The MoSi2 coating on C103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffus...The MoSi2 coating on C103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi2, the two phases’ transitional layer consists of NbSi2 and a few Nb5Si3 and the diffuse layer is composed of Nb5Si3. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating.展开更多
The microstructures of Ti47Al8NbxMn (mole fraction, %) alloy with x=1 or 2 were studied on as-cast, as-HIPped and on heat-treated samples. The results showed that Mn element is β phase stabilizer and promotes the for...The microstructures of Ti47Al8NbxMn (mole fraction, %) alloy with x=1 or 2 were studied on as-cast, as-HIPped and on heat-treated samples. The results showed that Mn element is β phase stabilizer and promotes the formation of β(B 2) phase. B 2 phase exists at the dendrite cores of the as-cast microstructures. This phase is metastable and can completely decompose into α and(or) γ during 1 200 ℃, 200 MPa, 4 h HIP process. In the as-HIPed alloy with 1% (mole fraction) Mn addition, the α 2+γ lamellae structure is interrupted and decomposed resulting in grain refined near gamma microstructure. The α 2+γ lamellae structure near interdendritic regions decomposes almost completely into γ grains, but near dendrite cores incompletely into γ grains with a small amount of α 2 particles or needles around or in it. However, compared with the alloy with 1% (mole fraction) Mn, more α 2 phase is retained in the alloy with 2% (mole fraction) Mn. α 2 phase contains more Mn and is more stable in the alloy with 2% (mole fraction) Mn than in that with 1%(mole fraction) Mn. This makes its grain refinement more difficult. The amount of α 2 phase decreases following (1 250 ℃, 7 h-1 150 ℃, 15 h) for 3 cycles heat treatment.展开更多
文摘The MoSi2 coating on C103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi2, the two phases’ transitional layer consists of NbSi2 and a few Nb5Si3 and the diffuse layer is composed of Nb5Si3. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating.
文摘The microstructures of Ti47Al8NbxMn (mole fraction, %) alloy with x=1 or 2 were studied on as-cast, as-HIPped and on heat-treated samples. The results showed that Mn element is β phase stabilizer and promotes the formation of β(B 2) phase. B 2 phase exists at the dendrite cores of the as-cast microstructures. This phase is metastable and can completely decompose into α and(or) γ during 1 200 ℃, 200 MPa, 4 h HIP process. In the as-HIPed alloy with 1% (mole fraction) Mn addition, the α 2+γ lamellae structure is interrupted and decomposed resulting in grain refined near gamma microstructure. The α 2+γ lamellae structure near interdendritic regions decomposes almost completely into γ grains, but near dendrite cores incompletely into γ grains with a small amount of α 2 particles or needles around or in it. However, compared with the alloy with 1% (mole fraction) Mn, more α 2 phase is retained in the alloy with 2% (mole fraction) Mn. α 2 phase contains more Mn and is more stable in the alloy with 2% (mole fraction) Mn than in that with 1%(mole fraction) Mn. This makes its grain refinement more difficult. The amount of α 2 phase decreases following (1 250 ℃, 7 h-1 150 ℃, 15 h) for 3 cycles heat treatment.
