定向凝固和单晶高温合金的再结晶研究

定向凝固和单晶高温合金具有优异的高温力学性能,是制造先进航空发动机涡轮叶片的主要材料.定向凝固和单晶高温合金的优异性能主要来源于消除了与主应力轴垂直的晶界,而再结晶的出现会显著降低合金的高温力学性能.针对近年来国内外对于定向凝固和单晶高温合金再结晶的研究,系统地介绍了定向凝固和单晶高温合金再结晶的特点、主要影响因素、再...     

Recrystallization During Thermo-Mechanical Fatigue of Two High-Generation Ni-Based Single Crystal SuperalloysEI北大核心CSCD

通过SEM和TEM等手段研究了经热机械疲劳变形后的第三代和第四代单晶高温合金的显微组织,了解高温合金在近服役条件下的变形组织,分析单晶高温合金近服役条件下的变形机制。结果表明,第三代和第四代单晶高温合金样品中在不同{111}面上产生了大量的变形孪晶,且在平行的孪晶片层中或者孪晶片层交截周围发现大量再结晶晶粒。再结晶晶粒的界面主要由变形后的孪晶界、小角度晶界以及孪晶相交产生的大角度晶界组成。借助像差校正透射电镜解析了变形后的孪晶界结构以及孪晶诱发动态再结晶的过程,揭示了单晶高温合金热机械疲劳断裂机制。     

C含量对单晶高温合金再结晶的影响

采用选晶法在真空定向凝固炉中,制备了C含量分别为0.019%、0.048%、0.074%和0.094%的单晶高温合金,合金表面吹沙后分别在1 250、1 300℃进行真空热处理,研究不同C含量对单晶高温合金再结晶的影响。结果表明:随着合金中C含量升高,碳化物含量增多,其形态由块状转变为骨架状、发达骨架状;随着C含量增加,合金再结晶层深度无明显变化趋势,这表明碳化物对合金再结晶无明显的抑制作用,随着热处理温度升高,再结晶厚层深度明显增加;C对单晶高温合金再结晶抑制作用与形成碳化物的形态和密度有关,在合金表层形成高密度的碳化物,从而对再结晶晶界形成钉扎作用,阻碍晶界的迁移,能够起到抑制再结晶的作用。     

Mg-9Gd-4Y-0.6Zr合金挤压T5态的高温组织与力学性能

研究了Mg-9Gd-4Y-0.6Zr合金挤压T5态在250~400℃之间的高温组织与力学性能。结果表明:该合金具有非常优异的高温力学性能,其力学性能明显优于WE54;该合金在250、300、350和400℃时的抗拉伸强度分别为348、262、150和62 MPa;该合金在400℃拉伸时还具有粗晶超塑性,晶界上有再结晶的细晶,晶内有大量孪晶同时共存。     

Re和W对铸态镍基单晶高温合金再结晶的影响

对不同Re和W含量的铸态镍基单晶高温合金通过Brinell硬度计压痕变形,分别在1230~1330℃保温1 h,研究了难熔元素Re和W对合金再结晶行为的影响.结果表明,再结晶晶粒在压痕表面形成,并沿枝晶干向内扩展,晶界迁移受到枝晶间粗大g'相和g+g'共晶阻碍.添加Re和W提高了铸态单晶高温合金的g'相溶解温度和g+g'共晶含量,导致单晶高温合金的再结晶温度升高.热处理温度升高,各单晶高温合金的再结晶面积随着枝晶间g'相和共晶含量的减少而增大.相同热处理温度下,由于不同成分单晶高温合金枝晶间粗大g'相和g+g'共晶含量不同,不含难熔元素Re和W的单晶高温合金再结晶面积最大,含Re单晶高温合金的再结晶面积大于含W单晶高温合金,同时添加Re和W的单晶高温合金再结晶面积最小.     

W和Re对固溶态镍基单晶高温合金变形和再结晶的影响

镍基单晶高温合金固溶处理后,分别经过Brinell硬度计压痕和吹砂引入变形,热处理后观察再结晶,研究了W和Re元素对固溶态镍基单晶高温合金变形和再结晶的影响.结果表明,加入W和Re后,单晶高温合金变形后位错密度增大,位错缠结增多,再结晶形核时间滞后,即再结晶孕育期延长,再结晶深度明显减小.单晶高温合金经吹砂后,表面显微硬度明显增加,加入W和Re后,变形深度减小,再结晶晶界迁移速率最大值降低,且沿着再结晶深度方向,晶界平均迁移速率的变化与显微硬度变化趋势一致.     

单晶高温合金再结晶的影响因素

研究变形量、退火温度及稀土Re元素对单晶高温合金再结晶行为的影响。结果表明:随变形量的增加,再结晶区域的面积和深度增加;低变形量下再结晶晶粒数量较少,高变形量下再结晶晶粒数量较多。退火温度影响γ’相溶解率,进而影响再结晶晶粒和晶界形貌,1270℃退火,再结晶晶粒呈枝晶状,晶界有少量M6C碳化物析出;1305℃退火,再结晶晶粒为等轴状,晶界上析出的碳化物尺寸较大、数量较多。随着Re含量的增加,合金的再结晶面积明显减小,Re元素具有抑制再结晶行为的作用。     

微量镱和过渡元素对Al-Zn-Mg-Cu超强合金晶界特征和性能的影响

通过电子背散射衍射实验对复合添加稀土元素Yb和过渡族元素Zr或Cr的Al-Zn-Mg-Cu超高强合金的晶粒尺寸、晶界类型、晶界特性进行研究,分析了微合金化后的晶界特性对合金力学性能和局部腐蚀(晶间腐蚀和剥落腐蚀)的影响。结果表明:与复合添加Yb和Cr相比,复合添加Yb和Zr的Al-Zn-Mg-Cu合金具有最佳的再结晶抑制效果;高温固溶-时效处理后,合金晶粒尺寸由40μm显著细化到2.3μm,再结晶程度从98%下降到3%,Σ27晶界所占比例大幅提高,小角度晶界所占比例由8%提高到51%;亚晶界上的析出相与晶内的析出相近似,合金的力学性能和耐腐蚀性能显著提高,使得合金拉伸断裂时,沿晶断裂分数大幅降低,合金的力学性能和耐腐蚀性能显著提高。     

定向凝固和单晶高温合金的再结晶

本文系统分析了定向凝固和单晶高温合金再结晶的特点、危害以及影响再结晶的主要因素,阐述了再结晶的物理本质.再结晶行为受合金元素、变形程度、热处理温度及时间、变形速率、变形工艺等的影响.再结晶的物理本质是由铸态γ'相溶解控制的高能态畸变组织向低能态无畸变组织转变的过程.镍基定向凝固高温合金再结晶开始温度在1050℃左右,钴基定向凝固高温合金在1100℃左右.定向凝固和单晶高温合金的动态再结晶行为主要与高温氧污染和自由表面有关.再结晶对高温持久、疲劳性能影响较大,含再结晶层的断口源区表现为沿晶断裂.目前定向凝固构件上的再结晶检测普遍采用金相方法.     

镍基单晶高温合金的研发进展

本文概述了近年来镍基单晶高温合金的研发进展。在合金研制方面,总结了单晶合金近几年的发展及其成分设计方法。针对单晶合金常见的变形和损伤、失效机制,分别介绍了单晶合金蠕变、疲劳、氧化及热腐蚀机理,以及单晶合金中常见缺陷对力学性能的影响。在单晶叶片制造工艺方面,总结了高速凝固、气冷、液态金属冷却、以及流态床冷却等几种常见定向凝固工艺的研发和应用现状,并介绍了单晶叶片中几种常见缺陷的形成机制和相关控制技术。此外,本文还讨论了单晶高温合金及单晶叶片在应用基础研究领域面临的困难和挑战。     

Surface recrystallization of single crystal nickel-based superalloy

As-cast single crystal (SC) superalloy samples were shot peened and then annealed at different temperatures to investigate the effect of annealing temperature on the surface recrystallization behavior of the SC superalloy. The results show that the depth of recrystallized layers increases with the increase of annealing temperature. Below 1200 °C, the recrystallization depth climbs slowly with temperature rising. Above 1200 °C, the recrystallization depth increases sharply with the rise of temperature. The morphology of recrystallized grains is significantly affected by annealing temperature. Below the γ′ solvus, cellular recrystallization may be observed. Above the γ′ solvus, recrystallization occurs through the growth of well developed recrystallized grains. In addition, the microstructure evolution of recrystallized grains at the homogenization annealing temperature was studied. It is found that recrystallized grains first nucleate in the dendritic core areas on the shot-peened surface and then grow inwards along the dendritic core areas. With the dissolution of the coarse γ′ precipitates and γ′/γ′ eutectics in the interdendritic areas, the recrystallized grain boundaries move through the interdendritic areas. Finally, the fully developed grains nearly have a uniform depth. The dissolution of primary γ′ precipitates is a critical factor influencing the recrystallization behavior of SC superalloys.     

单晶高温合金DD6再结晶晶界析出相特征及其形成机制

对单晶高温合金DD6进行表面吹砂处理,然后进行固溶与时效真空热处理,采用SEM,TEM,EPMA和Thermo-Calc的方法研究了单晶合金DD6再结晶晶界析出相的特征及其形成机制.结果表明,经过吹砂处理的DD6合金在固溶与时效热处理过程中发生再结晶.再结晶晶界出现析出相,分析表明析出相为M_6C碳化物,该碳化物呈粒状析出,尺寸约为0.5μm,数量极少,富含W,Re和Mo,且Cr,Nb和Co的含量与合金名义成分差别不大,而Al,Ta和Ni含量较低.由于再结晶晶界上C元素的聚集效应,C原子在晶界上达到一定浓度后即与一定数量的W,Mo等μ相形成元素发生相变反应,抑制了合金析出μ相的倾向.又因为DD6合金W含量较高,而Cr含量较低,抑制了M_(23)C_6碳化物,有利于析出M_6C碳化物.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

退火温度对冷轧商业纯钛板材组织和力学性能的影响（英文）

传统的商业纯钛(CP-Ti)合金强度往往不能满足结构材料的需求。为了提高其力学性能,对冷轧CP-Ti合金在不同温度下退火,并详细研究其再结晶行为和织构演变。结果表明,部分再结晶形成的双态结构(等轴和拉长的晶粒)表现出极限抗拉强度(702MPa)和总伸长率(36.4%)的优异结合。CP-Ti板材的再结晶形核优先发生在高应变和大角度晶界区域。同时,变形不均匀晶粒的内部取向差增大并转变成大角度晶界,进一步促进再结晶形核。主要再结晶织构是由冷轧基面RD-分裂织构转变而来的基面TD-分裂织构,再结晶过程中定向形核起主导作用。     

掺杂稀土元素镧对TZM合金板材再结晶行为的影响

采用粉末冶金和轧制工艺分别制备TZM合金和掺杂稀土元素镧的La-TZM合金板材,通过对其在1000、1100、1200、1300、1400、1500、1600℃退火后样品的金相组织、力学性能进行分析与对比,研究掺杂稀土元素镧对TZM合金再结晶温度的影响。研究表明,TZM合金的开始再结晶温度为1200℃左右,La-TZM合金的开始再结晶温度约为1300℃。La2O3在TZM合金的晶界处形成细小的第二相,阻碍了晶界的迁移,提高了TZM合金板材的再结晶温度。     

热轧工艺对高铝铁素体耐热不锈钢组织性能的影响

电站锅炉的连接件需具有优良的抗高温氧化性、冲击韧性和塑性，主要采用铁素体耐热不锈钢0.1C-18Cr-1Al-1Si制造，热轧工艺对其塑韧性能有至关重要的影响。采用热轧试验，研究了不同热轧工艺对铁素体耐热不锈钢0.1C-18Cr-1Al-1Si组织性能的影响规律。结果表明，热轧温度为700 ℃时，0.1C-18Cr-1Al-1Si热轧板的再结晶最充分；横纵轧和纵轧不同轧制方式下0.1C-18Cr-1Al-1Si板材小角度晶界比例均较高，边部小角度晶界出现频率更高；采用横纵轧工艺可以明显减轻0.1C-18Cr-1Al-1Si板材力学性能的各向异性。     

Effects of deformation parameters on microstructure and mechanical properties of magnesium alloy AZ31B

Plastic deformation and dynamic recrystallization (DRX) behaviors of magnesium alloy AZ31B during thermal compression and extrusion processes were studied.In addition, effects of deformation temperature and rates on the microstructure and mechanical properties were investigated.The results show that the DRX grains nucleate initially at the primary grain boundaries and the twin boundaries, and the twinning plays an important role in the grain refinement.The DRX grain size depends on the deformation temperature and strain rate The average grain size is only 1 μm when the strain rate is 5 s-1 and temperature is 250 ℃.It is also found that the DRX grain can grow up quickly at the elevated temperature.The microstructure of extruded rods was consisted of tiny equal-axis DRX grains and some elongated grains.The rods extruded slowly have tiny grains and exhibit good mechanical properties.     

热轧工艺对高铝铁素体耐热不锈钢组织性能的影响

电站锅炉的连接件需具有优良的抗高温氧化性、冲击韧性和塑性,主要采用铁素体耐热不锈钢0.1C-18Cr-1Al-1Si制造,热轧工艺对其塑韧性能有至关重要的影响。采用热轧试验,研究了不同热轧工艺对铁素体耐热不锈钢0.1C-18Cr-1Al-1Si组织性能的影响规律。结果表明,热轧温度为700 ℃时,0.1C-18Cr-1Al-1Si热轧板的再结晶最充分;横纵轧和纵轧不同轧制方式下0.1C-18Cr-1Al-1Si板材小角度晶界比例均较高,边部小角度晶界出现频率更高;采用横纵轧工艺可以明显减轻0.1C-18Cr-1Al-1Si板材力学性能的各向异性。