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Non-destructive electrical characterization of controlled Waspaloy microstructures.

机译：受控的Waspaloy显微组织​​的无损电特性。

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摘要

In this research, controlled Waspaloy microstructures are produced with the objective of studying microstructural evolution in this alloy via non-destructive electrical measurements. Waspaloy is a precipitation-hardenable gamma-gamma' nickel-base superalloy that is used in turbine blade applications demanding superior strength retention capabilities at elevated temperatures. Overall, three different sets of microstructures were produced that varied systematically as a function of the matrix (gamma) grain size and gamma' precipitate size distribution or just the latter. Initial solutionizing treatments conducted at 1045°C, 1090°C and 1145°C resulted in average y grain size of 13, 52 and 89 mum respectively. A vacancy stabilization treatment at 1045°C followed the solutionizing treatments in Set I experiments, after which the specimens were aged at 800°C for durations ranging from 0.1 hrs to 100 hrs. In Sets 11 and III, the matrix grain size was kept unchanged by an initial solution-treatment at 1145°C. The stabilization treatment at 1045°C was only conducted in Set II after the solution-treatment. Aging experiments were then conducted at 700°C (or 725°C in Set III), 800°C and 875°C to study the growth kinetics of gamma' precipitate distribution. The specimens with controlled microstructures were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray diffraction (XRD), ultra small-angle x-ray scattering (USAXS) and dc four-point probe resistivity. The applicability of two and four-probe ac impedance techniques was also investigated.;Characterization of heat-treated specimens. Solution-treated SEM microstructures showed the presence of polygonal etch-pit shapes, which was proposed to be due to the condensation of excess quenched-in vacancies along crystallographically soft-directions in the gamma phase. The etch-pits evolved upon subsequent aging into progressively irregular shapes, concurrent with gamma' growth inside the pits. SEM and USAXS observations clearly supported the progressive coarsening of the gamma' distribution with increasing aging time at 800°C. Complementary evidence from do resistivity measurements confirmed that gamma' nucleation was complete by 0.1 hrs of aging at 800°C. Coarsening studies indicated an LSW type volume diffusion mechanism of coarsening in Waspaloy, with an average coarsening rate constant of 3.25x10-29 [m3/sec] for Set I specimens aged at 800°C. This is an order of magnitude smaller compared to values quoted in the literature for binary Ni-Al systems. The resistivity decrease during initial stages of gamma' coarsening was attributed to the progressive decrease in the 'scattering power' of the gamma' distribution. The variations in the later stages were related to compositional changes during coarsening. Resistivity variations as a function of the aging time at 800°C did not show any dependence on the prior solution-treatment temperature in Set I specimens. Resistivity measurements also showed a drastic shortening in the first-stage coarsening with increase in the aging temperature from 725°C to 800°C, while this stage was completely absent at 875°C. In Set III specimens, the gamma-gamma' lattice misfit was found to increase with increasing aging duration at the same temperature or with increasing kinetics.
机译：在这项研究中，生产受控的Waspaloy显微组织​​的目的是通过无损电学测量研究这种合金的显微组织演变。 Waspaloy是一种可沉淀硬化的γ-γ'镍基高温合金，用于涡轮叶片，需要在高温下具有出色的强度保持能力。总体而言，产生了三组不同的微结构，它们随基体（γ）晶粒尺寸和γ'沉淀物尺寸分布或仅后者而系统地变化。最初在1045°C，1090°C和1145°C进行固溶处理的平均y晶粒尺寸分别为13、52和89微米。在组I实验中的固溶处理之后，进行1045℃的空位稳定化处理，此后将样品在800℃下老化0.1小时至100小时。在组11和组III中，通过在1145℃下进行初始固溶处理，使基体晶粒尺寸保持不变。仅在固溶处理后的组II中在1045℃下进行稳定化处理。然后在700°C（或III组中为725°C），800°C和875°C下进行老化实验，以研究γ'沉淀分布的生长动力学。使用扫描电子显微镜（SEM），原子力显微镜（AFM），x射线衍射（XRD），超小角度x射线散射（USAXS）和dc四点探针电阻率研究了具有受控微观结构的标本。还研究了两种和四种探针交流阻抗技术的适用性。固溶处理的SEM显微结构显示出多边形蚀刻坑的形状，这是由于在γ相中沿结晶软性方向的过量淬火空位的缩合所致。腐蚀坑在随后的时效过程中逐渐演变为不规则形状，同时在坑内出现伽马生长。 SEM和USAXS观察结果清楚地表明，随着800°C时效时间的增加，γ分布逐渐变粗。电阻率测量的补充证据证实，在800°C时效0.1小时后，γ'形核就完成了。粗化研究表明，在Waspaloy中，LSW型的体积扩散机制是粗化，对于800℃时效的I组试样，平均粗化速率常数为3.25x10-29 [m3 / sec]。与文献中针对二元Ni-Al系统引用的值相比，该值小一个数量级。 γ'粗化初始阶段的电阻率降低归因于γ'分布的“散射能力”逐渐降低。后期的变化与粗化过程中的成分变化有关。电阻率随800°C时效时间变化而变化，与Set I样品中先前的固溶处理温度无关。电阻率测量还表明，随着时效温度从725°C到800°C的升高，第一阶段粗化的急剧缩短，而在875°C时则完全消失。在第三组试样中，发现γ-γ'晶格失配随着在相同温度下老化时间的增加或动力学的增加而增加。

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