【作者】 潘晓林；

【导师】 胡壮麒； 杨洪才； 孙文儒；

【作者基本信息】 东北大学 ， 材料学， 2009， 博士

【摘要】 GH742合金是一种具有非常优异高温性能的高合金化镍基高温合金,使用温度可以达到750～800℃。但是由于合金化程度很高,合金的热加工问题非常突出,是合金发展的瓶颈。本文系统分析了铸态GH742和GH742y合金的凝固偏析行为与均匀化工艺,并重点研究了铸态GH742与轧态GH742M合金的热变形规律与动态再结晶形核机制,同时对锻态和轧态GH742M合金的热处理组织与力学性能进行了探讨,以期提高我国在难变形高温合金及其加工方面的研究水平,为高性能航空涡轮盘的国产化提供研究基础和实际指导。铸态GH742合金存在明显的枝晶偏析,Cr、Co、Al偏析于枝晶干,Nb、Ti、Mo偏析于枝晶间。γ’相在枝晶内的析出形貌和尺寸不同,枝晶干处为细小球形,枝晶间处为粗大方形或十字花形。MC碳化物,(γ+Y’)共晶,Laves相,δ相的枝晶间析出与Nb和Ti偏析有关；高Mo含量及其枝晶间偏析是析出σ相的重要原因；稀土元素La、Ce在枝晶间的富集促使含氧硫稀土相和Ni5Ce相的析出。合金的凝固温度区间为1346-1190℃,各相的凝固顺序为：γ基体,MC碳化物,(Y+γ’)共晶,Laves相,Ni5Ce相。合金中的δ相,σ相和γ’相是在凝固结束后固态析出的。稀土元素的加入可以有效降低O、S等杂质元素含量,改善碳化物的尺寸和分布,细化合金晶粒；同时,加剧Nb、Ti等元素的偏析,降低合金的终凝温度,促使许多有害相析出。铸态GH742合金中的稀土相在1120℃开始熔化,γ’相的完全溶解温度在1120℃以上,采用一种在1100℃低温预处理和1160℃高温扩散的二次均匀化工艺,可以获得无初熔、偏析极低的均一组织。元素扩散计算表明,提高均匀化温度显著提高元素的扩散系数,比增加均匀化时间更有效；Nb的扩散系数比Ti小得多。经过1100℃×30h+1160℃×40h均匀化处理可以提高合金的热变形塑性,降低变形抗力,有利于动态再结晶的发生。铸态GH742和轧态GH742M合金热压缩变形时的流变应力随着应变速率的降低和温度的升高而降低。高温、高或低应变速率均有利于动态再结晶的发生,相应的再结晶晶粒尺寸也较大。增加应变,有利于提高再结晶体积分数,但低应变速率变形时晶粒粗化。γ’相的存在恶化了合金的塑性,大幅提高了合金的变形抗力。γ’相能够钉扎晶界,阻碍晶界迁移,抑制动态再结晶形核。铸态GH742和轧态GH742M合金热变形时的应力指数和表观变形激活能随着温度的升高而降低,应变速率敏感性指数比较小,难以实现超塑性成形。应变对合金热加工图的影响很大,尤其是轧态GH742M合金,其变化反映了变形过程中的微观组织演化。低温、高应变速率变形容易产生绝热剪切带,而高温、低应变速率容易产生“楔形”裂纹。轧态GH742M合金高温变形的本构方程为：ε=8.89×10σp5.33exp(-809.48×103/RT)。合金热变形后的位错形态和位错密度是动态发展的,跟变形温度、应变速率以及应变均有关系,是加工硬化、动态回复和动态再结晶三者综合作用的结果。动态再结晶优先在原始晶界上形核,形成“项链”组织。铸态GH742和轧态GH742M合金再结晶开始形核机制均为不连续动态再结晶,但铸态合金随着应变的增加同时发生连续动态再结晶。孪生在动态再结晶形核和长大过程中发挥重要的作用,MC碳化物有利于再结晶的形核。应变速率不改变合金动态再结晶形核机制,但影响再结晶的形核和长大速率轧态GH742M合金在高温104～1s-1拉伸变形时,随着初始应变速率的提高,合金塑性和拉伸强度均逐渐提高。低应变速率时的拉伸断口为明显的沿晶断裂,而高应变速率拉伸时表现出一定的穿晶断裂特征。轧态GH742M合金在1080℃以下固溶处理时,晶粒尺寸变化不大；当固溶温度超过1080℃后,晶粒长大非常明显。经过1020℃×8h,AC+780℃×16h,AC热处理后,合金存在大量晶界强化相和三种尺寸的γ’相,可以提高在两相区变形的塑性,细化变形后的再结晶晶粒,有利于提高合金的热加工性能。GH742M合金经过标准热处理后的组织主要由γ基体,γ’相,MC和M23C6碳化物以及M5B4硼化物等组成。轧态GH742M合金的室温拉伸性能优于锻态GH742M合金,但持久寿命低于锻态合金；复合添加0.10%La和0.01%Ce对GH742合金室温拉伸性能影响不大。GH742y合金的合金化程度比GH742合金更高,其枝晶偏析也更严重,析出相也更复杂,枝晶间存在一次和二次析出γ’相,MC碳化物,(γ+γ,’)共晶,Laves相,σ相,μ相,δ相,M6C碳化物及Ni5Ce相等。合金凝固温度区间为1348～1167℃,各析出相的凝固顺序为γ基体,MC碳化物,一次γ’相,(γ+γ’)共晶,Laves相,Ni5Ce相和M6C碳化物。相分计算表明,枝晶间严重偏析区域的电子空位数已超过了2.30,导致σ相和μ相的析出。合金在1120℃发生初熔,γ,’的全溶温度在1140℃以上,采用1100℃+1160℃+1180℃三次均匀化工艺可以很好地消除第二相和元素偏析,获得均一的奥氏体组织。更多还原

【Abstract】 GH742 alloy is a high alloying nickel-base superalloy with excellent performance at high temperature, serving up to 750～800℃as turbine disc materials. However, the hot workability becomes quite poor due to the higher alloying level, which limits the further development of the alloy. In this dissertation, the solidification and segregation of as-cast GH742 and GH742y alloys were firstly analyzed, and then the hot deformation behavior and nucleation mechanism of dynamic recrystallization both in as-cast GH742 and as-rolled GH742M alloys were investigated, and finally the microstructure after standard heat treatment and mechanical properties of as-forged and as-rolled GH742M alloys were discussed. The main results are summarized as following:The as-cast GH742 alloy exhibits severe dendritic segregation with Cr, Co, Al segregated to the dendrite cores and Nb, Ti, Mo segregated to the interdendritic regions. The morphology ofγ’ particles is varied at different regions, which are spherical and small in the dendrite cores, while cubical and coarse in the interdendritic regions. The precipitates such as MC type carbide, (γ+γ’) eutectic, Laves phase andδphase are precipitated in the interdendritic regions because of the intensive segregation of Nb and Ti. High content of Mo as well as its segregation is a significant reason for the precipitation ofσphase. Two phases containing rare earth elements, Ni5Ce phase and a RE-O-S phase, are precipitated in the interdendritic regions due to the enrichment of La and Ce. The solidification temperature of GH742 alloy ranges between 1346℃and 1190℃, and the solidification sequence isγmatrix, MC carbide, (γ+γ’) eutectic, Laves phase, Ni5Ce phase. Theδphase,σphase andγ’ phase are precipitated after solidification. Additions of La and Ce decrease the contents of O and S, modify the distribution and dimension of MC carbide, and refine the grain size. However, they aggravate the segregation of Nb and Ti, lower the solidus temperature, and cause the precipitation of many detrimental phases.The incipient-melting temperature of RE-O-S phase in as-cast GH742 alloy is 1120℃, while the absolutely soluble temperature of coarseγ’ phase is over 1120℃, thus a two-step homogenization treatment via low temperature pretreatment at 1100℃followed by high temperature diffusion at 1160℃is established to eliminate the dendritic segregation and obtain uniform austenitic microstructure without incipient-melting. Increasing the homogenization temperature can remarkably increase the rate of elemental diffusion, which is more effective than increasing the homogenization time. The diffusion coefficient of Nb is much smaller than that of Ti, requiring more time to be absolutely homogenized. The two-step homogenization treatment by 1100℃x 30h+1160℃x 40h for the ingot improves the hot deformation plasticity, decreases the flow stress, and favors the dynamic recrystallization process.The flow stress of as-cast GH742 and as-rolled GH742M alloys during hot compression deformation decreases with the increasing of temperature and the decreasing of strain rate. Dynamic recrystallization is easier to take place at higher temperature and higher or lower strain rate, and the corresponding recrystallized grain size is larger. Increasing strain can increase the volume fraction of the recrystallized grains, but the grains become coarser at low strain rate. The existence of y’particles decreases the hot deformation plasticity, and significantly increases the flow stress. Theγ’particles can pin the grain boundaries, which restrain the migration of grain boundaries and the nucleation of dynamic recrystallization.Both the stress exponent and apparent activation energy of as-cast GH742 and as-rolled GH742M alloys decrease as the temperature increases during hot compression deformation. The strain rate sensitivity is less than 0.3, indicating that the GH742 alloy is very difficult to achieve superplastic forming. The strain has an obvious influence on the hot processing map, especially for the as-rolled GH742M alloy, which reflects the microstructure evolution during deformation. The adiabatic shear bands occur at lower temperature and higher strain rate, while the wedge type cracks are prone to produce at higher temperature and lower strain rate. The constitutive equation of as-rolled GH742M alloy during hot deformation is as follows:ε= 8.89x1017σp5.33 exp(-809.48 x 103/RT)The dislocation morphology and density develop dynamically, relating with the deformation conditions such as temperature, stain rate and strain, which are the integrative effect of work hardening, dynamic recovery and dynamic recrystallization. Nucleation of new recrystallized grains starts preferentially at the initial grain boundaries to form a necklace structure. The starting nucleation mechanism of dynamic recrystallization in as-cast GH742 and as-rolled GH742M alloys is discontinuous dynamic recrystallization, though continuous dynamic recrystallization simultaneously takes place as the strain increases in the as-cast GH742 alloy. Twinning plays an important role in the nucleation and growth of dynamically recrystallizaed grains. The MC carbide in the alloy can accelerate the nucleation of new grains. The strain rate does not affect the nucleation mechanism of dynamic recrystallization, but can affect the rate of nucleation and growth of recrystallized grains.The elongation to failure as well as the tensile strength of as-rolled GH742M alloy increases with the increasing initial strain rate ranging from 10-4 to Is-1 when deformed at high temperatures. The tensile rupture surface at lower strain rate is a typical intergranular fracture, while it exhibits some characteristic of transgranular fracture at higher strain rate.The grain size of as-rolled GH742M alloy does not change obviously when the temperature of solid solution treatment is below 1080℃, and it begins to grow intensively when the temperature is over 1080℃. Three dimensions ofγ’ phase and lots of strengthening phases along the grain boundaries by 1020℃x 8h, AC+780℃x 16h, AC treatment exist in the alloy, which can not only improve the deformation plasticity below 1080℃, but also refine the recrystallized grains.The microstructure of as-rolled GH742M alloy after standard heat treatment consists of y matrix, y’phase, MC and M6C carbides, M5B4 boride. The tensile property at room temperature of as-rolled GH742M alloy is better than that of as-forged GH742M alloy, but its stress rupture life is less than that of as-forged GH742M alloy. The co-addition of 0.10%La and 0.01% Ce does not influence the tensile property at room temperature of as-forged GH742M alloy.GH742y is a derivative of GH742 alloy, which has a higher level of Al, Ti, Nb, and W, V in addition. The higher alloying level of GH742y alloy makes the dendritic segregation more serious and the precipitates more complicated. The primary and secondaryγ’ phases, (γ+γ’) eutectic, MC and M6C carbide, Laves phase,8 phase, a phase,μphase and Ni5Ce phase are precipitated in the interdendritic regions. The solidification temperature of GH742y alloy ranges between 1348℃and 1167℃, and the solidification sequence is y matrix, MC carbide, primary y’phase, (γ+γ’) eutectic, Laves phase, Ni5Ce phase and M6C carbide. The phase calculation reveals that the electron vacancy number of severe segregation regions is beyond 2.30, causing the precipitation of TCP phases such asσphase andμphase. The incipient-melting temperature of Ni5Ce phase is 1120℃, and the absolutely soluble temperature of coarseγ’phase is over 1140℃. A three-step homogenization treatment via 1100℃+1160℃+1180℃is established to obtain uniform austenitic microstructure.更多还原