The precipitation behavior of carbide in K416 B superalloy was investigated by means of creep measurement and microstructure observation. The results show that nanometer M6 C particles discontinuously precipitate in the γ matrix or along the γ/γ′ interface of the alloy during high temperature tensile creep. Thereinto, the amount of fine M6 C carbide increases as creep goes on, and the coherent interfaces of M6 C phase precipitating from the γ matrix are {100} and {111} planes. The thermodynamics analysis indicates that the solubility of element carbon in the matrix decreases when the alloy is deformed by the axial tensile stress during creep, so as to cause the carbon segregating in the regions of stress concentration and combining with carbide-forming elements M（W, Co）, which promotes the fine M6 C carbide to precipitate from the γ matrix.
The pattern of dendritic growth and distortion of dendritic network in the platform have been investigated by one mold casting with different platform length during directional solidification. As the platform length elongates, the symmetry of dendritic growth along left and right edges gradually worsens in platform base. While the platform length reaches 14 mm, the distortion of dendritic network is first observed in outward platform. It is found that the distortion of dendritic network along platform inside is more serious than that along platform edges. Both [001] deviation and accumulated misorientation along platform inside, up to 9~ and 16.3~, respectively, are far greeter than those along left-outward-right edges. The deformation of dendritic network in a platform may be caused by the asymmetry of the solidification front at the mush zone.
Xiaoli ZhangYizhou ZhouYanyun HanTao JinXiaofeng Sun
The effect of long-term thermal exposure on the tensile behavior of a high W content nickel-based superalloy K416B was investigated.The microstructure and the deformation characteristics were observed by scanning electron microscopy and transmission electron microscopy,and the phase transformation of the alloy during long-term thermal exposure was analyzed by X-ray diffraction patterns and differential thermal analysis.Results showed that after thermal exposure at 1000℃,the MC carbides in the K416 B alloy decomposed into M_(6)C.During tensile deformation,dislocations slipping inγmatrix crossed over the M_(6)C by Orowan bowing mechanism.With the increase of thermal exposure time,the secondary M_(6)C reduced greatly the yield strength of the alloy at room temperature.Meanwhile,the continuous distribution of the secondary M_(6)C with great brittleness in the grain boundary could become the main source of crack,which might change the fracture characteristic of the alloy from trans-granular to intergranular.
With the development of nucleation theory,the calculation of phase diagrams(CALPHAD)method and microscopic elasto-plasticity mechanics,it is possible to apply the phase-field method to simulate the γrafting behavior of nickel-base single-crystal superalloys under the conditions of heat treatment,fatigue and creep testing.Based on the experimental progress in γrafting behavior and in combination with the microstructural simulation of the precipitation and rafting of c0phase,this paper reviews the phase-field investigation of γrafting in nickel–base single-crystal superalloys.
Yan ZhaoHongyu ZhangHua WeiQi ZhengTao JinXiaofeng Sun
The effect of solidification rate on grain structure evolution during directional solidification （I J：5） ot a Ni-oasea superalloy was explored. It was found that a high solidification rate led to sharper 〈001〉 texture and smaller grain size in the DS samples. One of the most important findings in this work was that such result was not in accordance with the general concept, and the sharper 〈001〉 texture was accompanied by the larger grain size. To explain the contradiction, the modeling samples with five grains were produced and the effect of solidification rate on the evolution of grain texture was illustrated based on the modeling samples.
K416B Ni-based superalloy with high W content has good high temperature properties and low cost,which has a great development potential.To investigate the room temperature tensile property and the deformation feature of K416B superalloy,tensile testing at room temperature was carried out,and optical microscopy (OM),scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the deformation and damage mechanisms.Results show that the main room temperature tensile deformation features of the K416B nickel-based superalloy are dislocations slipping in the matrix and shearing into γ’ phase.The <110> super-dislocations shearing into γ’ phase can form the anti-phase boundary two coupled (a/2)<110> partial-dislocations or decompose into the configuration of two (a/3)<112> partial dislocations plus stacking fault.In the later stage of tensile testing,the slip-lines with different orientations are activated in the grain,causing the stress concentration in the regions of block carbide or the porosity,and cracks initiate and propagate along these regions.
Based on a 5% （mass fraction） Re-containing single crystal nickel-based superalloy with 3% （mass fraction） Cr, the microstructural variation with Cr addition was investigated. The experimental results show that segregation of alloying elements was enhanced in as-cast microstructure with Cr addition; and the volume fraction of eutectic is increased. However, the solidus and liquidus temperatures are remarkably reduced. With the increase of Cr content, the average γ＇ size and volume fraction are decreased in the fully heat treated microstructure. X-ray diffraction results indicate that γ/γ＇ lattice misfit becomes more negative. According to the energy dispersive spectroscopy （EDS） results, Cr is mainly distributed in the γ matrix; and more y-forming elements, such as Re and W, enter the γ matrix, while the γ/γ＇ partition ratio of Cr is inversely decreased.
