Tian Wenming, Li Zhonglei, Kang HuiFeng, Cheng Fasong, Chen Fangfang, Pang Guoxing
School of Materials Engineering, North China Institute of Aerospace Engineering, No.133 Aimindong Road, Langfang 065000, China.
Heibei Key Laboratory of Trans-Media Aerial Underwater Vehicle, North China Institute of Aerospace Engineering, No.133 Aimindong Road, Langfang 065000, China.
Materials (Basel). 2020 Jul 21;13(14):3236. doi: 10.3390/ma13143236.
The bimodal-grain-size 7075 aluminium alloys containing varied ratios of large and small 7075 aluminium powders were prepared by spark plasma sintering (SPS). The large powder was 100 ± 15 μm in diameter and the small one was 10 ± 5 μm in diameter. The 7075 aluminium alloys was completely densified under the 500 °C sintering temperature and 60 MPa pressure. The large powders constituted coarse grain zone, and the small powders constituted fine grain zone in sintered 7075 aluminium alloys. The microstructural and microchemical difference between the large and small powders was remained in coarse and fine grain zones in bulk alloys after SPS sintering, which allowed for us to investigate the effects of microstructure and microchemistry on passive properties of oxide film formed on sintered alloys. The average diameter of intermetallic phases was 201.3 nm in coarse grain zone, while its vale was 79.8 nm in fine grain zone. The alloying element content in intermetallic phases in coarse grain zone was 33% to 48% higher than that on fine grain zone. The alloying element depletion zone surrounding intermetallic phases in coarse grain zone showed a bigger width and a more severe element depletion. The coarse grain zone in alloys showed a bigger electrochemical heterogeneity as compared to fine grain zone. The passive film formed on coarse grain zone had a thicker thickness and a point defect density of 2.4 × 10 m, and the film on fine grain zone had a thinner thickness and a point defect density of 4.0 × 10 m. The film resistance was 3.25 × 10 Ωcm on coarse grain zone, while it was 6.46 × 10 Ωcm on fine grain zone. The passive potential range of sintered alloys increased from 457 mV to 678 mV, while the corrosion current density decreased from 8.59 × 10 A/cm to 6.78 × 10 A/cm as fine grain zone increasing from 0% to 100%, which implied that the corrosion resistance of alloys increased with the increasing content of fine grains. The passive film on coarse grain zone exhibited bigger corrosion cavities after pitting initiation compared to that on fine grain zone. The passive film formed on fine grain zone showed a better corrosion resistance. The protectiveness of passive film was mainly determined by defect density rather than the thickness in this work.
通过放电等离子烧结（SPS）制备了包含不同比例大尺寸和小尺寸7075铝粉的双峰粒度7075铝合金。大尺寸粉末的直径为100±15μm，小尺寸粉末的直径为10±5μm。7075铝合金在500℃烧结温度和60MPa压力下完全致密化。在烧结后的7075铝合金中，大尺寸粉末构成粗晶区，小尺寸粉末构成细晶区。经过SPS烧结后，大尺寸和小尺寸粉末之间的微观结构和微化学差异保留在块状合金的粗晶区和细晶区中，这使我们能够研究微观结构和微化学对烧结合金上形成的氧化膜钝性的影响。粗晶区金属间相的平均直径为201.3nm，而细晶区的值为79.8nm。粗晶区金属间相中的合金元素含量比细晶区高33%至48%。粗晶区金属间相周围的合金元素贫化区显示出更大的宽度和更严重的元素贫化。与细晶区相比，合金中的粗晶区表现出更大的电化学不均匀性。在粗晶区形成的钝化膜具有更厚的厚度和2.4×10m的点缺陷密度，而在细晶区的膜厚度更薄，点缺陷密度为4.0×10m。粗晶区的膜电阻为3.25×10Ωcm，而细晶区为6.46×10Ωcm。随着细晶区从0%增加到100%，烧结合金的钝性电位范围从457mV增加到678mV，而腐蚀电流密度从8.59×10A/cm降低到6.78×10A/cm，这表明合金的耐腐蚀性随着细晶粒含量的增加而提高。与细晶区相比，粗晶区的钝化膜在点蚀引发后表现出更大的腐蚀空洞。在细晶区形成的钝化膜表现出更好的耐腐蚀性。在这项工作中，钝化膜的保护性主要由缺陷密度而非厚度决定。
