Statistical considerations on uniform grain size

The distribution of grain size was determined in four alloys: ferritic steel, austenitic steel, zinc, and ferrite-pearlitic steel, using samples that can be considered as having uniform grain size (UGS), in order to delimit this term. It was found that the grain volume distribution in the four samples was log-normal. The concept of UGS was related to the characteristics of the distribution obtained and, specifically, to the value of the standard logarithmic deviation which was equal to 1 in the four samples studied. This enables us to know the volume distribution exactly by knowing only the mean geometric volume of the grain.     

On the relation between grain size and grain topology

Experimental studies of the topology of grains in polycrystals have indicated that the topological complexity of a grain is related to its diameter, as opposed to its surface area or volume. This paper presents additional experimental documentation of this correlation and a theoretical derivation of the empirically observed relationship.     

The effect of grain size, strain rate, and temperature on the mechanical behavior of commercial purity aluminum

Commercial purity aluminum AA1050 was subjected to equal channel angular extrusion (ECAE) that resulted in an ultrafine-grained (UFG) microstructure with an as-received grain size of 0.35 μm. This UFG material was then annealed to obtain microstructures with grain sizes ranging from 0.47 to 20 μm. Specimens were compressed at quasi-static, intermediate, and dynamic strain rates at temperatures of 77 and 298 K. The mechanical properties were found to vary significantly with grain size, strain rate, and temperature. Yield stress was found to increase with decreasing grain size, decreasing temperature, and increasing strain rate. The work hardening rate was seen to increase with increasing grain size, decreasing temperature, and increasing strain rate. The influence of strain rate and temperature is most significant in the smallest grain size specimens. The rate of work hardening is also influenced by strain rate, temperature, and grain size with negative rates of work hardening observed at 298 K and quasi-static strain rates in the smallest grain sizes and increasing rates of work hardening with increasing loading rate and grain size. Work hardening behavior is correlated with the substructural evolution of these specimens.     

Acceleration of mechanical size reduction of chromium carbide powders and mixtures based on chromium carbide

We have investigated how mechanical activation by rolling on smooth rollers affects the dispersion kinetics of chromium carbide powders and mixtures of chromium carbide powder with nickel in a ball mill. We have shown that preliminary double rolling of chromium carbide powder on a rolling mill accelerates particle size reduction in the ball mill down to a size of 1.4–1.5 μm by a factor of 4.6 compared with unrolled powder. In this case, the size reduction increases from 36 to 91 in relative units. The mean particle diameter for the powders and mixtures decreases exponentially as the grinding time increases from 12 h to 84 h. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 1–8, January–February, 2000.     

Influence of grain size variability on the strain rate dependence of the stress exponent in mixed-mode power law and diffusional creep

The relationship between the strain rate and the stress in power law and diffusional creep has usually been derived with the assumption that all the grains have the same size, which predicts a sharp transition from power law creep, with a stress exponent of about four to five, to diffusional creep, where the stress exponent is equal to one. We show that the use of distributed grain size can lead to a transition from power law to diffusional creep that is spread over several orders of magnitude in strain rate. The breadth of this transition depends on the standard deviation of the grain size probability density function. The experimental values for the stress exponent that are apparently greater than one, when measured over two or three orders of magnitude in strain rate, can result from a very gradual change in the stress exponent with the strain rate for a distributed grain size. Data sets from copper are compared to the model.