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Stress relaxation of nickel-based superalloy helical springs at high temperatures

机译：镍基高温螺旋弹簧的应力松弛

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摘要

The creep resistance of materials in spring applications is generally acknowledged to be well below that observed in other applications. Helical springs formed from three candidate nickel-based superalloys, Nimonic 90, Rene 41 and Haynes 282, have been tested under compression in order to gain some insight into this phenomenon. Stress relaxation tests conducted at 600-700℃ found that, under constant displacement, the degradation of the spring force is one to three orders of magnitude faster than would be predicted from creep data from extruded samples under equivalent tensile loading. An analytical model for torsional creep in helical springs is derived from a modified version of the Dyson creep model. The effects of various microstructural features on the deformation rate are considered. Effects such as the coarsening of the precipitate-strengthening gamma-prime phase, tertiary creep due to dislocation multiplication, damage evolution and hardening due to transfer of the stress to the particles from the matrix are concluded to make negligible contributions. It is predicted that the poor performance of the springs is due to the very high population of geometrically necessary dislocations that result from the bending and twisting of the wire into a helical coil. It is expected that these dislocations are resistant to conventional heat treatments, resulting in a persistent residual stress field and a large number of dislocations to facilitate the creep process. In some cases, the stress relaxation is found to be so fast that the precipitate hardening of the alloy is too slow to prevent significant initial degradation of the spring.
机译：人们普遍认为，弹簧应用中材料的抗蠕变性远低于其他应用中观察到的抗蠕变性。为了对这种现象有一定的了解，已经对由三种候选的镍基高温合金（Nimonic 90，Rene 41和Haynes 282）形成的螺旋弹簧进行了压缩测试。在600-700℃下进行的应力松弛测试发现，在恒定位移下，弹簧力的降解要比在等效拉伸载荷下挤压样品的蠕变数据所预测的快一到三个数量级。螺旋弹簧扭转蠕变的解析模型是从戴森蠕变模型的修改版本中得出的。考虑了各种微观结构特征对变形率的影响。得出结论认为，诸如强化沉淀的γ-初生相的粗化，位错倍增引起的第三蠕变，由于应力从基体转移到颗粒上而引起的损伤演化和硬化等作用可忽略不计。可以预见的是，弹簧的不良性能是由于将线弯曲和扭曲成螺旋线圈而导致大量必要的几何错位。预期这些位错可抵抗常规热处理，从而导致持久的残余应力场和大量位错，从而有利于蠕变过程。在某些情况下，发现应力松弛是如此之快，以至于合金的沉淀硬化太慢而无法阻止弹簧的显着初始退化。

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