K416B镍基高温合金组织和性能研究

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	K416B镍基高温合金组织和性能研究
	杨彦红
学位类型	博士
导师	孙晓峰 ; 于金江
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	K416b镍基高温合金长期时效拉伸持久蠕变热疲劳铪 Nickel-base Superalloy K416b Long Term Thermal Exposure Tensile Stress Rupture Creep Thermal Fatigue Hafnium
摘要	"本文研究了铪元素对K416B合金组织和力学性能的影响，以及K416B合金瞬时拉伸性能、持久蠕变行为、热疲劳行为和950℃长期时效对组织及性能的影响，得到如下主要结论： Hf元素含量的增加能够显著减小合金中g¢相的尺寸、显微疏松的体积分数、合金元素的偏析系数，并且还能够降低合金的初熔温度。随Hf元素含量的增加，合金的力学性能有所提高，这与Hf能够强化晶界和g¢相有关。与此同时，Hf元素的添加可以抑制长期时效过程中TCP相的析出，这与Hf能够促进M6C型碳化物的析出和降低基体的过饱和度有关。合金的屈服强度及抗拉强度随温度变化规律基本相同，随拉伸温度的升高，屈服强度逐渐升高，并在800℃时达到峰值；当温度继续升高时，拉伸强度随温度的升高而迅速降低。合金的延伸率在中温区间700℃和800℃呈现两次低谷。在不同的温度区间内合金有不同的变形机制：低温区，合金的变形机制主要是位错切割g¢相；高温区位错绕过g¢相；而在中温区则为两者的过渡态。在测试温度范围内K416B合金蠕变曲线均呈现出蠕变三阶段特征。中温区(800-900℃)蠕变变形机制为位错切入γ′相并形成层错；高温区(975-1100℃)蠕变变形机制为位错以攀移机制越过γ¢相。蠕变过程中M6C型碳化物在γ/γ′相界面处的析出有利于提高合金的蠕变抗力。在不同温度下合金的断裂特征也不同：中温条件下，合金的断裂特征为混合型断裂，碳化物和晶界成为主要裂纹源；高温条件下，合金的断裂特征为微孔聚集型断裂，疏松和γ/γ′相界面成为主要的裂纹源。上限温度对K416B合金的热疲劳性能有显著影响，随上限温度的升高，裂纹萌生周次缩短，并且裂纹的扩展速率增加，合金的热疲劳性能下降。热疲劳裂纹主要沿枝晶间及碳化物界面处扩展，高温氧化和循环热应力的作用是热疲劳损伤的主要机制。热疲劳变形机制在900℃条件下为位错切割g¢相，随上限温度升高至1050℃时，位错在热激活的作用下以攀移机制越过g¢相，并在g¢相上形成位错网。长期时效过程中g¢相尺寸随时效时间延长而增大，并发生明显的形筏。g¢相之间相互连接形成不规则的筏形组织。MC型碳化物的形貌由“汉字体”状退化为颗粒状或棒状。同时，富Cr的M23C6型碳化物在晶界析出并长大。长期时效后合金的室温拉伸性能随时效时间增加而降低。持久寿命随时效时间延长而降低，而持久延长率基体保持不变。长期时效后合金室温拉伸变形机制主要为位错切割g¢相，长期时效后合金持久变形机制则由位错攀移越过g¢相向位错切割g¢相转变。"
其他摘要	"The effect of Hf on the microstructure and mechanical properties of K416B alloy are investigated. The tensile, creep and stress rupture properties, thermal fatigue and the stability of the microstructure and mechanical properties after long-term thermal exposure are also investigated. The main results are summarized as following: With the addition of Hf element, the size of g¢ precipitates, the volume fraction of pores, the incipient melting temperature and element segregation are significantly reduced. Furthermore, the mechanical properties are significantly influenced by the addition of Hf element. The tensile strength increases with the addition of Hf element, due to the strengthen grains boundaries and g¢ precipitates. Additionally, the Hf addition can promote the precipitates of M6C carbides, which depresses the degree of supersaturation. Therefore the TCP phase would be depressed during long-term thermal exposure. Both the yield strength and ultimate tensile strength increases with the temperatures. The yield strength increases with temperature until it reaches a peak at 800℃. Beyond this point, the strength decreases sharply. The elongation shows ductility minimum at 700℃and 800℃, separately. At low temperatures, the dominant deformation mechanism is g¢ shearing by dislocations. At high temperatures, the deformation is dominated by g¢ by-pass. Intermediate temperatures exhibited transitional behavior. The creep curves of K416B alloy exhibit strong temperature dependence. In this test temperature range, the creep curve exhibits obvious three stages. With the increasing in temperature, the deformation mechanism transfer from g¢ shearing by dislocations and forming stacking faults to dislocation climbing. At intermediate temperature, the fracture shows both cleavage feature and quasi-cleavage feature. The cracks can initiate at the interface of g/g¢ eutectic and M6C carbide/matrix interface and grain boundaries. At high temperature, the fracture mode is microviod coalessence fracturecracks. The cracks can initiate at the interface of g/g¢ precipitates and porosity. Thermal fatigue tests were performed on the K416B alloy. The crack growth rate increases with the rise of upper temperature. It indicates that the thermal fatigue property decreases with the rise of upper temperature. The thermal cracks propagate mainly at the interface of carbide/matrix and interdendrite region. The elevated temperature oxidation and cycle thermal strain leads to the failure of thermal fatigue. There are some dislocations owing to large plastic deformation. At 900℃, the thermal fatigue deformation mechanism is g¢ shearing by dislocations. At 1100℃, the dislocation climbing over the g¢ precipitates is the main deformation mechanism. The g¢ precipitates coarsen and the adjacent g¢ precipitates meet and link together and form irregular morphologies during aging. The MC carbides with Chinese-script shape decompose to blocky or granular shape after thermal exposure. The Cr-rich M23C6 carbides nucleate and grow at grain boundaries. The room temperature tensile deformation mechanism is mainly g¢ precipitates shearing by dislocations While, the deformation mechanism transforms from dislocations gliding g¢ precipitates to dislocations shearing g¢ precipitates after stress rupture test."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64501
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	杨彦红. K416B镍基高温合金组织和性能研究[D]. 北京. 中国科学院金属研究所,2012.