Types Of Grain Structure Research Articles

On the possibility of fabricating fully austenitic sub-micron grained AISI 304 stainless steel via equal channel angular pressing

The paper concerns possibility of increasing yield and ultimate stress of austenitic stainless steels by equal channel angular pressing (ECAP). To this end, an AISI 304 stainless steel subjected to eight passes of ECAP in a 105◦ die at 350◦ C. The evolution of microstructure as a function of induced plastic strain (number of passes) was studied in different length scales using light and scanning transmission electron microscope (STEM). An insight into fine details of dislocation structure was possible via X-ray diffraction profile analysis. Changes in the structure of processed samples were correlated with their mechanical properties as determined by tension tests and microhardness measurements. Light microscopy revealed a shear banded microstructure. STEM observations demonstrated co-existence of three types of grain structure: (a) short elongated grains/subgrains of 71 nm average width formed within the shear band regions, (b) a mixture of coarse (200–350 nm in size) and nanocrystalline (15– 70 nm) equiaxed grains developed outside the shear bands, and (c) nano-twin lamellae of 3–20 nm width inside nanocrystalline grains. XRD peak profile analysis using modified Warren–Averbach approach yielded dislocation density as ρ = 4.71 × 1015 m−2 after eight passes. ECAP-induced changes in the microstructure led to a remarkable increase in yield strength of the alloy (YS=1498 MPa, about 3.5 times the initial amount). At the same time, a moderate ductility of εf = 12% was maintained, primarily due to a large post-necking elongation. This behavior was explained by an increase in the strain rate sensitivity of ECAP-ed material in combination with the contribution from transformation induced plasticity, TRIP effect, caused by formation of ∼ 22 vol% martensite during room temperature straining.

Effect of Friction Time on the Mechanical and Microstructural Properties of AA6061 Joints by Continuous Drive Friction Welding

Friction welding is becoming a viable replacement of conventional joining methods. Continuous Drive Friction Welding (CDFW) is a type of friction welding used to join rods, tubes and similar shapes. Usually, the process contains a friction stage and a forging stage and the process parameters would be ticked accordingly. AA6061 is an Mg and Si aluminum alloy that is widely used in many industries. This research investigates the effect of friction time on the mechanical properties of AA6061 joints made with CDFW and the relation to the microstructure of the material and thermal profiles. It was found that AA6061 does not require a forging stage where solid joints are obtained without forging and did not fracture within the welding zones. Also, it was concluded that the process parameters are to be tailored in a way that produces a specific type of grain structure within the welding areas.

Effects of grain structure related precipitation on corrosion behavior and corrosion fatigue property of Al–Mg–Si alloy

An Al–Mg–Si alloy with three types of grain structure (fully, partly, and none recrystallized grains) were prepared to determine the effects of grain structure related precipitation on corrosion resistance and corrosion fatigue property in this paper. Corrosion immersion tests, corrosion fatigue tests were conducted for the purpose. Through the investigations, a mechanism different from the existent mechanisms for expounding the corrosion behavior of Al–Mg–Si alloys is proposed in this paper. It was found that low angel grain boundaries (LAGBs) with continuous Mg-Si segregation play a dominant role on the penetration depth of corrosion; while the intragranular dislocations induced precipitates determine the corrosion morphology on the surface of the sample; in addition, the high angel grain boundaries (HAGBs) and precipitation free zones (PFZs) in the periphery do not decrease the corrosion resistance of the experimental alloy in the conditions of this paper. Furthermore, a deeper penetration of corrosion rather than the severer attack of corrosion on the surface results in a shorter corrosion fatigue life, as well as a corrosion dominated fracture failure during corrosion fatigue tests.These results may supplement the existing corrosion mechanisms of Al–Mg–Si alloy.

Micromagnetic simulations of reversal magnetization in cerium-containing magnets**Project supported by the National Natural Science Foundation of China (Grant Nos. 51590882 and 51871063).

Single-grain models with different cerium contents or structural parameters have been introduced to investigate the reversal magnetization behaviors in cerium-containing magnets. All the micromagnetic simulations are carried out via the object oriented micromagnetic framework (OOMMF). As for single (Nd,Ce)2Fe14B type grain, the coercivity decreases monotonously with the increase of the cerium content. Four types of grain structure have been compared: single (Nd,Ce)2Fe14B type, core ((Nd,Ce)2Fe14B)–shell (Nd2Fe14B) type with 2 nm thick shell, core (Ce2Fe14B)–shell (Nd2Fe14B) type, and core (Nd2Fe14B)–shell (Ce2Fe14B) type. It is found that core ((Nd,Ce)2Fe14B)–shell (Nd2Fe14B) type grain with 2 nm thick shell always presents the largest coercivity under the same total cerium content. Furthermore, the relationship between the coercivity and the shell thickness t in core ((Nd,Ce)2Fe14B)–shell (Nd2Fe14B) type grain has been studied. When the total cerium content is kept at 20.51 at.%, the analyzed results show that as t varies from 1 nm to 7 nm, the coercivity gradually ascends at the beginning, then quickly descends after reaching the maximum value when t = 5 nm. From the perspective of the positions of nucleation points, the reasons why t affects the coercivity are discussed in detail.

Ultraviolet induced switchable surface wetting behavior of NiFe2O4

The surface wettability switching of nickel ferrite (NiFe2O4, NFO) thin films has been demonstrated, using a simple method via the ultraviolet (UV) irradiation process. Firstly, the as-deposited NFO thin films showed a hydrophobic wettability state with a water contact angle (WCA) value of 89 ± 1°. After a UV irradiation process for 60 min, the WCA value of as-deposited NFO was dramatically decreased from 89 ± 1° to 69 ± 1°, and then the surface free energy (SFE) was highly increased by 78.7% ranging from 18.8 to 33.6 mJ m−2. Simultaneously, the field emission scanning electron microscope images show the different type of grain structure with a sub-nanograin and micrograin type relating to the without and with post-heat treatment at 800 °C for 60 min, respectively. In addition, the X-ray diffraction patterns and optical transmittance spectra for NFO show the totally different on crystallinity and optical transmittance, and it reveals that the crystallinity and optical transmittance have been significantly enhanced with a simple post-heat treatment. In this research work, it was found the switchable wettability behavior and the corresponding SFE of NFO thin films could be changed by UV irradiation. Meanwhile, the different microstructure and optical property could be tuned via a simple post-heat treatment.

Modulation of the local grain structure in laser beam welds to inhibit the propagation of centerline hot cracks

The morphology and the size of the grains of a weld result from the solidification rate and the temperature gradient during solidification. The heat input and the velocity of the welding process directly affect these quantities. The laser power, the welding velocity and the beam diameter were periodically modulated to achieve a periodical change of the solidification conditions during welding. It leads to a continuous change of the type of grain structure over the welded length. The metallographic analysis of welds in close edge position shows the periodical interruption of centerline cracks at local changes of the grain structure.

Simulation of Centrifugal Casting and Structure of Fe-Ni-Co Super-Invar Alloy

This paper is focused on multi-scale modeling of solidification and alloy microstructure evolution through computer-aided simulation of the technological processes. Microstructure characterization of cast alloy after centrifugal casting including grain size, dendrite cell size and porosity is of interest in this paper. The centrifugal technological process was simulated using special LVMFlow program for computer-aided analysis of casting technologies to predict shrinkage porosity in axi-symmetric part blanks. The cast blanks were made of Fe-Ni-Co alloy with low thermal expansion and after turning could be used as bearing parts in contact with glass or ceramics. A method of combined simulation of technological processes and operating regimes of cast components was developed. The proposed method of simulating the complicated model allows separating the component from casting geometry in CAE postprocessor and the plot distribution of calculated parameters over the component. Also, this method allows varying diameters of hollow cylindrical casting and choosing the best location of components relative to the predicted porosity in the blank for high quality turning. The distribution of temperature and porosity could be used for more precise calculation of internal stresses in cast component under loading conditions with respect to the structural factors. The structural parameter was stated on a base of the calculated temperature fields to associate grain structure of cast alloy with solidification conditions. The austenitic microstructure of Fe-Ni-Co super-invar alloy was studied on the samples, which were cut from the axi-symmetric part after turning the blank. The calculated structural parameter was assigned with the types of grain structure.

Electrochemical behavior of Sn-Zn alloys with different grain structures in chloride-containing solutions

In the present research the electrochemical behavior of the Sn-Zn alloys (Sn-1wt.%Zn, Sn-4wt.%Zn and 8.9wt.%Zn) in 3% NaCl solution is analyzed using potentiodynamic cyclic polarization measurements and Electrochemical Impedance Spectroscopy (EIS) technique. Specimens were longitudinally solidified with simultaneous heat extraction in two opposite directions. Working electrodes were constructed using longitudinal and cross sections of the specimens with both types of structure: columnar and equiaxed.Results obtained from the polarization curves indicated that the two types of grain structures of Sn-Zn alloys (Sn-1wt.%Zn, Sn-4wt.%Zn and Sn-8.9wt.%Zn) corresponding to longitudinal section present a pseudo passive zone. In the case of specimens from cross sections of the samples, the columnar and equiaxed zones of Sn-8.9wt.%Zn are the only ones that do not have this pseudo passive region. In addition, the interdendritic zone of alloys is susceptible to corrosion by dealloying because this phase is zinc-rich. This type of corrosion also occurs in the zinc rich lamellar structure present in the eutectic. The percentage of zinc in the alloy increases with increasing susceptibility to pitting corrosion. The EIS values obtained revealed that the susceptibility to corrosion increases with increasing zinc content in alloys, for both the columnar and equiaxed zones. In addition, the columnar zones of Sn-4wt.%Zn and Sn-8.9wt.%Zn specimens are more resistant to corrosion than the equiaxed grain specimens. However, the equiaxed zone of Sn-1wt.%Zn alloy is less susceptible to corrosion than the columnar zone. After adjustment by equivalent circuits it is revealed that the equiaxed zone of Sn-8.9wt.%Zn alloy has a second porous layer composed of corrosion products on the electrode surface.

3D CAFE modeling of grain structures: application to primary dendritic and secondary eutectic solidification

A three-dimensional model is presented for the prediction of grain structures formed in casting. It is based on direct tracking of grain boundaries using a cellular automaton (CA) method. The model is fully coupled with a solution of the heat flow computed with a finite element (FE) method. Several unique capabilities are implemented including (i) the possibility to track the development of several types of grain structures, e.g. dendritic and eutectic grains, (ii) a coupling scheme that permits iterations between the FE method and the CA method, and (iii) tabulated enthalpy curves for the solid and liquid phases that offer the possibility to work with multicomponent alloys. The present CAFE model is also fully parallelized and runs on a cluster of computers. Demonstration is provided by direct comparison between simulated and recorded cooling curves for a directionally solidified aluminum–7 wt% silicon alloy.

Structure and Properties of Mg-Al-Ca Alloy after Severe Plastic Deformation

Severe plastic deformation of a Mg-Al-Ca alloy resulted in different types of grain structure. High pressure torsion (HPT) was shown to lead to the formation of a nanocrystalline structure with a grain size of 100-200 nm, while equal channel angular pressing (ECAP) produced ultrafine grained (UFG) or submicrocrystalline (SMC) structures, depending on the ECAP temperature. An UFG structure with a grain size of 2-5 -m was formed at 300°C, as distinct from a finer SMC structure with a grain size of 300-800 nm formed at a lower temperature (220°C). The possibility of increasing the strength of the alloy in the UFG condition by a factor of 1.5-2, combined with a reasonable level of ductility and enhanced functional properties was thus demonstrated. ECAP of annealed Mg-Al-Ca with the formation of UFG structure was shown to lead to increased yield strength (by a factor of 2) and enhanced tensile ductility (by a factor of 3).

Growth and Electronic Properties in Hot Wire Deposited Nanocrystalline Si Solar Cells

AbstractWe report on the growth and properties of nanocrystalline Si:H grown using a remote hot wire deposition system. Unlike previous results, the temperature of the substrate is not significantly affected by the hot filament in our system. The crystallinity of the growing film and the type of grain structure was systematically varied by changing the filament temperature and the degree of hydrogen dilution. It was found that high hydrogen dilution gave rise to random nucleation and <111> grain growth, whereas lower hydrogen dilution led to preferable growth of <220> grains. Similarly, a high filament temperature gave rise to preferential <111> growth compared to lower filament temperature. The electronic properties such as defect density and minority carrier diffusion length were studied as a function of the degree of crystallinity. It was found that the lowest defect density was obtained for a material which had an intermediate range of crystallnity, as determined from the Raman spectrum. Both highly amorphous and highly crystalline materials gave higher defect densities. The diffusion lengths were measured using a quantum efficiency technique, and were found to be the highest for the mid-range crystalline material. The results suggest that having an amorphous tissue surrounding the crystalline grain helps in passivating the grain boundaries.

Channeling effects during focused-ion-beam micromachining of copper

The rapid introduction of copper metallization for semiconductor devices has prompted increased research into focused-ion-beam micromachining of copper. Studies with the aim of increasing the material removal rate of Cu by focused-ion-beam micromachining have been complicated by variable micromachining behavior apparently resulting from differing Cu film morphologies produced by the various Cu deposition procedures. This work examined the micromachining behavior of thin copper films produced by physical-vapor deposition (PVD) and electroplating, as well as single-crystal copper samples. PVD copper films were found to be preferentially textured along 〈111〉, with a columnar grain structure. Channeling effects within this type of grain structure provide a geometric enhancement of the material removal rate of 30% when the sample normal is tilted 12° from the incident ion beam, regardless of sample rotation. Single-crystal (111) copper was found to exhibit similar material removal rate enhancement (averaged over 360° rotation) when tilted 12°, verifying that the etching enhancement observed in the PVD films is directly related to their 〈111〉 texture. Compared to the PVD film, electroplated (EP) copper thin films contained a significantly more random grain orientation. Consequently, the EP films did not exhibit any appreciable variation in material removal rate beyond the expected cosine dependence when tilted with respect to the incident Ga+ beam normal. Micromachining of the electroplated films, which have larger randomly oriented grains, results in grain decoration due to preferential etching producing severe micromachining-induced topography.

Polycrystalline Grain Structure of Sputtered Aluminum Nitride Films

ABSTRACTReactively-sputtered, polycrystalline thin film aluminum nitride (AlN) is an attractive material for use in acoustic wave devices, for which it requires a strong preferred orientation, similar to that found in epitaxial films. This investigation evaluated the grain structure including preferred orientation, grain size, and surface morphology of sputtered A1N films. The characterization techniques utilized included x-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results revealed two types of grain structure: 1) a single-grain columnar structure that is perfectly oriented in the [001] direction throughout the entire film thickness and 2) a multiple-grain columnar structure that possesses a strong [001] orientation at the bottom of the film and a tilted [001] combined with other orientations at the top of the film. Strong correlations between orientation and surface morphology, oxygen content, and grain size were observed, namely higher degrees of c-axis orientation correlated with lower mean surface roughness values, reduced oxygen concentration, and narrower grains.

Void Ratio of Noncohesive Soils and Similar Materials

A new approach to the topic of numerical description of void characteristics of noncohesive soils and similar granular materials is presented. Based upon a simple stochastic model of the void structure and void sizes, theoretical equations are derived by means of which the void ratio of a soil can be calculated from its grain‐size distribution. The calculations also give information about type of grain structure, and the grain size that separates fixed grains and possible loose grains is determined. The equations also consider grain shape, degree of densification, and size of compaction container. Results of numerous laboratory compaction tests on uniform to broadly graded sand, gravel, and crushed‐rock materials confirm the general forms of the derived equations and are the basis for evaluation of certain parameters. In a companion paper, the theory is extended to hydraulic conductivity (and, in a forthcoming paper, to filter performances).

Sintering and Microstructure of Mullite Aerogels

Mullite (3Al2O3·2SiO2) was prepared by a sol‐gel process and dried by supercritical extraction with CO2. The aerogel (<0.05 of the theoretical density of mullite) experienced a shrinkage of up to 0.6 and reached a density of only ∼0.5 of theoretical after 1 h at 1350°C Mechanically compacted aerogels, however, sintered to nearly theoretical density below 1200°C. This density is somewhat higher than those of gels prepared by conventional drying (i.e., exposure to the atmosphere) and is considerably higher than mullite prepared from mixed powders at higher temperatures. Although the X‐ray diffraction pattern of the sintered gels was almost identical to that of mullite, transmission electron microscopy and energy‐dispersive microanalysis showed two types of grain structure. Elongated grains with an Al/Si atomic ratio corresponding to that of stoichiometric mullite were surrounded by equiaxed grains with a lower Al/Si ratio.

Preparation and applications of thin film specimens containing grain boundaries of controlled geometry

A novel welding and annealing method for the convenient production of thin film specimens containing different types of controlled grain boundary structures has been developed. Bicrystals containing controlled grain boundaries in their midplane were produced by welding single-crystal thin films together face-to-face. Polycrystals containing columnar grains of only two crystal orientations were produced by annealing the previous bicrystals. Polycrystals with columnar grains of only three orientations were produced by extensions of the method in which films of three orientations were welded together and annealed. The types of grain structures which can be obtained by these techniques are described, and the potential uses of such specimens are discussed.