Effect of sampling direction on the stress corrosion cracking behavior of Al-Zn-Mg alloy
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摘要: 采用恒載荷拉伸應力腐蝕試驗和電化學試驗研究取向對Al-Zn-Mg合金型材的應力腐蝕(SCC) 開裂的影響, 腐蝕介質采用質量分數3. 5%的Na Cl溶液, 容器溫度維持在50±2℃, 并通過光學金相顯微鏡(OM)、掃描電子顯微鏡(SEM)、電子背散射衍射(EBSD) 等研究不同取向試樣應力腐蝕前、后的微觀形貌.結果表明橫向試樣在315 h時斷裂, 而縱向試樣在整個加載過程中未發生斷裂, 縱向試樣有更好的抗應力腐蝕開裂性能; 縱截面(L-S面) 的腐蝕電流密度為0. 980 m A·cm-2, 約為橫截面(T-S面) 的5倍, 腐蝕傾向于沿擠壓方向發展; 相比T-S面, L-S面晶粒間取向差較大, 大角度晶界多, 容易被腐蝕產生裂紋; 在應力腐蝕加載過程中, 試樣先發生陽極溶解, 形成腐蝕坑, 聚集的腐蝕產物所產生的楔入力和恒定載荷的共同作用促使裂紋在腐蝕介質中加速擴展, 兩種取向試樣均發生了明顯的晶間腐蝕, 存在應力腐蝕開裂的傾向.
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關鍵詞:
- Al-Zn-Mg合金 /
- 取樣方向 /
- 應力腐蝕開裂 /
- 陽極溶解 /
- 晶間腐蝕
Abstract: Thick-section Al-Zn-Mg aluminum alloy extrusions are key materials for manufacturing rail transit vehicles, and stress corrosion cracking (SCC) is an important engineering application problem during the service life of these materials. The effect of sampling direction on the stress corrosion cracking behavior of Al-Zn-Mg alloys was investigated through constant load tensile stress corrosion and electrochemical tests. The microstructures of specimens were analyzed in different sampling directions both before and after stress corrosion via optical microscopy, scanning electron microscopy, and electron backscatter diffraction. Specimens with their tensile axes parallel or perpendicular to the extrusion direction of the extruded profiles were labeled as longitudinal specimens and transverse specimens, respectively. The specimens were completely immersed in a corrosive solution, a mixture of 35 g Na Cl and 1 L deionized water, with a constant unidirectional loading of 225 MPa for 360 h at 50 ± 2 ℃. The experimental results show that the transverse specimen is fractured at 315 h, whereas the longitudinal specimen does not break during the entire loading process. Thus, the transverse specimens have poor resistance to stress corrosion cracking. The corrosion current density of the longitudinal section (L-S) is0. 980 m A·cm-2, which is approximately 5 times that of the transverse section (T-S). Thus, corrosion tends to propagate along the longitudinal direction. The L-S is more susceptible to corrosion than the T-S owing to the larger misorientation difference and higher energy of the grain boundary. During the stress corrosion loading process, anodic dissolution occurs and forms corrosion pits. Then, the cooperation of the wedge force produced by the accumulation of corrosion products and constant load causes the crack to propagate along the grain boundary. Intergranular corrosion of the two types of samples is obvious under all immersion corrosion conditions. Different specimens exhibit the tendency to undergo stress corrosion cracking. -
圖 1 試驗尺寸(a) 和取樣圖(b)
Figure 1. Specimen dimensions (a) and schematic of the sample processing modes (b)
圖 2 金相顯微組織照片. (a) L-S面; (b) T-S面
Figure 2. Optical micrographs: (a) longitudinal section (L-S); (b) transverse section (T-S)
圖 3 Al-Zn-Mg合金的掃描照片. (a) L-S面; (b) T-S面
Figure 3. SEM images of the Al-Zn-Mg alloy: (a) L-S; (b) T-S
圖 4 Al-Zn-Mg合金型材的透射電鏡明場像. (a) 晶內; (b) 晶界; (c) 衍射斑點
Figure 4. Bright-field TEM images of the Al-Zn-Mg alloy: (a) intragranular; (b) grain boundary; (c) diffraction spots
圖 6 L-S面和T-S面電子背散射衍射測試結果. (a, b) 取向成像圖; (c, d) 晶粒尺寸分布圖; (e, f) 取向差分布圖; (g; h) 再結晶組織統計柱狀圖
Figure 6. EBSD results of L-S and T-S surfaces: (a, b) orientation image map; (c, d) grain size distribution; (e, f) misorientation distribution; (g, h) recrystallization fraction chart
圖 7 試樣斷口側面金相. (a) 縱向試樣L-S面; (b) 橫向試樣T-S面; (c) graff試劑腐蝕后縱向試樣的L-S面; (d) graff試劑腐蝕后橫向試樣的T-S面
Figure 7. Optical microscopy images of the specimen fracture: (a) L-S surface; (b) T-S surface; (c) L-S surface corroded by graff reagent; (d) T-S surface corroded by graff reagent
圖 8 試樣斷口形貌照片. (a) 縱向試樣斷口宏觀形貌; (b) 橫向試樣斷口宏觀形貌; (c) 縱向試樣斷口腐蝕坑形貌; (d) 橫向試樣腐蝕區域低倍形貌; (e) 縱向試樣未腐蝕區域形貌; (f) 橫向試樣腐蝕區域A位置的高倍照片
Figure 8. SEM images of the fractography: (a) macro morphology of longitudinal specimen; (b) macro morphology of transverse specimen; (c) mor-phology of corroded area of longitudinal specimen; (d) morphology of corroded area of transverse specimen; (e) morphology of the uncorroded area of longitudinal specimen; (f) high magnification images of area A
圖 9 能譜分析. (a) 第二相能譜; (b) 線溝槽附近能譜
Figure 9. Energy spectrum analysis: (a) second-phase energy spectrum; (b) EDS near-line grooves
表 1 材料的化學成分(質量分數)
Table 1. Chemical composition of the materials ?
% Zn Mg Cu Mn Cr Zr Ti Si Fe Al 4.3~5.0 1.0~1.3 0.05~0.20 0.10~0.40 0.20 0.08~0.30 ＜0.035 ＜0.12 ＜0.12 余量 
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表 2 圖 3的第二相能譜分析結果(質量分數)
Table 2. EDS analysis results of second － phase particles shown in Fig.3 ?
% Fig.3位置 Mg Al Si Cr Mn Fe Cu Zn 其他 a1 0.24 67.65 3.96 1.43 5.20 20.43 — 1.11 0.00 a2 0.44 72.47 3.46 1.02 3.60 16.33 0.50 1.85 0.33 a3 1.52 92.35 — — — — 0.53 4.66 0.94 a4 1.40 93.71 0.71 — — 0.28 — 4.16 0.00 b1 0.08 65.79 4.59 1.37 4.36 21.92 — 1.90 0.00 b2 0.56 73.20 2.91 0.77 3.30 16.41 0.04 2.15 0.66 b3 1.19 93.58 — — — — 0.04 4.21 0.98 b4 1.51 92.29 — 0.34 0.32 — 0.67 4.54 0.33
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表 3 極化曲線擬合值
Table 3. Fitting results of the polarization curves
試樣 Ecorr/V Icorr/(mA·cm-2) T-S -0.912 0.219 L-S -0.906 0.980
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表 4 恒載荷實驗結果
Table 4. Constant load test results
試樣 載荷/MPa 伸長率/% 試驗時間/h 是否斷裂 橫向 225 2.69 319.45 是 225 2.82 311.28 是 225 2.73 315.12 是 縱向 225 4.56 360.00 否 225 4.92 360.00 否 225 4.88 360.00 否
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