【作者】 李亚敏；

【导师】 郝远；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2011， 博士

【摘要】 铸造K4169合金与美国Inconel718合金成分基本相同,是以体心四方γ"相和面心立方γ’相沉淀强化的铁-镍基高温合金,在-253～700℃温度范围内有良好的综合性能,但使用温度超过650℃时,合金中的主要强化相γ"会与基体失去共格,聚集粗化长大继而向稳定的δ相转变,导致合金的强度、塑性等一系列性能迅速下降。高温合金的性能主要取决于它的化学成分和组织,国内外研究工作者尝试通过调整Al、Ti、Nb、Fe、Mo、Cr、Ta、W等主要合金元素含量以期获得更好的组织稳定性和更高的使用温度。但是,K4169合金中除了上述主要合金元素外还含有少量的Co和微量的Zr、Cu等元素,这些元素对合金组织和性能也会造成一定的影响。目前这些元素在K4169合金中的作用还不是很清楚,因此有必要进行深入研究和分析。本文以K4169合金为研究对象,利用手工电弧炉熔炼制备了14种不同Al、Co、Zr、Cu含量的合金,并采用金相显微镜、扫描电镜、透射电镜、电子探针、X射线衍射和力学性能测试等方法,研究了Al、Co、Zr、Cu对铸态、标准热处理态以及长时时效态合金组织和力学性能的影响,着重研究了这些元素对合金晶界和晶内析出相的影响,得到以下主要结论:(1)合金中Co的加入量从0.1wt%增加到0.3wt%时,促使Mo、Ti、Nb等元素向晶界偏析。因此Co强烈促进合金铸态组织中Laves相的析出,缩小合金枝晶间偏析区,降低Nb、Mo、Ti在基体中的溶解度而提高这些元素在Laves相中的浓度;Co的加入提高了Laves相初熔温度,而使合金的熔点稍有降低。(2)同时调整合金中Al和Co的加入量时,由于Al减轻Nb、Mo、Ti的偏析,因此Al和Co的交互作用减少了Laves相的析出且基本不改变Laves相初熔温度,降低了合金的熔点。合金经标准热处理后,Al和Co之间的交互作用减小了Co对合金晶界析出相的不利影响,并且使晶内析出相以纳米多晶的形式析出,晶内析出相呈球形,但没能形成γ′/γ″包覆组织。在长时时效过程中,Al和Co的交互作用增加了γ″相的稳定性,降低了合金晶内析出相的粗化程度。(3)Zr促进碳原子向晶界偏聚,因而促进晶界碳化物析出,同时Zr促进Nb、Mo等原子向MC中偏聚,抑制了晶界Laves相的析出。Zr的加入提高了Laves相初熔温度,降低了合金的熔点。Zr强烈改变γ′和γ″相析出形貌和大小。在长时时效过程中,Zr的加入可以提高γ″相的高温稳定性;当Zr质量分数为0.03wt%时,合金中形成了γ′/γ″包覆组织,没有形成盘片状δ相。(4)Cu促进Nb、Mo等元素的偏析,因此强烈促进Laves相的析出;Cu的加入降低了Laves相初熔温度,同时降低了合金的熔点。Cu的加入改变了γ″相和γ′的析出形貌,随着Cu质量分数的增加,γ″相尺寸呈先增加后减小的趋势;在长时时效过程中Cu有抑制γ″相向δ相转变的作用,即Cu增加了γ″相的稳定性。(5)单独调整Co的加入量对K4169合金室温硬度的影响较小但降低了合金的屈服强度;同时调整合金中Al和Co的加入量时,Al含量的增加提高了合金的硬度和屈服强度,但在长期时效过程中出现了过时效,导致合金硬度降低了;Zr的加入降低了合金的硬度和屈服强度;Cu的加入也降低了合金的硬度和屈服强度。综上所述,对于K4169合金来说,Co的含量要严格控制;Al的加入量可以适当提高,本文中Al含量为1.1wt%时合金的组织和性能较好;Zr对合金晶内析出相的改善是有利的,但综合考虑晶界析出相及合金的性能,Zr的加入量应小于0.03wt%;考虑Cu促进Laves相的析出以及Cu增加γ″相的稳定性,Cu在K4169合金中可以适量存在,但含量应控制在0.1wt%左右为宜。更多还原

【Abstract】 As-cast K4169 alloy has the same composition with Inconel 718, which is the iron-nickel superalloy strengthened by precipitated phase B.C.T.γ" and F.C.C.γ’. K4169 alloy has good comprehensive performance between -253°C to 650°C. However, when the service temperature is above 650°C, theγ" phase becomes incoherent with matrix alloy, and then aggregates, grows up and transforms to stableδphase, so many properties such as strength and plasticity rapidly deteriorate. The properties of superalloys depend on the chemical composition and microstructure. To improve the properties at elevated temperature, alloying methods were applied to increase the structure stability of K4169 alloy by adjust the content of Al, Ti, Nb, Fe, Mo, Cr, Ta, W, et al. Except above elements, there are small amounts of Co and trace quantities of Zr and Cu in K4169 alloys to influence the microstructure and properties. However, the effect of these trace elements in K4169 alloys is not clearly understood and need further research and analyse. In this paper, 14 kinds of superalloys with different Al, Co, Zr and Cu addition were prepared by manual electric arc furnace. The effect of Al, Co, Zr and Cu on microstructure and properties of as-cast, standard heat treatment and long-time aging K4169 supperalloy, especially on the precipitates in grain and boundaries, were investigated by metallurgical microscopy (OM), scanning electronic microscope (SEM), transmission electron microscopy (TEM), electron microprobe analysis (EMPA), X-ray diffractometer (XRD) and mechanics performance testings. The research conclusions are as follows:(1) When the addition of Co element increases from 0.1wt% to 0.3wt%, the elements of Mo, Ti, Nb segregate to grain boundaries. So Co promotes the precipitation of Laves phase, reduces interdendritic microsegregation area, decreases the solubility of Nb, Mo, Ti in matrix and increases the solubility in Laves phase. The addition of Co increases the incipient melting temperature of Laves phase and slightly decreases the melting temperature of alloys.(2) When the additions of Al and Co vary simultaneously, the segregation of Mo, Ti, Nb are reduced. The interaction of Al and Co decreases the precipitation of Laves phase without changing the incipient melting temperature of Laves phase, but the melting temperature of alloys is reduced. After standard heat treatment, the interaction of Al and Co eliminates the unfavorable effects of Co on the precipitation phases at grain boundaries; the precipitation phases in grains are precipitated as nano polycrystalline; theγ″phases are precipitated as spherical shape but theγ′/γ″clad structure isn’t gotten. In the long-time aging process, the interaction of Al and Co increases the stability ofγ″phases and decreases the coarsening rate of precipitation phases in grains.(3) The addition of Zr element promotes the segregation of C atoms to grain boundaries and precipitation of carbides at grain boundaries. In the meantime, the segregation of Nb, Mo in MC phases inhibits the precipitation of Laves phases at grain boundaries. The addition of Zr increases the incipient melting temperature of Laves phase and decreases the melting temperature of alloys. The Zr element strongly changes morphology and size ofγ′andγ″phases. In the process of long-time aging, the addition of Zr can increase the high temperature stability ofγ″phase; theγ″/γ′clad structure is formed and no flakyδphase is generated when the Zr addition is 0.03wt%.(4) The addition of Cu element promotes the segregation of Nb, Mo and precipitation of Laves phase. The addition of Cu decreases the incipient melting temperature of Laves phase and decreases the melting temperature of alloys. The addition of Cu changes the morphology ofγ″andγ′precipitation. With the increasing of Cu addition, the size ofγ″phase shows the trend of increasing firstly and then decreasing. In the process of long-time aging, Cu shows the effect of inhibiting the change ofγ″toδphase, so the Cu element increases the high temperature stability ofγ″phase.(5) The separately adjusting of Co addition has slight effect on the hardness at room temperature but decrease the yield strength of the test alloy. When the content of Al and Co is adjusted simultaneously, the hardness and yield strength of tested alloy increase with Al content. However, after long-time aging, the hardness is decreased due to the overaging. The Zr addition decreases the hardness and yield strength of the test alloy. The addition of Cu element decreases the hardness and yield strength too.In conclusion, in K4169 alloys, the Co content should be strictly limited; the addition of Al element can be properly increased, and the better content of Al is 1.1 wt% according to the experiments in this paper; the Zr element helps to improve the precipitated phases in grains, so small amounts of Zr can be added, but the content should be less than 0.03 wt% in consideration of precipitated phases at grain boundaries and property of alloy; because the Cu element can promote the precipitation of Laves phase and improve the stability ofγ″phase, the Cu elements can be properly added but the content need be strictly control less than 0.1 wt%.更多还原