The effect of constrained groove pressing on grain size,dislocation density and electrical resistivity of low carbon steel

In this research, constrained groove pressing (CGP) technique is used for imposing severe plastic deformation (SPD) on the low carbon steel sheets. Using transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical microscopy, the microstructural characteristics of produced sheets are investigated. The results show that CGP process can effectively refine the coarse-grained structure to an ultrafine grain range. Dislocation densities of the ultrafine grained low carbon steel sheets are quantitatively calculated and it is found that the CGP can effectively enhance the dislocation density of the sheets. Measurements of their electrical resistivity values show that microstructure refinement and increasing the dislocation density can efficiently increase the electrical resistivity of the CGPed sheets up to ∼100%.     

Austenite and ferrite grain sizes in interstitial free steel

Abstract

An oxidation method has been employed to reveal prior austenite grain boundaries in C–Mn and interstitial free (IF) steels. The ability of this technique to reveal prior austenite grain boundaries is assessed by comparing its results with those of an etching method applied on the C–Mn steel. Optimum conditions were established by trial and error. The conditions varied with different steels and with heat treatment temperature. In the IF steel rapid grain growth at high temperatures in the ferrite range made a significant contribution to the prevention of grain refinement through transformation. Attempts to obtain the smallest prior austenite grain size in the IF steel to assess the ability of the oxidation technique to reveal fine austenite grains led to an average austenite grain size of 80 μm in warm rolled samples after the shortest holding time at 950°C.

MST/3203     

The influence of grain size on the ductility of micro-scale stainless steel stent struts

Vascular stents are used to restore blood flow in stenotic arteries, and at present the implantation of a stent is the preferred revascularisation method for treating coronary artery disease, as the introduction of drug eluting stents (DESs) has lead to a significant improvement in the clinical outcome of coronary stenting. However the mechanical limits of stents are being tested when they are deployed in severe cases. In this study we aimed to show (by a combination of experimental tests and crystal plasticity finite element models) that the ductility of stainless steel stent struts can be increased by optimising the grain structure within micro-scale stainless steel stent struts. The results of the study show that within the specimen size range 55 to 190 μ m ductility was not dependent on the size of the stent strut when the grain size maximised. For values of the ratio of cross sectional area to characteristic grain length less than 1000, ductility was at a minimum irrespective of specimen size. However, when the ratio of cross sectional area to characteristic grain length becomes greater than 1000 an improvement in ductility occurs, reaching a plateau when the ratio approaches a value characteristic of bulk material properties. In conclusion the ductility of micro-scale stainless steel stent struts is sensitive to microstructure and can be improved by reducing the grain size.     

The influence of temperature and grain size on substructure evolution in stainless steel 316L

The flow stress of polycrystals is controlled by the processes occurring in the grain interior as well as in the mantle, i.e. at the grain boundary and its immediate vicinity. The early stages of evolution of dislocation substructure in these two regions with strain in 316L stainless steel polycrystals have been studied at 293 K, 673 K and 1123 K representing the low temperature thermal, the intermediate temperature athermal and the high temperature thermal regimes respectively. Specimens with grain sizes of 4 and 12 m were employed to determine the effect of grain size.Transmission electron microscopy studies on deformed specimens show the different roles of grain boundary and grain interior in different temperature regimes. In the low temperature regime grain boundaries act as obstacles to moving dislocations and as such high density of dislocation is found in the grain boundary vicinity. In the intermediate temperature regime the dislocations which are easily spread into the grain interior rearrange to form cell walls. In the high temperature regime grain boundaries transform to the equilibrium state and do not contain any grain boundary dislocations, and the distribution of dislocations within grains is homogeneous at all strains. Significantly higher values of dislocation densities in the vicinity as well as in the grain interior were found in the finer grain size material in the whole strain region employed.     

Determination of dislocation density from hardness measurements in metals

For the first time the Nix-Gao model for indentation size effect (ISE) is used to estimate the dislocation density in a metal. The estimate of dislocation density obtained by this method, using Ni as a case study, is compared with the values obtained from direct observation by transmission electron microscopy. It is shown that the estimate of dislocation density from indentation hardness measurements, adjusted by the Nix-Gao model, gives values consistent with those obtained by TEM, provided that the proper procedures to minimize errors are adopted. Although the direct observation of dislocations by TEM gives additional structural information, the indirect method to estimate dislocation density based on hardness measurements is more efficient, since the sample preparation method, measurement procedure and analysis of results are easier and faster.     

Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering

Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300-1800 °C) and pressure (90-266 MPa) conditions. Pure tungsten powders with an average particle size of about 1 μm were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400 °C and an applied pressure up to 266 MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530 kg/mm²). Above 1500 °C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders.     

Relation of ductile-to-brittle transition temperature to phosphorus grain boundary segregation for a Ti-stabilized interstitial free steel

Equilibrium grain boundary segregation of phosphorus in a Ti-stabilized interstitial free (IF) steel is measured using Auger electron spectroscopy (AES) after the specimens are aged for adequate time at different temperatures between 600 and 850 °C. Based on the experimental data of equilibrium grain boundary segregation along with the McLean equilibrium segregation theory, the free energy of segregation of phosphorus is evaluated to be ∼44.8 kJ/mol, being independent of temperature. With the AES results being combined with the ductile-to-brittle transition temperatures (DBTTs) determined by impact tests, a relationship between DBTT and phosphorus boundary concentration is established. Predictions with the relationship indicate that cold work embrittlement may be severe if the steel is annealed at relatively low temperatures after cold rolling.     

The influence of grain size on the creep of uranium dioxide

The creep of uranium dioxide has been investigated as a function of grain size. At high stresses, when creep is controlled by dislocation movement, grain boundaries exert a strengthening effect and this strengthening is correlated with the Hall-Petch equation. The degree of strengthening diminishes with increases in temperature. At lower stresses, when creep is controlled by mass transport, grain boundaries exert a weakening effect owing to the reduction in diffusion path length as grain size is reduced. In this range behaviour is correlated with the Nabarro-Herring equation with stress replaced by an effective stress _E=–₀ where ₀ is a threshold stress for diffusional creep associated with the limitation of the ability of boundaries to emit and absorb vacancies. ₀ appears to decrease as grain size is increased.     

The influence of plate hardness on the ballistic penetration of thick steel plates

This investigation describes and analyses the experimental results pertinent to the penetration of steel plates of varying hardness in the range HV295–HV520 and of thickness 20 and 80 mm by ogive-shaped 20-mm-diameter projectiles over the velocity range 300–800 m s⁻¹. All the tests were carried out at normal impact angle, i.e. zero obliquity. The experimental results presented include the variation of depth of penetration, crater volume, lip height, bulge height and diameter, plugged length and diameter and specific energy absorption capacity with impact velocity for tests on each plate of a given hardness and thickness. Selected data and observations relating to the plastic zone size and shape surrounding the penetrating projectile, incidence and extent of adiabatic shear band (ASB) formation and plate spalling have also been presented. These experimental data have been interpreted in terms of the appropriate penetration mechanisms like ductile hole formation, bulging followed by star cracking, ASB-induced shear plugging, etc., and also by making use of the fact that the projectile undergoes substantial deformation when penetrating the harder plates (HV450 and HV520). It is also demonstrated that the resistance to penetration and hence the mechanism of penetration is very much dependent on whether the penetration occurs under plane strain or plane stress conditions. For example, ASB-induced plugging occurs only under plane stress conditions while projectile deformation is dominant only under plane strain conditions even in harder plates.     

Measurement of grain size and size distribution in engineering materials

Abstract

Methods to characterise grain size distribution in engineering materials are evaluated, and compared with measurements on two representative materials, an equiaxed aluminium alloy and a heterogeneous nickel alloy. Computer simulations from a model tetrakaidecahedron structure have also been obtained using a personal computer program to rotate and section the model randomly, generating distribution data on section area and intercept lengths. Comparisons have been made between the model distribution data and intercept distribution data from the material microstructures.

It is recommended that size distributions of area plotted against size (intercept) or normalised size provide an effective method for discriminating between different grain size distributions. An estimate of area can be used indirectly by squaring intercept values rather than making direct measurements of grain areas.     

Evaluation of fatigue life of steel using steel grain size

The insightful discussion of the relationship between the construction of metallographic and fatigue life and proposition of mathematical relationships describing this relationship was conducted in the paper. This paper presents a method for estimating the fatigue life, based on the construction of the microstructural material taking into account the grain size and the participation of ferrite and pearlite phase.     

连续粒径水泥2矿粉双组分体系在浆体中的堆积密度计算公式

首先推导出双组分连续粒径粉体在浆体中的堆积密度公式;通过测定多组粒径分布不同的复合水泥在流动度相同的情况下的需水量,得到不同粒径分布对应着的不同堆积密度。所采用的15 种试样的需水量相差4%～10%。用该公式计算上述不同粒径分布的复合水泥浆体的堆积密度及需水量与试验值基本吻合。对公式的适用性进行了验证,说明该公式可以用来模拟诸如水泥和磨细矿粉体系、水泥和粉煤灰体系等胶凝材料细颗粒的堆积密度。     

Fracture toughness and hardness of zinc sulphide as a function of grain size

The erosion properties of brittle materials depend upon plastic deformation and crack generation at an impact or indented site. Vickers indentations have been used to investigate the plastic processes and crack systems in chemical vapour deposited zinc sulphide of different grain sizes. The hardness,H, and the local fracture toughnessK _c, are dependent upon the grain size of the material. For small grain size material (<50 m) the Vickers hardness was found to increase with decreasing grain size in accord with the Petch mechanism, i.e.H=H ₀ +kd ^–1/2 wherek andH ₀ are constants andd is the grain diameter. A maximum hardness of ca. 4 GPa has been observed for material with an average 0.5 m grain diameter. In large grain size material, hardness anisotropy within the grains causes significant experimental scatter in the hardness measurements because the plastic impression formed by the indenter (load 10 N and 100 N) is smaller than the grain diameter. The values ofK _c obtained using an indentation technique show that for grain sizes less than 8 mK _c decreases with decreasing grain size. For materials with a grain size in the range 500 m to 8 m, well developed median cracks were not observed, however, the radius of the fracture zone was measured in order to estimate an effectiveK _c. The effectiveK _c was found to increase approximately linearly with the reciprocal root of the grain size. Consideration of the models for elastic/plastic impact and micromechanics of crack nucleation in conjuction with the variation ofK _c andH, indicate that zinc sulphide with a mean grain size of 8 m will give the optimum solid particle and rain erosion resistance.     

Combined hardening and non-hardening embrittlement of an interstitial free steel

The ductile-to-brittle transition temperature (DBTT) and hardness of an interstitial free steel solid-solution-strengthened by phosphorus are experimentally evaluated under different conditions. The results demonstrate a linear relationship between DBTT and hardness under constant phosphorus grain boundary concentration and grain size. The DBTT increases with increasing hardness, showing a hardening embrittlement effect. With reference to a previous study concerning the effect of phosphorus boundary segregation on the ductile-to-brittle transition for the same steel, which shows a non-hardening embrittlement effect, it is seen that the two embrittlement effects are independent of each other and that the combined embrittlement effect under an average grain size of ~ 70 μm can be expressed as DBTT (°C) = 0.24H_V + 1.60C_P − 34.8, where H_V is the Vickers hardness value and C_P is the phosphorus boundary concentration in at.%.