Zr-2.5Nb合金不同工艺预生膜的显微组织和摩擦磨损性能

Microstructure and Friction and Wear Properties of Pre-formed Films on Zr-2.5Nb Alloy Prepared by Different Processes

摘要:

微动磨损是限制重水堆用压力管寿命的主要原因之一。为提高重水堆用Zr-2.5Nb压力管的服役可靠性，采用3种预氧化处理工艺在Zr-2.5Nb合金表面制备了约1 μm厚的预生氧化膜，并研究了其力学性能与显微组织的关系。3种预氧化处理工艺分别为在400 ℃/10.3 MPa除氧过热蒸汽、300 μg/kg溶解氧过热蒸汽以及400 ℃/2 MPa低压蒸汽中氧化24 h。结果表明，Zr-2.5Nb合金由α-Zr和β-Zr双相组成，α-Zr和β-Zr的形貌均呈长条状，β-Zr连续分布在α-Zr晶界处。不同预氧化条件下生成的膜显微结构存在差异：除氧条件下的预生膜中的微裂纹相对较多，其柱状晶尺寸较为短小并且排列杂乱；溶解氧条件下的预生膜最致密，缺陷最少；低压条件下的预生膜厚度最大，孔隙和裂纹相对更多。与原始合金相比，预生膜使合金纳米硬度提高了50%~180%，硬度与弹性模量之比（H/E）提高了56%~81%，磨损率降低了31%~44%，预生膜显著提高了合金表面的硬度和耐磨损性能，使磨损机制由严重的磨粒磨损转变为轻微黏着磨损。其中，溶解氧条件下制得的预生膜最为致密，硬度提升效果最突出，这是因为预生膜的强化作用与其显微形貌密切相关。致密完好的氧化膜不仅硬度更高，且与金属基体结合更牢固，不易在局部受力时剥落或开裂。

Abstract:

Micro-motion wear is one of the primary factors limiting the service life of pressure tubes used in heavy-water reactors. To enhance the operational reliability of Zr-2.5Nb pressure tubes for heavy-water reactors. A pre-formed film with thickness of approximately 1 μm on the surface of Zr-2.5Nb alloy by three pre-oxidation treatment processes was prepared, and the relationship between its mechanical properties and microstructure was investigated. The three pre-oxidation treatment processes were conducted at 400 °C for 24 h: (1) in deoxidized superheated steam at 10.3 MPa; (2) in superheated steam with dissolved oxygen of 300 μg/kg; (3) in low-pressure steam at 2 MPa. The results indicate that the Zr-2.5Nb alloy consists of α-Zr and β-Zr phases, with both α-Zr and β-Zr phases exhibiting elongated morphologies. And β-Zr phase continuously distributes at the α-Zr grain boundaries. Microstructures of the films formed under different pre-oxidation conditions exhibit differences. Among them, the pre-formed film prepared under deoxidized conditions contains relatively more microcracks, with shorter and more randomly arranged columnar grains; the pre-formed film prepared under dissolved oxygen conditions is the densest with the fewest defects, while the pre-formed film prepared under low-pressure conditions has the greatest thickness with relatively more pores and cracks. Compared with the original alloy, the pre-formed film increases the nano-hardness of the alloy by 50%–180%, improves the hardness-to-modulus ratio (H/E) by approximately 56%–81%, and reduces the wear rate by 31%–44%. The pre-formed film significantly enhances the surface hardness and wear resistance of the alloy, transforming the wear mechanism from severe abrasive wear to mild adhesive wear. Among three pre-formed films, the pre-formed film prepared under dissolved oxygen conditions is the densest and exhibits the most pronounced hardness enhancement. This is because the strengthening effect of the pre-formed film is closely related to its microstructure. A dense and intact oxide film not only has higher hardness, but also adheres more firmly to the metal substrate, making it less prone to peeling or cracking under localized stress.