B含量对K417G合金凝固过程中组织演变和力学性能的影响

Effect of Boron Content on Microstructure Evolution During Solidification and Mechanical Properties of K417G Alloy

研究了B含量对K417G合金凝固过程中相析出以及铸态组织和力学性能的影响。研究不仅证实了B显著促进元素偏析以及凝固后期(γ+γ')共晶析出,还发现B对K417G合金凝固早期基体γ相的析出和长大具有明显影响。B降低基体γ相的析出温度,抑制γ相的形核,并阻碍γ相生长。当B含量低于0.036%时,由于B降低γ相形核率,导致K417G合金的晶粒组织随B含量增加明显粗化。当B含量增加至0.060%时,尽管B仍然降低γ相形核率,但由于其阻碍γ相枝晶的早期生长,使熔体内部过冷度升高,局部发生晶粒细化。B对K417G合金力学性能的影响决定于硼化物的晶界析出形态,当B含量低于0.036%时,硼化物以颗粒状在晶界析出并对晶界产生强化作用,合金的900℃拉伸性能和900℃、315 MPa持久性能随B含量的增加而显著提高。当B含量达到0.060%时,共晶态硼化物在(γ+γ')前沿析出,导致共晶态硼化物与(γ+γ')的界面显著弱化,拉伸和持久性能显著降低。

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 inves tigated, providing some fun damental in formation for the control of boron additi on in the alloy. It has been found that boron aggravated the elemental segregation and promoted the eutectic (y+y1) precipitation at the final stage of the solidification of K417G alloy. In addition, boron decreased the precipitation temperature, and hence reduced the nucleati on rate of the y matrix. When the boron con tent was below 0.036%, the grai n size was increased with the in creme nt of B content, which is caused by the decreased nucle ation of the y 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 y 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 in creasing B content in this additi on range. When the B conte nt was in creased to 0.060%, the boride was precipitated as eutectic form in front of the eutectic (y+y′). The tensile and stress rupture properties were decreased due to the weak cohesion between the eutectic (y+y′) and the eutectic form borides.