| 摘要 | 磷、硼显著提高IN718（GH4169）合金持久和蠕变性能的有益作用已逐渐地得到承认，但是影响机理有待深入研究，以促进其实际应用。IN718合金的使用温度一般低于650℃，这一缺点限制了其在现代航空发动机中的应用。过去的研究证实，磷、硼可明显提高718合金的使用温度，具有良好的应用前景。因此，本文在过去研究的基础上开展以下两个方面的研究：一是研究磷、硼在DA718合金中的作用机理；二是研究磷、硼微合金化合金的热加工及长期时效组织和性能稳定性。通过以上研究为合理地利用磷、硼的强化作用提高IN718合金的使用性能提供指导。
本文首先研究了磷、硼对DA718合金蠕变性能及蠕变变形机理的影响。进一步地验证了复合添加磷、硼可以更显著地提高DA718合金的蠕变性能。加入磷、硼提高DA718合金的蠕变激活能、降低应力指数，增加蠕变变形抗力，延长合金的蠕变寿命。利用透射电镜对蠕变组织进行观察表明，合金的蠕变组织中存在大量光滑平直且互相平行的滑移带。随着蠕变的进行，晶粒内部的形变带密度增加，间距变窄，添加磷、硼能有效地抵抗变形。部分形变带被确定为孪晶，其它也有可能为位错滑移带。不全位错切过基体和γ″强化相后产生一个单层孪晶，γ″强化相的析出是产生孪晶变形的根源，因为在固溶态合金蠕变变形组织中并未观察到孪晶变形。
磷、硼对DA718合金室温及高温拉伸性能、室温冲击性能的影响不明显。单独添加磷、硼对合金室温及高温拉伸性能影响不明显，而且拉伸断口裂纹源处的形貌没有差别，但是复合添加磷、硼能改善高温拉伸时裂纹源的形貌。单独添加磷稍降低DA718合金的室温冲击韧性，但是合金在650℃时效100h后，加磷合金的室温冲击功反而上升，硼并不降低合金的室温冲击韧性。所有的室温冲击断口形貌都是穿晶韧窝型断裂，磷、硼对此没有明显影响。
研究了单独添加磷、硼对DA718合金热处理态组织的影响，磷、硼能稍细化晶内γ″相的尺寸。晶界处析出很少的δ相，呈短棒状或颗粒状，且没有明显差别。磷、硼对合金直接时效态晶粒组织影响亦不大，各合金晶粒大小均匀平均尺寸约10μm。在680℃时效3000h过程中，γ″相沿半径方向直线长大，沿厚度方向按抛物线规律长大。磷、硼显著地降低γ″相的长大速率。时效过程中磷、硼能抑制晶界δ相的粗化，并阻碍其周围无析出区的宽化。分析认为其主要原因是磷、硼能降低合金中铌的扩散系数。DA718合金在680℃时效3000h过程中，其硬度变化曲线表明，磷稍抑制过时效。磷、硼对合金在680℃时效600h后的室温拉伸性能的影响不明显，但仍然能提高蠕变性能。表明在DA718合金中用磷、硼作为有益微合金元素是合适的。
磷、硼的作用机理分为晶界机制和晶内机制。计算分析表明，磷降低纯镍的晶界结合力，但是硼提高纯镍的晶界结合力；磷、硼的晶界偏聚将阻碍晶界扩散、并最终阻碍晶界δ相的粗化及其周围无析出区的宽化；同时磷、硼抑制晶界扩散也有利于抑制蠕变时的晶界滑移及裂纹萌生。磷、硼也可以偏聚于位错等晶内缺陷处，更可能占据大量的空位或间隙位置，降低合金的层错能，产生固溶强化作用；磷和硼还能减缓合金中元素的扩散，提高强化相的稳定性。
热模拟试验中，718合金随变形温度的提高，变形的峰值应力降低，对应的应变值变小。加入磷、硼以后，在1000℃以下压缩，合金的峰值应力和对应的应变值更低。此时磷、硼抑制晶界δ相的析出，降低热加工的激活能，合金变形抗力降低，有利于获得再结晶细晶组织；在高于1000℃时，添加磷、硼的影响很小。合金在1060℃变形时，随着变形量的增加，位错密度不断增加，促进再结晶的发生。对于不同磷、硼含量的合金，当温度高于1000℃时，只要变形量达到30%以上，就可以获得完全的再结晶组织。
在较低的温度变形结合磷、硼微合金化的方法，可以进一步提高合金的强度和蠕变性能。在680℃时效3000h过程中其硬度高于在较高温度变形的合金。在680℃时效600h后，在拉伸强度及蠕变性能方面的优势均能保留。磷、硼复合强化结合热加工可在合金组织中形成弯曲晶界，且晶内γ″强化相细化，体积分数增多，并形成少量<100>的显微织构，因此明显提高合金的拉伸强度和蠕变性能。; It has been revealed by many researchers that doping proper phosphorus and boron is significantly beneficial to the creep property of IN718 alloy, while the mechanism is still unclear, which limits the application of the beneficial effect of phosphorus and boron in industrial production. The service temperature of IN718 alloy is generally below 650℃, which can not satisfy the demand of modern aircraft engines. However, it has been found that phosphorus and boron can increase the service temperature of IN718 alloy noticeably. Based on these backgrounds, two points were investigated in this article. One is to demonstrate the mechanism of phosphorus and boron on the microstructure and mechanical properties of DA718 alloy. The other is to investigate their behavior during the thermal processing and long term aging of DA718 alloys. The aim of the study is to provide useful message for application of the micro-alloying.
Firstly, the effect of phosphorus and boron on the creep property of DA718 alloy and the creep deformation mechanism of the alloy was studied. Results show that the co-addition of phosphorus and boron can improve the creep property of IN718 alloy more significantly than the single addition of phosphorus or boron. Phosphorus or boron increases the creep activation energy and decreases the stress exponent, which increasing the creep resistance and prolonging creep rupture time. Observing by TEM, it is proved that there are many parallel deformation bands appearing as straight smooth lines in the creep microstructure. The average spacing between two deformation bands is getting narrower with the creeping time. Addition of phosphorus or boron can resist the creep deformation effectively. Some of the deformation bands were determined to be twins, and others might be the slip bands of dislocations. Analysis shows that gliding of partial dislocation may produce a single layer stacking fault when it cuts through matrix and γ″phase, which is also a twin of a single layer atoms. The existence of phase should be responsible for the formation of deformation twins because no deformation twins were observed in the creep microstructure of a solutionized(γ″-free) IN718 alloy.
Either addition of phosphorus or boron does not affect the tensile strength at room temperature and 720℃ of DA718 alloy. No difference in fracture character at the crack initiating area was observed, but co-addition of phosphorus and boron can improve the morphology of tensile crack source at 720℃. The impact work of DA718 alloy was slightly decreased by the doping of phosphorus, but increased after being subjected to the ageing at 650℃ for 100h. Boron does not decrease the impact work of the alloy as heat treated or aged. Phosphorus and boron doping does not affect the morphology of rupture surfaces of the impact samples, on which exhibiting a typical transgrannular fracture with a lot of plastic dimples.
For the direct aged 718 alloys, addition of phosphorus or boron refined γ″particles. The δ phases are scattered at grain boundaries and little difference was observed. The average grain size of the alloys is about 10μm and phosphorus or boron did not influence the grain size. When DA718 alloy is aged at 680℃ for 3000h, the γ″ phase is coarsening gradually. The particles grow linearly along radium direction and parabolic along thickness direction. It is determined by analysis that phosphorus or boron decreases the growth rate of γ″phase significantly by decreasing the diffusion coefficient of niobium. Because of the same reason, addition of phosphorus or boron resists the coarsening of grain boundary δ phase and the widening of precipitate-free zone around δ phase. According to the variation of hardness curves during ageing, it shows that phosphorus can retard over aging slightly. Phosphorus or boron can still improve creep property markedly and do not harm to the tensile property at room temperature, even after the alloys had been subjected to 600h aging at 680℃, which suggests that phosphorus and boron can be used as beneficial micro-alloying elements to improve the creep property of DA718 alloy.
Doping of phosphorus or boron can affect the alloy by influencing grain interior and grain boundary. Phosphorus atoms may decrease the grain boundary cohesion of pure nickel, while boron atoms may not. The segregation of phosphorus or boron at grain boundary may also hamper the grain boundary diffusion, which is helpful to resist the growth of grain boundary δ phase and widening of precipitate-free zone. For the same reason, the grain boundary sliding and initiating of cracks during creep are also retarded. Phosphorus or boron atoms are liable to be segregated at defects such as dislocations or vacancies. As a result, it may decrease stacking fault energy and strengthen the matrix of the alloy. In addition, phosphorus and boron may hamper the diffusion process by occupying the rapid diffusion path such as dislocations or grain boundaries, and finally improve the stability of γ″phase and δ phase.
The influence of phosphorus and boron on the hot processing of IN718 alloy was investigated. With the increase of deformation temperature, both the peak stress and the corresponding strain at which stress reaches the peak value are decreased. When the deformation temperature is below 1000℃, the addition of phosphorus or boron decreases the peak stress and the corresponding strain. When the deformation temperature is higher than1000℃, the effect of phosphorus or boron on the true stress-true strain curves disappeared. The reason is that addition of phosphorus or boron resists the precipitation of δ phase in the temperature range between 900-1000℃, and therefore decreases the deformation resistant stress and activation energy. Through metallographic observation of the deformation microstructure, it is proved that addition of phosphorus or boron is helpful to refine grains during thermal processing. When the alloy is deformed above 1100℃, the grains will coarsen and the effect of phosphorus or boron will disappear. The evolution of hot deformation microstructure at 1060℃ was observed by TEM. Results show that with the increasing of deformation ratio, the dislocation density is enhanced, which accelerates dynamic recrystallization. When the percentage of hot deformation is more than 30%, completely recrystallized fine grains can be acquired. Doping of phosphorus or boron does not affect the recrystallization process under this condition.
Combining phosphorus and boron complex micro-alloying method with low temperature thermal processing, the strength and creep property are improved further. Through the measuring of hardness variation of the alloys during ageing at 680℃ for 3000h, it is revealed that the high strength can be held to long time at high temperature. It is observed that the grain boundaries are curved, a little bit micro-texture is produced, and the phases are refined in the alloy micro-alloyed with phosphorus and boron and thermal processed at lower temperature. During the ageing, the coarsening of γ″ phase is slower than that of the alloy processed at higher temperature, but the grain boundary phase is rapidly growing and the co-addition of phosphorus and boron has litter effect to suppress the growth of δ phase. |
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