不同相组成近β钛合金Ti-3Al-5Mo-4Nb-4Cr-2Zr的应力腐蚀行为

Stress Corrosion Behavior of Near-β Titanium Alloy Ti-3Al-5Mo-4Nb-4Cr-2Zr with different phase compositions

摘要:

海洋工程用近β钛合金在海水中易发生应力腐蚀开裂，合金相组成对其应力腐蚀行为具有显著影响。通过对近β合金Ti-3Al-5Mo-4Nb-4Cr-2Zr进行三种不同工艺的热处理，使合金具有不同的三种相组成，利用扫描电子显微镜和X射线衍射仪分析显微组织，并进行电化学实验与慢应变速率拉伸实验，研究相组成不同时合金的应力腐蚀行为。结果表明，在β相区固溶处理后，合金为全β相组织(Pβ)；500℃/6h时效处理后，于β基体析出细小且间距较小次生α(αs)相(Pβ+fα)；650℃/4h时效处理后，于β基体析出了粗大且间距较大αs相(Pβ+cα)。Pβ+cα的自腐蚀电位相对最小、腐蚀电流密度相对最大、容抗弧仰角及0.01Hz下阻抗模量相对最小，其耐腐蚀性相对最差，相对更优的依次为Pβ+fα和Pβ。应力腐蚀敏感性指数由高到低依次为，Pβ+cα、Pβ+fα、Pβ。应力腐蚀断口表面出现韧窝、微裂纹、撕裂棱以及小平面，表明发生了高度位错运动和局部塑性变形。应力腐蚀开裂主要由氢吸附诱导位错发射(AIDE)和氢加速局部塑性变形(HELP)机制主导。αs相间距减小有利于降低合金应力腐蚀开裂倾向。

Abstract:

Near-β titanium alloys for marine engineering applications may be suffered from stress corrosion cracking. The phase composition has a significant impact on stress corrosion behavior. Different phase compositions of a near-β titanium alloy, Ti-3Al-5Mo-4Nb-4Cr-2Zr, were obtained by undergoing various heat treatments. The microstructures were characterized by scanning electron microscope and X-ray diffraction. The electrochemical measurements and slow strain rate testing were carried out to investigate the effect of phase composition on stress corrosion behavior. The results indicate that the single β phase is obtained by solution treatment at β phase field (Pβ). After aged at 500℃ for 6h, fine secondary α phase with narrow spacing precipitate within β matrix (Pβ+fα). After aged at 650℃ for 6h, coarsened secondary α phase with wide spacing precipitate within β matrix (Pβ+cα). For the Pβ+cα, the corrosion potential and elevation angle of capacitive impedance arc as well as impedance modulus at 0.01Hz are relatively minimum, and the corrosion current density is relatively maximum. The Pβ+cα exhibits relatively poor corrosion resistance. The relatively better order is the Pβ+fα and Pβ. The stress corrosion susceptibility index order from high to low is Pβ+cα、Pβ+fα、Pβ. The fracture morphology exhibits a mixed characteristic with shallow dimples and micro-cracks as well as tear ridges and flat facets. The combination of Absorption Induced Dislocation Emission (AIDE) and Hydrogen Enhanced Localized Plasticity (HELP) is the main mechanism for stress corrosion cracking. The decrease of secondary α phase spacing has a beneficial effect on reducing the stress corrosion susceptibility.