【作者】 于兴福；

【导师】 田素贵； 崔树森；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2008， 博士

【摘要】 本文通过热力学及TEM方法测算了Ni基合金的层错能,并对[001]取向的镍基单晶合金、P-型结构单晶合金进行蠕变性能测试和SEM、TEM形貌观察,研究了合金元素、层错能对镍基单晶合金稳态蠕变期间位错运动内摩擦应力及蠕变特征的影响,考察了组织结构对单晶合金蠕变期间组织演化规律及蠕变寿命的影响,并对合金的变形机制进行了深入讨论。得出如下结论:单晶合金在凝固期间形成的共晶组织由条状粗大和细网状γ/γ’两相构成,其中粗大的γ/γ’相由包晶反应生成,细网状γ/γ’共晶组织形成于共晶反应。铸态单晶合金在枝晶臂/间存在明显的成分偏析和γ/γ’两相的尺寸差别,致使铸态合金有较大的晶格错配度。通过差热曲线分析及尝试法,制定出合金的热处理工艺。经完全热处理后,元素的偏析程度明显减少,且立方γ’相以共格方式嵌镶在γ基体相中,且均匀分布,可使合金的晶格错配度减小。元素Al可明显降低Ni-Al-M合金的层错能,随着Al含量的增加,合金层错能降低的幅度增加,随温度提高,合金的层错能增加;其中元素Al降低合金中原子的偏聚自由能,促进γ’有序相的形成是降低Ni-Al-M合金层错能的主要原因。高层错能合金有较低的内摩擦应力和蠕变抗力,随层错能降低,单晶合金的内摩擦应力和蠕变抗力提高,并延长蠕变寿命;与其它合金比较,设计及制备的低层错能无Re镍基单晶合金具有较高的蠕变抗力,在1040℃、137MPa条件下的持久寿命达到1280h。在中温高应力蠕变期间,该合金的变形机制是位错在γ基体中运动和剪切γ’相,切入γ’相内的<11O>超位错即可在{111}八面体晶面滑移,也可在{100}立方体晶面滑移;当(1/2)[110]位错在γ’/γ两相界面切入γ’相发生分解,可形成(1/3)<112>超肖克莱不全位错加层错的位错组态。而在高温低应力的蠕变初期,合金的变形机制是(1/2)<110>位错在γ基体通道的八面体滑移系中以交滑移方式运动;γ’相形成筏状结构后,合金的变形机制是位错攀移越过筏状γ’相;而蠕变后期,合金的变形机制是<110>螺、刃型超位错剪切γ’相。在蠕变期间,P-型结构合金发生了复杂的组织演化,其P-型γ’相转变成具有较短尺寸的N-型筏状结构,使位错易于滑移越过γ’相,是使P-型结构合金具有较高应变速率和较短蠕变寿命的主要原因。在实验的温度和应力范围内,与P-型结构合金相比,完全热处理态合金具有较低的应变速率和较长的持久寿命;分别计算出热处理态合金和P-型结构合金在稳态蠕变期间的激活能分别为Q_a=462kJ/mol和Q_a=412.5 kJ/mol,表观应力指数分别为n_a=3.5和n_a=5.2。在拉伸蠕变期间,不同成分合金中γ’相具有不同的定向粗化速率,随合金中Ta+Mo含量及Ta/W比值的增加,溶质元素(Al、Ta)的扩散及γ’相的定向粗化速率降低。拉伸蠕变期间,类立方γ’相中与施加应力轴垂直的界面受水平切应力,使晶格收缩可排斥较大半径的Al、Ta原子;与应力轴平行的界面受拉伸张应力,使晶格扩张可诱捕较大半径的Al、Ta原子,由此引起的原子偏聚形成γ’相是自由能降低的过程;其中,较大半径的Al、Ta原子扩散迁移至{100}晶面,形成异类原子结合键及稳定的堆垛方式,是促使γ’相形成N-型筏状结构的主要原因;而γ’相不同界面的应变能密度变化是元素扩散及γ’相定向粗化的驱动力。更多还原

【Abstract】 In this dissertation,the stacking fault energies of nickel-base superalloys are calculated by the thermodynamic and TEM methods,the influence of elements,stacking fault energy on the internal friction stress and creep features of single crystal nickel base superalloys during creep are investigated by means of the measurement of creep properties and SEM,TEM observation,to explore the microstructure evolution regularlity during tensile creep and the influence of the microstructure on the creep lifetimes of superalloys, and in the further,to discuss the creep mechanism of the designed alloy during creep.Some of the conclusions are given as following:The eutectic microstructure of the single crystal nickel-base superalloy formed during solidification consists of the thicker strip-likeγ/γ′phases and finer network-likeγ/γ′phases, thereinto,the thicker strip-likeγ/γ′phases result from the peritectic reaction,and the finer network-likeγ/γ′phases originate form the eutectic reaction.The obvious composition segregation and size difference ofγ′phase in as-cast single crystal nickel-base superalloy apprear in the dendrite and interdendrite regions,which results in a bigger lattice misfit betweenγandγ′phases in the alloy.The heat treatment regimes of the alloy are constituted by means of the DTA curve analysis and a trial and error method.After fully heat treatment,the composition segregation in the alloy was obviously improved,and the cubicalγ′phase was coherently embedded in theγmatrix phase,which decreases the lattice misfit betweenγandγ′phases in the alloy.The stacking fault energy（SFE） of Ni-Al-M alloys may be decreased by adding the element Al,the decreased extent of SFE increases with Al element,and the SFE of the Ni-base superalloys decreases with the elevated temperature.The element Al may decrease the accumulated Gibbs free energy of Ni-base alloy,and promotes the formation of theγ′-Ni₃Al ordered phase,this is a main reason of decreasing the SFE of the Ni-Al-M alloys. The alloys with higher stacking fault energy possess a lower internal fracture stress and creep resistance.The internal fracture stresses and creep resistance of the nickel-base single crystal supralloys are improved with the drop of stacking fault energy,this enhances the creep lifetimes of the superalloys.Compared with the other alloys,the designed Re-free nickel base single crystal superalloy possesses a better creep resistance,and the creep lifetimes of the alloy at 1040℃,137MPa conditions is enhanced to 1280 h.During intermediate temperature and high stress creep,the deformation mechanism of the alloy is theγ′phase sheared by the <110> super dislocations which may move both on {111} octahedral crystal planes and on {100} cubical crystal planes,the super-dislocations resulted from the reaction may be cross-slipped to {100} the cubical crystal planes from the {111} ones.If（1/2）<110> dislocation shears into theγ′phase from theγ′/γinterface to occur the reaction,this may promote the formation of（1/3）<112> super partial dislocation+stacking fault configuration.During the initial stage of high temperature and low stress creep,the deformation mechanism of the superalloy is the slipping of（1/2）<110> dislocations activated on the octahedral slipping planes of theγmatrix channel in the form of cross-slip.After theγ′phase transformed into the rafted structure,the deformation mechanism of the superalloy during creep is the dislocations over the raftedγ′phase by climbing.And in the later stage of creep,the deformation mechanism of the superalloy is the screw or edge <110> super-dislocations in character shearing into the raftedγ′phase.In the range of experimental temperatures and stresses,compared to the P-type structure alloy,the fully heat treated superalloy displays a lower strain rate and longer stress rupture lifetime,the creep activation energies and apparent stress exponents of the fully heat treated and P-type structure alloys are calculated to be Q_a =462 kJ/mol and Q_a=412.5 kJ/mol,respectively,and the apparent stress exponents being n_a=3.5 and n_a=5.2,respectively.During tensile creep,a complicated microstructure evolution occurs during creep of P-type structure alloy in which theγ′phase with the P-type structure is transformed into the N-type rafted structure with a shorter size,so that the moving dislocations are easily over the rafts by slipping.This is a main reason of P-type structure alloy possessing a higher strain rate and shorter creep lifetime.During tensile creep,the superalloys with different compositions display a different rate ofγ′phase directional coarsening.The diffusion rates of the solutes elements andγ′phase directional coarsening are reduced with the increase of the Ta+Mo content and Ta/W ratio in the superalloys.During tensile creep,a shearing stress is applied on the cubical-likeγ′phase interface vertical to the stress axis,which results in the lattice constriction of theγ′phase to repel out the Al,Ta atoms with bigger radius. At the same time,a tension stress is applied on theγ′phase interfaces along the direction parallel to the stress axis,which results in the lattice expansion ofγ′phase to trap the Al,Ta atoms with the bigger radius.This brings out the accumulation of the solute atoms（Al,Ta） to form the N-type rafted structure.Al,Ta atoms with bigger radius diffuse to the {100} plane to form the linked bond of the heterogeneous atoms and the stable stacking mode,this is a main reason of promoting the transformation ofγ′phase into the N-type rafted structure.And the change of the strain energy density in different interfaces of the cubical-likeγ′phase is thought to be the driving force of the elements diffusion and theγ′phase directional growth during creep.更多还原