Grain Boundary Precipitation Research Articles

Effect of Ti Microaddition on Cavitation Behavior During Uniaxial Hot-Tensile of Fe-22Mn-1.5Al-1.3Si-0.5C Austenitic TWIP Steel

ABSTRACTIt is well-known that metal and alloys develop internal cavities when subjected to uniaxial or multiaxial tensile strains at elevated temperature. In most cases, cavitation may lead to premature failure during forming. Therefore, damage and fracture behavior imposes significant limitations in hot metal-forming processes. Although high-Mn austenitic TWIP steels exhibit a unique combination of strength and ductility, cavitation during hot working is one issue that must be tackled. The aim of this research work is to determine the effect of Ti microaddition on cavity mechanisms of Fe-22Mn-1.5Al-1.3Si-0.5C TWIP steel under uniaxial hot-tensile condition at 800 °C and constant true strain rate of 10-3 s-1. For this purpose, light optical (LOM) and scanning electron (SEM) microscopies and image analysis were applied to quantify cavities formation along longitudinal section of deformed samples near to the fracture surface. The number of cavities greater than 10 µm (critical length) in non-microalloyed and Ti microalloyed TWIP steels were 2.75 and 3.75 cavities/mm2, respectively. On the other hand, average cavity area was 125 and 152 µm2, respectively. Both TWIP steels showed cavities type “r”, “l” and “A”. Finally, Ti microaddition to TWIP steel resulted in a predominant brittle fracture behavior due to finer grain-boundary precipitation, which weakens grains cohesion and accelerates crack growth by grain-boundary sliding. In this case, crack growth behavior is explained in terms of a void interconnection mechanism.

EVALUATION OF INTERGRANULAR CORROSION SUSCEPTIBILITY OF 11Cr FERRITIC STAINLESS STEEL BY DL-EPR METHOD

Ferritic stainless steel (FSS) containg (11%~13%)Cr with low C and N has excellent comprehen- sive performances and thus can be widely applied in extensive fields such as automobile exhaust systems, contain- ers, buses and so on. Among them, 409L steel containing 11%Cr has been increasingly used in applications for tail pipes in the cold end parts of automobile exhaust systems because of its good corrosion resistance and moderate price. During the manufacture process for these tail pipes, improper heat treatments and welding operations cause the precipitation of some detrimental phases such as carbides, nitrides, which leads to a reduction on the resistance to intergranular corrosion (IGC) due to the presence of Cr-depleted zone in the grain boundaries. In this work, the precipitates in grain boundaries of 409L steel aged at 600 ℃ were investigated using TEM, EDS and SAED. The

Comments on “Electropulsing induced evolution of grain-boundary precipitates without loss of strength in the 7075 Al alloy” by Xu et al. [Materials Characterization 105 (2015) 90–94

This letter attempts to comment on the article by Xu et al. [Materials Characterization 105 (2015) 90–94] in the aspects about the sizes of matrix precipitates (MPts) and Cr-bearing dispersoids in 7075 Al alloy.

Significantly enhanced the ductility of the fine-grained Al–Zn–Mg–Cu alloy by strain-induced precipitation

A fine-grained structure of Al–Zn–Mg–Cu alloy was produced by a successive two-step deformation (STD) process based on strain-induced precipitation (SIP). The fine-grained alloy treated by the STD process exhibited significantly superior tensile ductility than the conventional hot-deformed (CHD) alloy. Effects of the STD process on microstructure and mechanical properties were investigated, in conjunction with fracture characterizations. Numerous spherical precipitates and dense dislocations were induced by the SIP at 300°C. A fine lamellar structure was formed during subsequent heating and hot deformation of the STD process, finally contributing to the fine-grained T6-aged alloy. Due to the fine-grained structure, more dimples in the fracture surface of the STD treated alloy were produced than those of the CHD treated alloy. TEM in-situ testified that the grain boundary precipitates (GBPs) originated the initiation of micro-cracks, and the cracks propagated along the (sub)grain boundaries during the tensile loading. The initiation and propagation of micro-cracks were explained in terms of grain boundary precipitates (GBPs), precipitation free zones, and grain refinement. Although initiation and propagation of the cracks easily occur to coarse GBPs and grain boundaries, the fine-grained structure obtained by the STD treatment could effectively delay these behaviors and improve mechanical properties.

Microstructure, mechanical properties and stress corrosion cracking of Al–Zn–Mg–Zr alloy sheet with trace amount of Sc

Microstructural and property evolution of the Al–Zn–Mg–0.10%Sc–0.10%Zr alloy sheet during its preparation were investigated in detail by means of optical microscopy (OM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Vickers micro-hardness test and room temperature tensile test. Stress corrosion cracking (SCC) behavior of the Al–Zn–Mg–0.10%Sc–0.10%Zr alloy under different heat treatments was studied using slow strain rate test. The results showed that serious dendritic segregation existed in as-cast condition. The suitable homogenization treatment for Al–Zn–Mg–0.10%Sc–0.10%Zr alloy was 470°C/24 h. After homogenization treatment, dissoluble Zn and Mg enriched non-equilibrium phases dissolved into α-Al matrix completely. The suitable solid solution-aging treatment for Al–Zn–Mg–0.10%Sc–0.10%Zr alloy was solution treated at 470 °C for 60 min, followed by water quenching and then aged at 120 °C for 24 h. Under this aging temper, the grain structures were composed of sub-grains, ηʹ phases and nanometer-sized, spherical Al3(Sc, Zr) particles. Grain boundary precipitates (GBPs) area fraction was found to be an important parameter to evaluate the SCC susceptibility. The improved corrosion resistance from increasing aging temperature or prolonging aging time was due to the discontinuous η precipitates along the grain boundary and the high area fraction of GBPs. The main strengthening mechanisms of Al–Zn–Mg–0.10%Sc–0.10%Zr alloy are precipitation strengthening derived from ηʹ precipitates, dispersion strengthening, sub-grain strengthening and grain refinement caused by coherent Al3(Sc, Zr) particles.

Tempering effect on the localized corrosion of 13Cr4Ni0.02C and 13Cr1Ni0.15C steels in a synthetic marine environment

Primarily, tempering is used in martensitic stainless steels with low carbon in order to change their mechanical properties. However, it also modifies the corrosion properties; usually depending on the temperature and duration of the heat treatment. This work investigated the impact of tempering on the corrosion of 13Cr4Ni0.02C and 13Cr1Ni0.15C (% weight) steels. Samples were tempered at 650o, 700o and 750oC, characterized by optical microscopy and cyclic anodic polarization tests in a synthetic marine environment. The results showed differences in the microstructure of the 13Cr4Ni0.02C tempered samples, which changed from lath to lamellar with precipitates in the prior austenite grain boundaries. On the other hand, tempering did not affect the 13Cr1Ni0.15C microstructure. The anodic polarization tests showed that the pitting potential of the 13Cr4Ni0.02C samples decreased with the tempering temperature increasing. Similar values of pitting potential were found with the 13Cr1Ni0.15C tempered samples, which were lower than the 13Cr4Ni0.02C samples after tempering. It shows that even though the corrosion resistance of the 13Cr4Ni0.02C reduces when the tempering temperature was increased, it is better than that presented by 13Cr1Ni0.15C.

A study of the influence of precipitate-free zones on the strain localization and failure of the aluminium alloy AA7075-T651

Age-hardened aluminium alloys have various degrees of precipitate-free zones (PFZs) along grain boundaries (GBs). The PFZs are weak zones and their existence promotes combined transgranular and intergranular fracture, thus reducing the ductility of the alloy. In this study, transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of the PFZs in the AA7075-T651 aluminium alloy. PFZs are found along grain and sub-GBs and their widths are about 40 and 20 nm, respectively. The PFZs are depleted of alloying elements compared with the nominal composition due to GB precipitation, but still they contain a certain amount of such elements in solid solution which will contribute to increase the yield strength and the work hardening compared to pure aluminium. Based on the results from the TEM study, a micromechanical finite element model of an idealized microstructure including grains and soft zones along the GBs is established. The Gurson model was used to represent the behaviour of the material in the grains and in the soft zones, using different initial void volume fractions to account for GB precipitation. Several loading conditions were applied to the micromechanical model in order to evaluate the localization of strains inside the soft zones and thus to get a better understanding of the role of the PFZs in ductile fracture of age-hardened aluminium alloys. It was found that the global failure strain varies non-monotonically with the global stress triaxiality due to the heterogeneity of the idealized microstructure.

Electropulsing induced evolution of grain-boundary precipitates without loss of strength in the 7075 Al alloy

The influence of electropulsing on the peak-aged (T6) microstructures in the 7075 Al alloy was investigated in this study. The results show that parts of the intragranular precipitates dissolve in the matrix during electropulsing, and the electropulsing treated sample exhibits discontinuous dissolution of the grain-boundary precipitates (GBP). After re-aging, the mechanical properties of the re-aged (RA) sample are nearly the same as that of T6 temper, meanwhile, the grown discontinuous GBP were obtained. Hence it is a promising method to change the distribution of GBP without loss the strength in the 7075 Al alloy.

Exfoliation corrosion of an extruded low-Cu-containing AA7003-T6 profile with a partially recrystallised grain structure

The effect of a partially recrystallised grain structure on the exfoliation corrosion susceptibility of a peak aged (T6) extruded low-Cu-containing AA7003 profile was evaluated using the ASTM G34 standard test method coupled with a metallographic examination. Both the as-extruded (recrystallised) and milled (non-recrystallised) starting surfaces were assigned the same ASTM G34 visual classification of EB (moderate exfoliation corrosion) after 48 h immersion. Damage initiated on the as-extruded starting surface as pitting corrosion in the recrystallised coarse-grained surface layer and propagated as exfoliation corrosion in the non-recrystallised fibrous interior once the surface layer was penetrated. A complementary scanning transmission electron microscopy (STEM) examination suggested that there was not likely a marked difference in the distribution of Mg, Zn and Cu across the precipitation free zone (PFZ) and grain-boundary precipitates between the recrystallised and non-recrystallised grain structures. This indicates that the transition from pitting corrosion to exfoliation corrosion in the peak aged AA7003 alloy was controlled more so by the variation in some other microstructure parameter across the grain-boundary region and less so by the variation in the microchemistry (Cu content in particular).

Influence of alloyed Sc and Zr, and heat treatment on microstructures and stress corrosion cracking of Al–Zn–Mg–Cu alloys

Abstract Stress corrosion cracking (SCC) behavior of Al–Zn–Mg–Cu alloys with different Sc, Zr contents and heat treatments was studied using slow strain rate test. Grain boundary microstructures were identified by transmission electron microscopy (TEM) and statistical analysis. It was found that the SCC resistance of alloys is improved by increasing Sc, Zr contents and aging degree. Grain boundary precipitates (GBPs) area fraction was found to be an important parameter to evaluate the SCC susceptibility. The results reveal that for Al–Zn–Mg–Cu–0.25Sc–0.10Zr (wt%) alloy with different aging degrees, hydrogen induced cracking dominates the SCC when the area fraction of GBPs is relatively low. For peak-aged Al–Zn–Mg–Cu alloy and Al–Zn–Mg–Cu–0.10Sc–0.10Zr (wt%) alloy, anodic dissolution dominates the SCC when the area fraction of GBPs is sufficiently high.

PRE-AGING OF RETROGRESSION AND RE-AGINGOF SPRAY FORMED 7075 ALLOY

Retrogression and re-aging(RRA) treatments are divided into pre-aging, retrogression and re-aging. Although peak aging was used as the pre-aging of RRA treatment in the past, some different opinions were reported in recent years. The effects of pre-aging of RRA treatment on microstructure, mechanical properties and conductivity of spray formed 7075 aluminum alloy were investigated by TEM, tensile and conductivity test. The results show that the mechanical properties and conductivity of spray formed 7075 alloy could be improved by the under aging at 120 ℃ for 16 h as the pre-aging of RRA treatment. The properties of the 7xxx series aluminum alloys depend on matrix precipitates(MPt), grain boundary precipitates(GBPs) and precipitate free zones(PFZs).The tiny MPt can increase the tensile strength. The connected GBPs and narrow PFZs will lower the conductivity and the elongation of the alloy. The under aging is more beneficial for the re-dissolution of the MPts at retrogression treatment at 200 ℃ for 10 min, and is more conducive to interrupt distributions of the GBPs than the early aging after RRA treatment. With the under aging as the pre-aging treatment, the growth of the MPts was actively suppressed and the GBPs at grain boundaries are continuous. During retrogression treatment, the MPts were re-dissolved absolutely and the GBPs were transformed from a chain to dissociation. After re-aging treatment, lots of tiny dispersive MPts precipitated out again in matrix, the GBPs were totally separated and the PFZ were widened.After pre-aging at 120 ℃ for 16 h and RRA treatment, the tensile strength and yield strength of the alloy are 782and 726 MPa, respectively, which are higher than that after peak aging treatment or conventional RRA treatment,the conductivity of the 7075 alloy is excellent with 22.7 MS/m.

Non-isothermal retrogression kinetics for grain boundary precipitate of 7A55 aluminum alloy

The retrogression kinetics for grain boundary precipitate (GBP) of 7A55 aluminum alloy was investigated by transmission electron microscopy (TEM) observation. The results reveal that the coarsening behavior of GBP obeys “LSW” theory, namely, the cube of GBP average size has a linear dependence relation to retrogression time, and the coarsening rate accelerates at the elevated retrogression temperature. The GBP coarsening activation energy Qd of (115.2±1.3) kJ/mol is obtained subsequently. Taking the retrogression treatment schedule of 190 °C, 45 min derived from AA7055 thin plate as reference, the non-isothermal retrogression model for GBP coarsening behavior is established based on “LSW” theory and “iso-kinetics” solution, which includes an Arrhenius form equation. After that, the average size of GBP r(t) is predicted successfully at any non-isothermal process T(t) when the initial size of GBP r0 is given. Finally, the universal characterization method for the microstructure homogeneity along the thickness direction of 7A55 aluminum alloy thick plate is also set up.

The impact of aging pre-treatment on the hot deformation behavior of alloy 800H at 750 °C

The influence of aging pre-treatment on the hot deformation of a commercial alloy 800H was investigated through uniaxial compression tests. Aging pre-treatments were performed at 750°C for 0h, 5h, 10h, 20h and 50h, followed by compression tests at 750°C with strain rates of 0.01s−1, 0.1s-1 and 1s−1. The flow curves signified that the peak stress decreased evidently with increasing aging time. The microstructural analysis of alloy 800H after aging at 750°C for 50h indicated that the predominant precipitates are block-shaped Cr23C6 and cube-shaped Ti(C,N). The formation of grain-boundary Cr23C6 results in the segregation of Cr and C with the depletion of Ni at the grain boundaries. The kernel average misorientation maps after hot deformation demonstrates that the grain-boundary precipitates induce the pinning force to change the distribution of local misorientation and two different deformation patterns were defined to characterize the substructure developed near the grain boundaries.

English

The corrosion properties of duplex stainless steel type SAF 2205 after long-term ageing at 300°C and 400°C were investigated. However, these steels show a degradation of their properties as submitted by long time, in low temperatures about 280 to 520°C. This work describes this behavior by analysis of the resistance to general corrosion of the SAF 2205 steel ageing in the temperatures 300°C and 400°C, during 2000, 3000 and 5000 h, for sample aged at 400°C for 5000 h. The experimental techniques used were the potential dynamic curves (EPR-Double Loop), scanning and transmission electronic microscopy for microstructure analysis and weight loss test. It is verified that the ageing treatments modify the corrosion resistance due to microstructural changes of precipitation in the matrix and not by precipitation in grain boundaries (sensitization). However the sample aged at 400°C for 5000 h, due to the precipitation of phases G and α’, showed an appreciable increase in the loss of mass. Key words: ageing, precipitation, corrosion, weight loss, EPR.

Correlations between stress corrosion cracking susceptibility and grain boundary microstructures for an Al–Zn–Mg alloy

The correlations between grain boundary precipitates (GBPs) area fraction and stress corrosion cracking (SCC) susceptibility for an Al–Zn–Mg alloy have been studied using TEM statistical investigations and image analysis. The results reveal that SCC susceptibility of the Al–Zn–Mg alloy first decreases and then increases with the increase of GBPs area fraction, which does not agree with Dix’s anodic dissolution theory, suggesting that SCC of the Al–Zn–Mg alloy is dominated by hydrogen induced cracking when GBPs area fraction is relatively low and is dominated by anodic dissolution when GBPs area fraction is relatively high.

Microstructural Evolution and Precipitation Mechanism of α-Phase in Ti-55531 Alloy Aged at High Temperature

After two-stage annealing heat treatment process, the near β-Titanium alloys reveal a mixed microstructure containing lath-like α phase and finer acicular α phase in β matrix, leading to the improvement of strength-ductility balance. In this paper, the microstructural evolution and the behaviour of α precipitate during high temperature ageing process were investigated by SEM in a near β-Titanium alloy called Ti-55531.The relationship between α precipitates and the β grain orientation in high temperature was investigated by EBSD. The results show that the α-phase precipitated only at some places of the β grain boundaries at higher ageing temperature (~780 °C); the amount of grain boundary α increased with the decreased of the ageing temperature; after ageing at 720 °C for 45 min, we found that the α-phase precipitated not only at grain boundaries but also within the grains. It seems that the precipitation of grain boundary α is strongly influenced by β grain boundary energy which means that grain boundary α tends to form preferentially at high energy grain boundaries (high-angle grain boundaries); The α-phase precipitates more easily at the grain boundaries where the {110} plane of adjacent β grains have the same orientation.

Quasi in situ analytical TEM to investigate electrochemically induced microstructural changes in alloys: AA2024-T3 as an example

A quasi in situ method using a specially developed holder is presented to follow electrochemically induced microstructural changes on a nanometre scale using analytical transmission electron microscopy (TEM). By following a specific specimen area after repeated exposure to oxygen bubbled through aqueous HCl of pH=3, de-alloying of a grain-boundary precipitate of ∼200nm was observed by monitoring the changes in the EELS copper L2,3-edge and EFTEM elemental maps. These results, supporting the existing models of de-alloying of copper rich precipitates in aluminium alloys, demonstrate the effectiveness of the quasi in situ specimen holder for analytical TEM studies of corrosion.

Finite Element Simulation of Carbide Precipitation in Austenitic Stainless Steel 304

A three dimensional transient finite element model with Gaussian heat source was proposed to predict the heat transfer in Gas Tungsten Arc Welding (GTAW) of stainless steel 304 and experimental tests were conducted to study chromium carbide precipitation and to verify the developed model. The position of chromium carbide precipitation band and its relation to welding parameters were studied. The results showed, increasing the heat input of weld, increases the distance in which precipitation in austenite grain boundaries occurs. After measurement of carbide-band distance from weld centre and using obtained time-temperature curves of the developed model, the distance of carbide-band from the weld centre line was simulated. Finally, good agreement was observed between experimental and simulation results in terms of prediction of thermal history and carbide precipitation.

Influence of grain-boundary pre-precipitation and corrosion characteristics of inter-granular phases on corrosion behaviors of an Al–Zn–Mg–Cu alloy

► Grain boundary pre-precipitation enhances distribution discontinuity of the precipitates. ► Increase of Cu content of grain-boundary precipitates. ► Potentials differences among inter-granular phases obviously decrease after pre-precipitation. In this paper the influence of high-temperature pre-precipitation treatment on corrosion behaviors of 7055 aluminum alloy is investigated. High-temperature pre-precipitation treatment enhances the discontinuing distribution of the coarse precipitates along the grain boundary, and increases the Cu content of grain-boundary precipitates, which obviously improves stress corrosion resistance, suppresses the serve fracture of sub-grain boundary under the stress corrosion and reduces inter-granular corrosion (IGC) susceptibility with the same strength and ductility. The electrochemical corrosion results of inter-granular phases indicate that the Cu-rich, discrete and coarse grain-boundary precipitates and the decreasing potential difference among inter-granular phases in the grain boundary are responsible for the enhancement of corrosion resistance of 7055 alloy.

Influence of dual retrogression and re-aging temper on microstructure, strength and exfoliation corrosion behavior of Al–Zn–Mg–Cu alloy

Influence of dual retrogression and re-aging (dual-RRA) temper on microstructure, strength and exfoliation corrosion (EC) behavior of Al–Zn–Mg–Cu alloy was investigated by hardness measurements, tensile properties tests, exfoliation corrosion tests, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) observation combined energy dispersive X-ray detector (EDX) analysis. Dual-RRA temper maintains the matrix precipitates (MPs) similar to RRA temper, meanwhile obtains∼coarser and sparser grain boundary precipitates (GBPs) as well as higher∼Cu and lower Zn content compared with T76 temper. Therefore, dual-RRA temper not only keeps strength equivalent to the RRA temper but also obtains higher EC resistance than T76 temper.