Area Fraction Of Grain Boundaries Research Articles

Improving the strength and SCC resistance of an Al-5Mg-3Zn alloy with low-angle grain boundary structure

The strength of traditional Al-Mg alloys is relatively low because it mainly relies on solid solution strengthening. Adding a third component to form precipitation can improve their strength, but it usually leads to high-stress corrosion cracking (SCC) sensitivity due to the formation of high-density precipitates at grain boundaries (GBs). So far, it is still challenging to improve the strength of Al-Mg alloys without reducing SCC resistance. Herein, a nanostructured Al-5Mg-3 Zn alloy with a good yield strength of 336 MPa and good elongation was successfully produced. By dynamic plastic deformation and appropriate annealing treatment, near-equiaxed nanograins were introduced in the nanostructured Al-5Mg-3 Zn alloy with a high proportion (71%) of the low-angle grain boundary. TEM statistical investigations show that the precipitation of active T' phase at GBs has been greatly suppressed in the nanostructured Al-5Mg-3 Zn alloy at sensitized conditions, and the area fraction of GB precipitates is reduced from 72% to 21%, which significantly decreases the SCC susceptibility. This study provides guidance for developing advanced Al-Mg alloy with high SCC resistance.

Microstructure, Mechanical Properties, and Corrosion Behavior of Laser‐Welded Dissimilar Joints Between DP980 and QP980 Steel

Herein, fiber laser welding experiments are performed to evaluate the microstructure, mechanical properties, and corrosion behavior of dissimilar/similar joints between DP980 and QP980 steel. Results show that the fusion zone (FZ) of all joints consists of lath martensite with high microhardness (≈480 HV); the retained austenite content in the subcritical heat‐affected zone (HAZ) in the QP980 side of the dissimilar weld joint (WJ) is less than base metals (BMs). Lower hardness in the subcritical HAZ is found compared with BMs in all joints. The microstructure of softening zones is tempered martensite, and the reduction in hardness is about 8.5−10.8% of BMs, which has a great influence on the tensile fracture position of WJs. All joints fracture at softening zones with a typical feature of ductile fracture, and the joint efficiencies are above 96%. The electrochemical corrosion test reveals that the welded samples exhibit a higher corrosion rate compared with the BMs due to the increase in martensite content and area fraction of grain boundary.

Effect of repetitious retrogression and re-aging treatment on the microstructure, strength and corrosion behavior of powder hot-extruded 7055 Al alloy

The precipitation evolution, mechanical properties and corrosion behavior of powder hot-extruded 7055 Al alloy treated by repetitious retrogression and re-aging (RRA) were investigated. The results suggest that the dominant intragranular precipitates after different numbers of RRA treatment change from GP zones +η' phase (RRA1) to η' phase (RRA2, RRA3) and then to η' phase + η phase (RRA4), and both strength and corrosion resistance deteriorate with the increase of RRA repetition times. Transmission electron microscope (TEM) observation indicates that the coarsening and transformation of intragranular precipitates during repetitious RRA treatment induce the decrease of strength, while the increases in the area fraction of grain boundary precipitates (GBPs) and the width of precipitate free zones (PFZs) are detrimental for the corrosion resistance of the alloy. Therefore, the RRA1 treatment can achieve the best comprehensive performance of the powder hot-extruded 7055 Al alloy with excellent strength and good corrosion resistance.

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

This study demonstrates that the precipitation behavior of β-Mg17Al12 phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.

Correlation of grain boundary precipitate characteristics with fracture and fracture toughness in an Mg-8Al-0.5 Zn alloy

In the present work, the fracture and fracture toughness properties of the Mg-0.8Al-0.5Zn alloy have been studied. In view of this, four different aging temperatures were selected to vary the size and area fraction of grain boundary precipitates and the associated precipitate-free zones (PFZs) width. The results suggest that the precipitation state at the grain boundary plays an important role in deciding the exact mode of failure (i.e., intergranular or transgranular type). The role of precipitate characteristics in determining the fracture mode has been ascertained. Moreover, the significance of preferential deformation within PFZ regions has been evaluated using TEM. The correlation between the various microstructural features and fracture toughness value is established following the Hornbogen and Graf model. The measured values of fracture toughness are found to be in good agreement with the values predicted by the model.

Influence of orientation on corrosion behavior of directionally solidified cobalt‐base superalloy

Oxidation behavior on intersecting and longitudinal surfaces of directionally solidified cobalt‐base superalloy DZ40M was studied respectively at 1050 °C for 0.1 h, 1 h and 10 h and electrochemical property before and after oxidation on two orientations in 3.5% sodium chloride solution at room temperature was investigated. Grain boundaries of DZ40M are conducive to ion diffusion at elevated temperature and the higher area fraction of grain boundaries on intersecting surface results in higher oxidation rate and more CoO in the oxide layers. Carbides with Cr depletion zone around vulnerable to corrosion are located in grain boundaries, and unoxidized intersecting surface with higher area fraction of grain boundaries is prone to more serious uneven general corrosion. After oxidation, the corrosion resistance of the alloy is enhanced because oxide layer preferentially covers the grain boundaries and relieves the unevenness of corrosion. The protective effect of oxide layer is mainly provided by the innermost compact and continuous chromium oxide (Cr2O3). The intersecting surface has lower Ecorr and higher icorr, for the protective Cr2O3 layer on this orientation is largely consumed by excessive cobalt oxide to form CoCr2O4.

Influences of metallurgical factors on the corrosion behaviour of extruded binary Mg–Sn alloys

The corrosion behaviour of extruded Mg–(2–8wt%)Sn alloys was investigated in aqueous chloride solutions, and the influences of the volume fraction of Mg2Sn intermetallics, solutionised Sn, and area fraction of grain boundaries on the corrosion behaviour were discussed. The Sn addition to α-Mg increased the solutionised Sn content in the matrix, increased the volume fraction of Mg2Sn intermetallics to 7.3%, and changed the grain size distribution. Regarding the corrosion behaviour, the Sn addition up to 8wt% increased the overall dissolution rate of the alloys, although it promoted the passivity, which resulted from the increased H2 evolution rate and increased numbers of initiation sites for pitting corrosion. The overall corrosion behaviour primarily depended on the amount of Mg2Sn intermetallics, which promoted passivity and notably increased the H2 evolution rate, and functioned as pitting corrosion initiation sites. As the area fraction of grain boundaries increased, the H2 evolution rate was also accelerated. In contrast, the solutionised Sn decreased the H2 evolution rate.

STEM-HAADF tomography investigation of grain boundary precipitates in Al–Cu–Mg alloy

The high angle annular dark field scanning transmission electron microscopy (STEM-HAADF) tomography technique was applied to understand the three dimensional (3D) morphology and distribution characteristic of grain boundary precipitate (GBP) in peak-aged Al–Cu–Mg alloy. The results indicate that GBPs show both spherical and lenticular shapes and triangularly distribute along grain boundary (GB). Based on 3D observations from various directions, the values of GBP relevant parameters such as GBP size, center to center distance, number of GBPs per unit GB area and area fraction of GB covered by GBPs are further determined. The 3D method for GBP relevant parameter determination seems an effective way to avoid misunderstandings in the conventional two dimensional (2D) methods induced by GBP overlapping and projection effect as well as curved GB surface.

Improvement in rollability of AZ91 magnesium alloy by carbon addition

The present study aims to investigate the effect of carbon addition on the hot rolling behavior of as-cast AZ91 alloy. The AZ91 and C-added AZ91 alloys were subjected to hot rolling at 400°C with a reduction of 30% per one pass. The as-cast C-added AZ91 alloy with very fine equi-axed grains of approximately 75μm exhibited excellent hot rollability compared to as-cast AZ91 alloy with coarse dendrite structure, although the final grain size of the rolled C-added AZ91 alloy sheet was slightly larger than that of the rolled AZ91 alloy sheet. The side-crack occurrence on the surface during hot-rolling is mainly affected by the existence of twin boundary and the area fraction of grain boundaries. Based on the results, the improvement in rollability of the C-added AZ91 alloy is attributed to fine equi-axed grains and the polygonal Al8Mn5 phase located inside grains, which can homogeneously distribute and effectively absorb strain energy and prohibit crack growth.

Mechanical Behavior of AA6061 Aluminum in the Semisolid State Obtained by Partial Melting and Partial Solidification

The tensile properties of a 6061 aluminum alloy have been studied in the semisolid state at large solid fractions. The tests have been carried out either after a partial melting treatment or after partial solidification. Results show the following: (1) the mechanical behavior depends on the liquid-phase distribution and, therefore, on the way the semisolid state has been achieved (melting or solidification); (2) there is a critical solid fraction range where the semisolid alloy is relatively brittle; and (3) the mushy alloy exhibits viscoplastic behavior with the occurrence of micro-superplasticity at low strain rate. Modeling of this behavior is carried out by considering either the area fraction of grain boundaries wetted by the liquid or a cohesion parameter of the solid phase, which depends on solid fraction and thermal treatment.

Effects of high magnetic fields on the microstructures and grain boundaries in binary Al–Li alloy

The effects of high magnetic fields on the as-cast microstructure and grain boundaries of binary Al-Li alloy were investigated. It was found that secondary dendrite arm spacing (SDAS) decreased and the distribution of the SDAS became uniform under high magnetic fields. The application of a high magnetic field had no effect on the type of precipitated phases in the alloy, although delta' (Al(3)Li) precipitates coarsened under the field of 11.5 T. Area fraction of grain boundary (AFGB) slightly decreased at first and then increased with increasing magnetic flux density under uniform magnetic fields, while it decreased under magnetic field gradients. (C) 2008 Elsevier B.V. All rights reserved.

Radiation tolerance in a nanostructure: Is smaller better?

While previous experimental studies suggest that the presence of nanostructure is detrimental to the phase-transition resistance under an irradiation environment, we have recently found an opposite effect – nanostructure enhances phase-transition resistance. Here we analyze the change in free energy of an irradiated single-phase nanocrystalline material and explain the radiation tolerance (characterized by the resistance to phase-transition) in terms of two competing effects: (i) a smaller grain size tends to lower the free energy because the accumulation of point defects (mainly vacancies) in the grain interior is suppressed and (ii) a smaller grain size tends to increase the free energy because the area fraction of grain boundary is larger. For a two-phase nanocrystalline material, the heat of mixing between the two-phases often needs to be sufficiently positive so that the ion-beam mixing is avoided. Our analysis explains all previous experimental results where a nanostructure is found to either enhance or lower the phase-transition resistance.

Effect of dispersed liquid B 2O 3 particles on high-temperature mechanical behavior of copper bicrystals

Orientation controlled Cu bicrystals with dispersed B 2O 3 particles, where B 2O 3 behaves liquid-like at high temperature, were tensile tested at elevated temperatures. The fracture strain tended to decrease with increasing temperature and misorientation angle, and with decreasing strain rate. At lower temperatures where B 2O 3 behaves like a solid, the occurrence of grain-boundary fracture was understood by the difference of the time constant for saturated grain-boundary sliding to provide stress concentration sites to promote voids and cracks. Furthermore, at higher temperatures where B 2O 3 behaves liquid-like, the occurrence of grain-boundary fracture was explained by the loss of grain-boundary strength due to the increasing density of the grain-boundary area fraction of liquid particles and/or formation of liquid film at the grain boundary induced by grain-boundary sliding and migration. The liquid particles on the grain boundary seemed not to impede extensive grain-boundary sliding.

Fracture of 8090 AlLi plate I. Short transverse fracture toughness

Abstract “Low density” AlLi alloys are attractive replacements for conventional aluminium alloys but can have a low resistance to intergranular fracture under overload and creep conditions. The effects of aging treatments and testing temperature on the short transverse fracture toughnesss of AlLiCuMgZr 8090 alloy plate are reported in this paper (Part I). Studies of creep crack growth are reported in a companion paper (Part II). In Part I, double-aging treatments which substantially increase the toughness with only a small loss of strength are described in particular. Observations made using a variety of metallographic and fractographic techniques, consideration of the effects of the temperature on toughness, and determination of the activation energy for re-embrittlement of toughened material by subsequent aging suggest that toughness is mainly determined by the extent of segregation of lithium at grain boundaries. The area fraction of grain boundary precipitates and the extent of strain localization in precipitate-free zones at grain boundaries are also significant factors.