1. AECC Sichuan Gas Turbine Establishment, Chengdu 610500, China 2. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 3. Laboratory of Nickel and Cobalt Resources Comprehensive Utilization, Jinchuan Group Co. , Ltd. , Jinchang 737100, China
Cite this article:
Qiaomu LIU,Shunzhou HUANG,Fang LIU,Yan YANG,Hongqiang NAN,Dong ZHANG,Wenru SUN. Effect of Boron Content on Microstructure Evolution During Solidification and Mechanical Properties of K417G Alloy. Acta Metall Sin, 2019, 55(6): 720-728.
Boron is a key element in superalloys and many other metallic materials for strengthening the grain boundaries. However, it also has harmful effect on aggravating the solidification segregation of the alloys. Although the mechanism for the influences of B on the alloys has been studied extensively, it is still required to study in some alloys currently because the compositive effects of boron in different alloys are sometimes distinct. K417G, a cast superalloy with good comprehensive properties, has been applied in aero engines of China. In the present work, the effects of boron content on the microstructure evolution during the solidification and the mechanical properties of the as cast K417G alloy have been investigated, providing some fundamental information for the control of boron addition in the alloy. It has been found that boron aggravated the elemental segregation and promoted the eutectic (γ+γ') precipitation at the final stage of the solidification of K417G alloy. In addition, boron decreased the precipitation temperature, and hence reduced the nucleation rate of the γ matrix. When the boron content was below 0.036%, the grain size was increased with the increment of B content, which is caused by the decreased nucleation of the γ phase. When the B addition was increased up to 0.060%, the grain was refined at some local places, because the growth of the dendrites was inhibited and the γ phase could nucleate at the inner part of the subcooled liquids. The mechanical properties of K417G alloy were significantly influenced by the precipitation of the boride at the grain boundaries. The borides were precipitated as fine particles at the grain boundaries when the B addition was below 0.036%, and the tensile properties at 900 ℃ and the stress rupture properties at 900 ℃ and 315 MPa were markedly improved with the increasing B content in this addition range. When the B content was increased to 0.060%, the boride was precipitated as eutectic form in front of the eutectic (γ+γ'). The tensile and stress rupture properties were decreased due to the weak cohesion between the eutectic (γ+γ') and the eutectic form borides.
Fig.1 OM images of the as cast K417G alloy with 0.0017%B (a), 0.0059%B (b), 0.012%B (c) and 0.060%B (d)
Fig.2 OM images of the eutectic (γ+γ') precipitates in K417G alloy with 0.0017%B (a), 0.025%B (b) and 0.060%B (c)
Fig.3 Relationship between B content and area fraction of eutectic (γ+γ')
Fig.4 Effects of B content on the precipitation of the borides at the grain boundaries (a~d) and in front of the eutectic (γ+γ') (e) in K417G alloy with 0.0017%B (a), 0.036%B (b, c) and 0.060%B (d, e)
Fig.5 DSC curves of K417G alloy with 0.0017%B (a) and 0.036%B (b)
Fig.6 Effects of B content on the tensile properties of K417G alloy at 900 ℃
Fig.7 SEM images showing the macrostructures at fracture surfaces (a, c, e) and longitudinal sections (b, d, f) of tensile fractographs at 900 ℃ in K417G alloy with 0.0017%B (a, b), 0.036%B (c, d) and 0.060%B (e, f)
Fig.8 Stress rupture properties at 900 ℃ and 315 MPa of K417G alloy with different B contents
Fig.9 SEM images showing the macrostructures at facture surfaces (a, c, e) and longitudinal sections (b, d, f) of the samples tensiled to rupture at 900 ℃ and 315 MPa in K417G alloy with 0.0017%B (a, b), 0.036%B (c, d) and 0.060%B (e, f)
Fig.10 SEM images showing the microstructures at fracture surfaces (a, b, d) and longitudinal section (c) of the samples tensiled to rupture at 900 ℃ in K417G alloy with 0.0017%B (a), 0.036%B (b, c) and 0.060%B (d)
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