Investigation the effect of crystal orientation of nickel‐based single crystal superalloys on bending creep tests

The relationship between the three points bending creep test and the uni‐axial creep test on the single crystal superalloy was investigated by using crystal plasticity slip theory with a three‐dimensional (3D) finite element model. The purpose of the present work is to build the relationship between bending creep and conventional uni‐axial tensile creep in determining crystallographic creep parameters for face centered cubic (FCC) nickel‐based single crystal superalloys. To this aim, the bending creep performed on [001]‐, [011]‐, and [111]‐oriented nickel‐based single crystal superalloys were respectively investigated, and the data was compared with those obtained with uni‐axial tensile creep counterparts. It shows that the determination of crystallographic creep stress exponent is independent of crystallographic orientations, and the results agree reasonably well between bending creep test and uni‐axial tensile creep test. The findings may shed some light on understanding of the crystal structures and its time‐dependent deformation mechanisms with the bending creep method.     

Creep–fatigue crack growth behaviour based on EBSD observations for notched specimen made of directionally solidified Ni-base superalloy

Abstract

In order to characterise the creep–fatigue crack growth behaviour of a directionally solidified Ni-base superalloy, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method using notched specimens has been carried out. The average misorientation in the vicinity of notches increases linearly up to the initiation of cracks with the increasing creep strains under creep or creep–fatigue conditions, whereas under fatigue conditions fatigue cracks grew without an increase in misorientation. The stress holding time clearly influenced the growth behaviour of creep–fatigue cracks and the appearance of misorientation development. However, it was shown that the relationship between the average misorientation and the relative notch opening displacement was independent of test conditions such as temperatures, stresses and stress wave forms.

It is concluded that the misorientation analysis of damaged samples based on the EBSD method allows the prediction of the initiation of creep–fatigue cracks and their growth behaviours.     

镍基单晶高温合金扩散阻挡层的研究进展

综述了镍基单晶高温合金扩散阻挡层的国内外研究进展。简要介绍了高温防护涂层/镍基单晶高温合金基体元素的互扩散现象,说明了扩散阻挡层的作用机理,阐述了扩散阻挡层的设计原则和要求,通过对比分析了各类扩散阻挡层的优缺点。最后对镍基单晶高温合金用扩散阻挡层的研究做出了展望。     

Ru对镍基单晶热暴露组织演变及持久性能的影响

对比研究了不含Ru的USTB-F7及添加2.5 wt％ Ru的USTB-F8两种镍基单晶高温合金的组织稳定性和持久性能.标准热处理与1100℃长期热暴露组织研究表明:合金USTB-F7中γ′相形貌介于球形和立方形之间,属中间态形貌;热暴露2000 h后,其形貌仍保持稳定,仅发生粗化而未发生筏排化.Ru的添加使Re元素在γ/γ′中的分配比增大,提高了合金USTB-F8的γ/γ′点阵错配度和γ'相的立方度,从而加速了长期热暴露过程中的筏排化进程,经2000 h热暴露发生了明显的筏排化现象.同时,合金USTB-F7热暴露700 h后在枝晶干处析出了富集Re、W和Cr元素的TCP相,Ru的加入有效地抑制了TCP相的析出,合金USTB-F8直至2000 h仍未析出TCP相.1100℃/140MPa持久性能测试表明,Ru显著提高合金的持久寿命,这与Ru增加合金中的y相体积分数和γ/γ′点阵错配度,促进筏排组织的形成,并减小时效组织中的γ通道宽度有关.     

合金元素钨对新型Co-Al-W合金热腐蚀行为的影响

研究Co-Al-W和商用MAN900合金在800℃75%Na2SO4+25%NaCl混合熔盐中的腐蚀动力学及热腐蚀行为。结果表明:7.5 W、9.8 W和10.7 W合金经热腐蚀后质量增加量比MAN900合金的质量增加量少,Co-Al-W合金的耐热腐蚀能力比MAN900合金的耐热腐蚀能力强。Co-Al-W合金在熔盐中腐蚀膜结构分成3层,即呈蓬松状由富钴氧化物Co3O4组成的腐蚀膜最外层,由Co、Al、W复杂氧化物组成的中间过渡层和主要由Al及Co氧化物组成较致密的腐蚀膜内层。随着腐蚀时间的增加,合金腐蚀膜最外层由于脱落逐渐变薄;中间过渡层厚度逐渐增加,该层中各元素分布趋于均匀、稳定;腐蚀膜内层致密性增加使该层增厚不明显。     

孔洞形态对镍基单晶合金蠕变行为的影响

通过蠕变性能测试及组织形貌观察,研究不同形态孔洞缺陷对单晶镍基合金蠕变性能的影响,针对合金中存在有/无裂纹孔洞等铸造缺陷,对高温蠕变期间近有/无孔洞区域的应力分布进行有限元分析,根据拉应力载荷下近孔洞区域的微观应力分布特性,分析不同形态孔洞区域应力分布对单晶合金蠕变行为及组织演化的影响.结果表明:在高温蠕变期间,近有/无裂纹孔洞区域的应力分布对合金中)γ'相筏形化的形态有明显影响,在有/无裂纹孔洞两侧极点处,存在最大应力值,且可致使其裂纹沿垂直于应力轴方向发生萌生与扩展;与无裂纹孔洞相比,在有裂纹孔洞两侧的极点处应力值较大,随着蠕变的进行,在较大应力处易于发生裂纹的扩展是合金具有较低蠕变寿命的主要原因.     

几种微量元素在高温合金中的作用与机理

研究几种微量元素对高温合金力学性能和微观组织结构的影响;从晶界结合力、晶界沉淀和凝固偏析等方面对微量元素的不同作用进行讨论.结果表明:适量的B、C、Y、La、Ce、Zr和Mg明显改善高温合金的持久性能;Si和S则明显降低高温合金的持久性能;而P对一些高温合金特别是变形高温合金表现出有益作用,而对另一些高温合金特别是铸造合金表现出有害作用.     

Condition assessment techniques for degraded gas turbine superalloy materials

Direct and indirect methods of quantifying structural damage or residual mechanical properties have been investigated and compared for virgin Nimonic 115 and ex-service material from gas turbine blading. Whilst the indirect technique of X-ray diffraction line profile analysis exhibits a significant increase in line width with increasing creep strain, the current level of experimental scatter would need to be reduced for it to be a useful life assessment tool. The direct technique of temperature accelerated stress rupture (isostress) testing is shown to produce data which can be extrapolated to service conditions with confidence and which can be used in an assessment procedure. Stress accelerated rupture testing is demonstrated to give an unduly conservative indication of residual rupture properties at service conditions.     

600MW反应堆压力容器径向支承块特厚件镍基合金焊接

600MW反应堆压力容器(RPV-600)的径向支承块的焊接存在较多的困难因素:属于异种金属焊接接头;焊接材料可焊性差;焊缝位置较深等。在多次试验、讨论后,提出了一套合理的完整的焊接规范,使焊接质量达到了规定的技术要求。重点对镍基合金的焊接性、打磨工艺等其它辅助工艺措施作了重点的研究。