Effect of boron addition on the microstructure and mechanical properties of K4750 nickel-based superalloy

The effect of boron addition at 0,0.007 wt.% and 0.010 wt.% on the microstructure and mechanical properties of K4750 nickel-based superalloy was studied.The microstructure of the as-cast and heat-treated alloys was analyzed by SEM,EPMA,SIMS and TEM.Lamellar M₅ B₃-type borides were observed in boroncontaining as-cast alloys.After the full heat treatment,boron atoms released from the decomposition of M₅ B₃ borides were segregated at grain boundaries,which inhibited the growth and agglomeration of M₂₃C₆ carbides.Therefore,the M₂₃C₆ carbides along grain boundaries were granular in boron-containing alloys,while those were continuous in boron-free alloys.The mechanical prope rty analysis indicated that the addition of bo ron significantly improved the tensile ductility at room tempe rature and stress rupture properties at 750℃/430 MPa of K4750 alloy.The low tensile ductility at room temperature of 0 B alloy was attributed to continuous M₂₃C₆ carbides leaded to stress concentration,which provided a favorable location for crack nucleation and propagation.The improvement of the stress rupture properties of boron-containing alloys was the result of the combination of boron segregation increased the cohesion of grain boundaries and granular M₂₃C₆ carbides suppressed the link-up and extension of micro-cracks.     

Microstructure evolution and mechanical properties of GH984G alloy with different Ti/Al ratios during long-term thermal exposure

GH984G alloy is a low cost Ni–Fe based wrought superalloy designed for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the microstructure evolution and tensile properties of GH984G alloy with different Ti/Al ratios during thermal exposure at different high temperatures are investigated. Detailed microstructure analysis reveals that the Microstructure of alloys with different Ti/Al ratios are similar after standard heat treatment, and the primary precipitates are γ′, MC, M₂₃C₆ and M₂B. However, η phase precipitates at grain boundary in the alloy with high Ti/Al ratio after thermal exposure at 750 °C for 570 h. By contrast, the microstructure stability of the alloy with lower Ti/Al ratio is excellent. There is no detrimental phase even if after thermal exposure at 750 °C for 5000 h in the alloy with lower Ti/Al ratio. γ′ coarsening plays a great role on the tensile strength, and the critical size range of γ′ could be defined as approximately 27–40 nm. The influence of η phase on tensile strength has close relationship with its volume fraction, the high volume fraction results in the decrease of tensile strength. The tensile strength of the alloy with lower Ti/Al ratio is obviously higher than the alloy with higher Ti/Al ratio and the yield strength has no obvious decrease during long-term thermal exposure at 700 °C. It is demonstrated that the thermal stability of microstructure and mechanical properties of GH984G alloy can be improved by moderately decreasing Ti/Al ratio to satisfy the requirement of A-USC plants.     

Stability of microstructure and mechanical properties of K17G cast nickel-base superalloy during prolonged aging

K17G cast nickel-base superalloy has an attractive combination of mechanical properties: high strength and good ductility as well as low density. In particular, after prolonged exposure at 750, 850 (both up to 10000 h) and 950°C (up to 1500 h), this alloy proved to have excellent phase stability and no sigma phase was found. The effects ofprolonged exposure on high temperature tensile and stress rupture properties were not serious and decreased gradually with aging time. The change in properties of specimens taken directly from the turbine blades followed those ofthe cast-to-size specimens. The rupture lives, tensile and rupture ductilities were approximately one third lower compared with those ofthe cast-to-size specimens, but still remained at reasonably high levels.     

等温时效对新型Sn-Ag基复合钎料显微组织和力学性能的影响

研究了等温时效对Sn-3.5Ag共晶钎料及其复合钎料的力学性能和显微组织变化的影响。为了弥补传统复合钎料制备和服役中强化颗粒容易粗化的问题, 制备了不同种类最佳配比的具有纳米结构的有机无机笼型硅氧烷齐聚物(POSS)颗粒增强的Sn-Ag基复合钎料。对钎焊接头在不同温度（125、150、175℃）下进行时效,通过SEM和EDAX分析了钎料与基板间金属间化合物层(IMC)的生长情况。结果表明, 经过不同温度时效,复合钎料钎焊接头界面处金属间化合物的生长速率比Sn3.5Ag共晶钎料慢, 复合钎料的IMC生长的激活能分别为80、97和77kJ/mol,均高于Sn3.5Ag共晶钎料。经过150℃时效1000h后,复合钎料钎焊接头的剪切强度分别下降了22%、13%和18%,下降幅度相当或明显小于Sn-3.5Ag钎料钎焊接头。      

Microstructure evolution and mechanical properties of 316H weld metal during aging

ABSTRACT

A standard 316H welding wire was used to obtain 6% δ-ferrite in the weld metal by the automatic tungsten inert gas welding. The effect of aging time at 750°C on microstructure and mechanical properties of the weld metal were analysed. The results showed that as the aging time increased, the δ-ferrite fraction decreased, and finally, δ-ferrite was no longer seen, the M₂₃C₆ fraction first increased and then decreased, and the σ phase fraction increased. Finally, the fractions of M₂₃C₆ and σ phase kept stable, but the size increased. Accordingly, as the aging time increased, the yield and tensile strengths first decreased significantly, then increased obviously and finally decreased slightly again, while the elongation and impact energy decreased all the time.     

Quantitative study of microstructure evolution and the effect on mechanical properties of Super304H during aging

In this paper, the evolution of microstructure and mechanical properties of Super304H during aging at 650 °C is investigated, and the effect between microstructure evolution and properties change are discussed. The results shows that the precipitated phases in the material during aging mainly include Cu-rich and MX particles with nano-size dispersed in matrix, M₂₃C₆ and σ particles with micron-size located at the grain boundary. These precipitates change significantly in quantity and size with the aging time, which affect the mechanical properties of the material. It helps to improve the tensile performance and hardness of the material, but it has an adverse effect on the impact energy. In addition, it is found that the degradation of impact energy is divided into two stages. Stage I witnesses the rapid drop in impact energy caused by Cr-depletion, which occurs between 0 to 1000 h during aging at 650°C. The content of Cr near the grain boundary rapidly drops from 21% to about 16% due to the precipitation of M₂₃C₆ particles on the grain boundary. Stage II is the stage of impact energy drop relatively smoothly, which occurs after aging for 1000 h. In this stage, although the Cr content increases from 16% to about 20% and remains stable due to intracrystalline diffusion, the impact energy decreased slowly, besides the M₂₃C₆ particles located at grain boundaries, the micron-sized brittle σ precipitated is another important reason.