Intercrystalline corrosion behavior of Mg, Ag, Zn microalloalized Al-Cu-Li aluminum lithium alloy T6-state esophpathy

Aluminum lithium alloy has the characteristics of low density, ratio strength and higher than mod, can reduce the weight of aerospace structure materials, effectively improve the carrying load of aircraft, is the ideal material for the modern aerospace industry. Since the birth of aluminum lithium alloy, has developed three generations of products, including the 1980s developed three generations of aluminum lithium alloy comprehensive performance is good, with high copper and low lithium content characteristics. With the addition of microalloalized elements, the characteristics of the aesthetic properties of the microalloalized aluminum lithium alloy can be comparable with the 7-series aluminum alloy, coupled with the low density characteristics of the aluminum lithium alloy itself, so that the aluminum lithium alloy has a very broad application prospect .
the microalloying of aluminum lithium alloy is one of the effective ways to improve the properties of aluminum lithium alloy, the addition of a small amount of microalloy elements and even traces will have a significant impact on the organization and performance of aluminum lithium alloy. For example, the addition of Mg, Zn and Ag alone has different degrees of reinforcement of the Al-Cu-Li alloy. Among them, the addition of Mg microalloalization reinforcement is the strongest, and the addition of Ag microalloalization is the weakest. Mg's microalloalization reinforcement is derived from accelerating the nucleus of the GP region, thus promoting the analysis of T1 phase, while Zn's microalloalization effect is to increase the misalming of the deformation phase and the substation, which can promote the analysis of the δ phase, and also promote the analysis of the S' phase and T1 phase. Mg and Ag add at the same time can promote the dispersion of T1 phase and the S' phase of the uniform breakdown, Mg and Zn at the same time add with and add Mg and Ag similar effects.
However, the presence of al-Cu-Li aluminum lithium alloys with lively Li makes them highly localized corrosion in harsh and complex environments, which can reduce the material's electron properties and potentially shorten the normal service life of the material. Although the local corrosion sensitivity of Al-Cu-Li aluminum alloy is significantly increased compared to other aluminum alloys, it can still change its corrosion sensitivity and improve the corrosion resistance of the alloy by selecting the appropriate heat treatment process and adding microalloalized elements. It has been pointed out in the literature that the most important influence on the performance of materials in various local corrosion forms of aluminum alloy is intercrystalline corrosion, which is mainly caused by the dissolution of anodes in crystalline corrosion microcrystals. The type of microalloalized elements and the esophedic heat treatment system added will affect the morphology and distribution of the crystal boundary and the electrochemical characteristics near the crystal boundary. Most of the intercrystalline corrosion behavior of Al-Cu-Li aluminum lithium alloy in domestic and foreign literature is based on heat treatment system and less on microalloalization. Therefore, it is of great significance to study the corrosion process of Al-Cu-Li aluminum lithium alloy in the esical state and the intercrystalline corrosion behavior under the condition of microalloalization of different elements.
1 Experimental Method
Aluminum lithium alloy is prepared by casting legal system, and the ingots are deformed into aluminum lithium alloy sheets about 2 mm thick after homogenization heat treatment, hot and cold rolling. The temperature of the homogenization heat treatment is 410 degrees C and the time is 24 h. The material warms up at a temperature of 470 degrees C when hot-rolled, and hot-rolled to 5 mm thick plates are cold-rolled after de-ignition treatment. Cold-rolled sheet samples are treated with solid solubility of 1 h (solid soluble temperature 510 degrees C), water is quenched to room temperature, and solid-soluble samples are placed in a 175-degree C drying tank for T6 time-based heat treatment at different times. The chemical composition of the four Al-Cu-Li alloys of microalloalized Zn, Mg, Mg-Zn, and Mg-Ag is set out in Table 1.
intercrystalline corrosion (IGC) experiment using GB/T 7998-2005 standard implementation, the production of esophageal samples by epoxy resin package, sandpaper polishing, polishing treatment to expose the surface, alkali washing, acid wash light after the sample into the corrosion medium soaked 6 h after being taken out for test. The corrosion medium is 57 g/L NaCl plus 10 mL/L H2O2 solution. The ratio of liquid product to corrosion area of the corrosive medium is 13.5 mL/cm2. Intercrystalline corrosion immersion temperature is (35±2) degrees C. After immersion, the corrosion cross-section of the sample is polished and polished under the Leica DMILM gold phase microscope, and the type of intercrystalline corrosion is determined according to the observation, and the maximum corrosion depth and corrosion distribution range are determined. A large number of corrosion status distribution statistics were obtained at least 3 times per intercrystalline corrosion sample. Each of these statistical sections varies no less than 3 mm to ensure the validity of a wide range of statistics. The transmission mirror (TEM) sample is thinned on a dual-injection PI-type dual-injection electrolytic reducer with a control voltage of 12 V, a mixture solution of nitric acid and methanol (volume ratio of 1:3) and a cooling electrolyte temperature of -25 degrees C. The TEM observation is performed on the Tecnai220 with an acceleration voltage of 200 kV.
2 results show different corrosion characteristics after the aluminum lithium alloy after the
2.1 corrosion type
has been soaked in the corrosive medium by 6 h. One is intercrystalline corrosion, which occurs on most alloy surfaces and is represented by a continuous intercrystalline corrosion mesh (Figure 1a), which can be called general intercrystalline corrosion (general IGC). The second is that intercrystalline corrosion is still occurring, but at this point intercrystalline corrosion is limited to local areas and can be called local intercrystalline corrosion (Figure 1b). Another corrosion feature is pitting (Figure 1c), where the intercrystalline corrosion appearance is not observed. Pit erosion is different from hole erosion in that the hole corrosion opening cross-sectional area is small and the internal hole is larger, the pit erosion is the largest opening cross-sectional area and deeper and gradually smaller. The last corrosion feature is shown in Figure 1d, where both pit erosion and intercrystalline corrosion occur, and intercrystalline corrosion occurs at the edge of the corrosion pit, which is shown as intercrystalline corrosion mesh attached to the pit.
2.2 Effect of the esophedic process on corrosion types
Several corrosion patterns in Figure 1 are closely related to esophat time, and the emerging caused time points of corrosion types should have some correlation with intercrystalline corrosion patterns. In addition, elemental microalloalization also affects the correlation between intercrystalline corrosion types and esophostical processes. Figure 2 shows a time-hardening curve of the 2-alloy (including Zn) and a typical cross-sectional corrosion photo after different time-of-time. It can be seen that the 2-year-old alloy reaches the peak esophonal state after the 28 h edict, and the esophedic effect speed is slow. As can be seen from the intercrystalline corrosion profiling image, local intercrystalline corrosion occurs at 0.5 h (corresponding to the initial estration period), 4 h (under-time) is characterized by comprehensive intercrystalline corrosion characteristics (Figure 2c), while the estration 28 h (peak ergoncation) has intercrystalline corrosion characteristics at the edge of pit erosion and etching holes (Figure 2d). At that time, the effective time reached 120 h, and the alloy corrosion cross-section was shown as pit erosion (Figures 2e and f).
of a typical cross-sectional corrosion appearance of the 3-inch alloy (including Mg) and the typical cross-sectional corrosion after different time-of-time periods is shown in Figure 3. The alloy reaches the peak tense state at 12 h, and the esophedic response rate is fast. It can be seen from the intercrystalline corrosion morphology photos that the estration of 0.5 h (corresponding to the initial estheat) is characterized by local intercrystalline corrosion (Figure 3b), while the estration of 4 h (under-time), the alloy surface is still full intercrystalline corrosion, but the area of intercrystalline corrosion is reduced (Figure 3c);
figure 4 shows the time-hardening curve of the 4-plus alloy (including Mg-Zn) and the typical cross-sectional corrosion appearance after different time-of-time. It can be seen that the hardening curve of the 4-alloy is basically the same as that of the 3-alloy, which reaches the peak tense after 12 h and has a faster esophedic response rate. At the same time, the corrosion pattern evolution of the 4-alloy is basically the same as that of the 3-alloy (Figure 3).
of the 5-plus alloy (including Mg-Ag) and the typical cross-sectional corrosion deformation after different time-of-age periods can be found in Figure 5. The 5-year alloy reaches its peak after 12 h and has a faster erration response rate. Intercrystalline corrosion patterns are: 0.5 h (corresponding to the initial erythration period), the emergence of comprehensive intercrystalline corrosion (Figure 5b), the erration 4 h (under-time), the alloy surface is still shown as comprehensive intercrystalline corrosion, but the intercrystalline corrosion area has been reduced (Figure 5c); The erration time is further extended to 36 h or even 120 h, and the alloy surface is re-represented as local intercrystalline corrosion (Figure 5e) and full intercrystalline corrosion (Figure 5f).
four types of microalloalized Al-Cu-Li aluminum lithium alloy corrosion type, intercrystalline corrosion average depth, intercrystalline corrosion depth, pitting average depth, pit erosion most depth statistics can be found in Tables 2 and 3. In general, these four Mg, Ag, Zn microalloalized Al-Cu-Li aluminum lithium alloys generally show two basic intercrystalline corrosion morphological characteristics. The corrosion morphological process of the three microalloalized Al-Cu-Li aluminum lithium alloys, including Zn, Mg and Zn-Mg, is basically the same, showing the development process of intercrystalline corrosion patterns related to the escical system process. The process of intercrystalline corrosion is: local intercrystalline corrosion, comprehensive intercrystalline corrosion, pit corrosion, pit corrosion and accompanied by weak local intercrystalline corrosion. Among them, the pit corrosion with Zn alloy and the time range with weak local intercrystalline corrosion is shorter than the other two, which shows that its intercrystalline corrosion sensitivity is weaker than that of the other two alloys, and shows slightly stronger intercrystalline corrosion resistance, indicating that adding Zn alone can improve the intercrystalline corrosion resistance of the alloy and reduce its intercrystalline corrosion sensitivity. However, unlike the corrosion morphological features of the first three alloys, the intercrystalline corrosion morphological process of al-Cu-Li aluminum lithium alloy containing Mg-Ag microalloalization is: comprehensive intercrystalline corrosion, pit corrosion and accompanied by obvious local intercrystalline corrosion, comprehensive intercrystalline corrosion.
figure 6 is a microstructure of the inner and crystal boundaries in the T6 peak aging (28 h) state of the alloys of 3 , 4 and 5 . Only the Mg-containing 3-alloy crystal boundary has a large size of T1 phase non-continuous analysis (Figure 6a), the crystal has T1 phase and S' phase analysis (Figure 6b). As can be seen from Figures 6c and d, there are a large number of small dispersion distributions in the T1 phase in the 4-plus alloy crystal containing Mg-Zn, accompanied by a small number of S'phases, and a large number of seine T1 phase distributions at the crystal boundary. The photo of the micro-organization of the inner and crystal boundaries of the 2-plus alloy containing only Zn is basically consistent with the micro-organization characteristics of the 4-plus alloy containing Mg-Zn, and will not be repeated here. There are a large number of T1 phases with a small dispersion distribution in the 5-plus alloy crystals containing Mg-Ag, accompanied by a small amount of S'phase (Figure 6f), while at the crystal boundary there is a small continuous distribution of T1 phases (Figure 6e).
The type and morphology of the desced phase in the Al-Cu-Li aluminum lithium alloy are closely related to the Cu and Li content (Cu/Li ratio) in the alloy, and the aluminum lithium alloy with al-3.2Cu-1.2Li-xMg-yZn-zAg content studied in this paper mainly desceds the phase T1 phase and there are a small number of S' phases. According to the chemical subtation of the second phase in the Al-Cu-Li alloy as already documented, the t1 phase has a capacitance of -1.089 VSCE, which is more negative than the pure Al-power (-0.746 VSCE). There is a bit difference between the T1 phase and the unmodulated precipitation belt (PFZ), which can cause the crystal boundary to continuously precipitate the priority dissolving of T1 phase, and the adhoal dissolution of the continuous T1 phase at the crystal boundary can reduce the corrosion resistance between crystals. Compared to PFZ (-0.612 VSCE) with pure Al potential, PFZ dissolves first in corrosive microcables formed by Al2Cu phase and PFZ.
Mg-Ag at the same time add changes to the alloy ergonomic analysis process, can promote the T1 phase fine uniform dispersion in the crystal and crystal boundary, the continuous T1 phase of the crystal boundary can affect the crystal boundary phase and the non-tissue precipitation belt between the chemical level distribution state, and the author believes that Ag's addition can also change the crystal boundary distribution T1 phase of the chemical composition and thus affect the crystal boundary corrosion microcillulation process in the microcrystal. Previously carried out on the addition of alloy elements to affect the crystalline phase of the chemical characteristics are mainly for Mg-Li-empty clusters, Ag replaced part of cu thus changed the chemical composition of T1 phase, resulting in crystallography and the position characteristics of the crystal boundary containing Cu intermediate phase changes, thereby changing its macrocrystalline corrosion behavior characteristics. The intercrystalline corrosion mechanism of the crystal boundary T1 phase is that the lively Li priority dissolves in the T1 (Al2CuLi) phase, causing the negative PFZ bit in the crystal boundary corrosion microcity formed by Al2Cu and PFZ to dissolve first, and forming a continuous wide corrosion dissolution channel, which leads to the occurrence of intercrystalline corrosion behavior, which is consistent with the literature results. However, a series of corrosion mechanisms, such as how the addition of Mg-Ag affect the formation of atomic clusters and the crystalline phase distribution and chemical characteristics, need to be studied in depth.
3 Conclusions
(1) Mg, Zn, Ag microalloalized Al-Cu-Li aluminum lithium alloys show four different forms of intercrystalline corrosion behavior in T6 escical states, including comprehensive intercrystalline corrosion, local intercrystalline corrosion, pit erosion, and pit erosion, accompanied by intercrystalline corrosion.
(2) Mg, Zn, Mg-Zn microalloalized Al-Cu-Li aluminum lithium alloy with the extended performance of the olecation time of the intercrystalline corrosion trend is consistent, as follows: the initial local intercrystalline corrosion, the under-time phase of comprehensive intercrystalline corrosion, peak olescence is manifested as pit erosion, obsolete effect pit erosion and accompanied by trace intercrystalline corrosion. Among them, the intercrystalline corrosion resistance of the lithium aluminum alloy of the Al-Cu-Li system, which adds Zn separately, is slightly greater than that of the other two alloys.
(3) Mg-Ag microalloalized Al-Cu-Li aluminum lithium alloys exhibit intercrystalline corrosion morphological processes that differ from the other three alloys, representing local intercrystalline corrosion or comprehensive intercrystalline corrosion from olex to obsolescence, and pitting and accompanying intercrystalline corrosion at peak olescence.
(4) The different intercrystalline corrosion morphology mechanism of the aluminum lithium alloy of the Mg-Ag microalloalized Al-Cu-Li system lies in the fact that Mg-Ag simultaneously adds a change in the alloy erythro-time analysis process, which can promote the T1 phase analysis of the inner and crystal worlds. It is pointed out that the continuous T1 phase of crystal boundary and the tissue precipitation belt PFZ near the crystal boundary have a large bit difference causing anode dissolution, and Ag can affect the continuous T1 phase electrochemical characteristics of crystal boundary, resulting in a wider time point range of intercrystalline corrosion.