Inconel 740H合金原位高温拉伸微裂纹萌生扩展研究

利用自主研发的SEM原位高温拉伸台,研究了750℃高温条件下镍基高温合金Inconel 740H单轴拉伸变形过程中微观组织演变规律及微裂纹萌生与扩展机制。结果表明,在室温和高温条件下,Inconel 740H合金变形过程中晶界是主要的裂纹萌生源,但是在室温时微裂纹也会在晶内萌生。通过对原位变形机制的分析表明,750℃高温不仅降低了滑移系的开启能量,使更多的滑移系容易开动,而且弱化了晶界强度,使晶界具有弯曲和滑移的变形特性,从而增强了合金的塑性协调变形能力,但是却降低了合金的屈服强度和抗拉强度,高温同时也使合金晶界的相对强度弱化,导致微裂纹更易从晶界处萌生并扩展。     

原位拉伸法研究GH3625合金退火孪晶界稳定性及断裂行为

孪晶界作为低能稳定界面易在低层错能金属中被调控而成为近年来研究的热点。固溶态GH3625合金组织中含有大量退火孪晶组织。本实验采用室温原位拉伸结合扫描电子显微镜(SEM)观察和能谱(EDS)分析的方法研究了固溶态GH3625合金中孪晶组织演变及断裂行为。结果表明,GH3625合金在原位拉伸变形过程中,孪晶组织内部主要以单滑移为主;在拉伸直至断裂的过程中,随变形量的增加,孪晶界逐渐发生弯曲,但孪晶界始终存在于合金组织中,起阻碍位错的作用,具有良好的室温机械稳定性。GH3625合金断裂时既有韧性断裂又有脆性断裂,碳化物偏析是造成晶界裂纹以及晶内孔洞形成的主要原因。     

K40S钻基高温合金的高温低周疲劳行为Ⅱ.疲劳断口分析

研究了K40S钴基高温合金在700℃和900℃温度条件下由应变控制的高温低周疲劳行为,对疲劳断口形貌进行观察,结果表明:在高温低周疲劳加载条件下,K40S合金疲劳裂纹萌生机制为表面滑移带开裂与表面碳化物相界面开裂的综合作用;疲劳裂纹萌生与扩展方式为穿晶型,瞬断区呈现枝晶断裂特征;碳化物可作为障碍,阻碍疲劳裂纹的扩展,且为主要的二次裂纹策源地;K40S合金高温低周疲劳断裂为机械疲劳与高温环境氧化共同作用的结果.     

K40S钴基高温合金的高温低周疲劳行为——Ⅱ.疲劳断口分析

研究了K40S钴基高温合金在700℃和900℃温度条件下由应变控制的高温低周疲劳行为，对疲劳断口形貌进行观察，结果表明；在高温低周疲劳加载条件下，K40S合金疲劳裂纹萌生机制为表面滑移带开裂与表面碳化物相界面开裂的综合作用；疲劳裂纹萌生与扩展方式为穿晶型，瞬断区呈现枝晶断裂特征；碳化物可作为障碍，阻碍疲劳裂纹的扩展，且为主要的二交裂纹策源地；K40S合金高温低周疲劳断裂为机械疲劳与高温环境氧化共同作用的结果。     

铸态U-10%Mo合金的高温力学性能及其显微结构

采用真空熔炼法制备了铸态U-10%Mo合金,研究了该合金的室温和高温下的拉伸性能。结果表明,合金的拉伸断裂极限强度随温度的升高呈下降趋势。铀钼合金在室温下发生脆性沿晶断裂,高温条件下则发生韧窝型沿晶断裂和撕裂型沿晶断裂,这是由断口晶界处聚集的大量铀的氧化物和碳化物造成的。     

一种镍基第三代单晶高温合金的横向拉伸性能

在760℃到1100℃条件下,研究了一种镍基第三代单晶高温合金的横向拉伸性能。采用光学显微镜(OM)、场发射扫描电子显微镜(FESEM)与扫描透射电子显微镜(STEM)观察了合金的显微组织与断口形貌。结果表明：随着温度的升高,合金的拉伸强度降低,而拉伸延伸率增加。在760℃与850℃条件下的拉伸断裂均为类解理断裂。在980℃,1070℃和1100℃条件下,试样断口出现了反映凝固方向的枝晶形貌特征,且随着温度的升高枝晶形貌在断口上的面积增加。在980℃条件下,拉伸断裂为类解理断裂与韧窝断裂的混合断裂。在1070℃与1100℃条件下,拉伸断裂均为韧窝断裂。随着温度的升高,塑性变形过程中开动了更多滑移系,导致形成了不同的位错形貌。760℃拉伸,合金中出现了高密度大致平行分布的a/2<110>位错;980℃拉伸,合金中出现了位错缠结;1100℃拉伸,合金中形成了位错网络。     

760℃长期时效对一种Ni-Cr-W-Fe合金组织和力学性能的影响

采用OM,SEM和TEM研究了一种Ni-Cr-W-Fe合金在760℃长期时效过程中的显微组织变化,测试了合金室温和高温力学性能,对拉伸断口进行了分析.结果表明,1100℃固溶后合金平均晶粒尺寸约为80 mm,晶内包含退火孪晶.760℃时效后合金中析出M23C6和g'相.g'相尺寸约为29 nm,体积分数约为19%.760℃长期时效后,g'颗粒平均尺寸与时间t满足Ostwald方程.固溶态合金具有优异的室温塑性,拉伸断口具有韧性断裂形貌.时效态合金室温屈服强度明显增加,塑性下降.随760℃保温时间延长,合金室温和高温屈服强度缓慢降低.与时效态合金相比,1000~3000 h时效后的合金室温塑性降低,高温塑性维持在15%左右,与时效态基本相当.     

低温下ZL101合金的裂纹扩展行为

对ZL101合金在20~-60℃拉伸过程中裂纹扩展行为进行研究,利用扫描电子显微镜观察断口形貌,并采用光学显微镜对断口附近组织和裂纹萌生和扩展形貌进行分析,研究裂纹扩展过程中铝基体内部应力的变化。结果表明:当温度由20℃下降至-60℃时ZL101合金的抗拉强度和屈服强度均有提高,而伸长率略有降低。ZL101合金在塑性变形过程中产生大量位错在Si相处塞积产生应力,使Si相发生解理断裂形成微裂纹,并且Si相断裂形成的解理面与附近的位错滑移带成135°角。在裂纹扩展过程中Si相的破裂使铝基体产生应力集中,在低温情况下Si相断裂所需的外加应力升高使断裂Si相之间铝基体的应力集中升高引发准解理断裂,导致合金伸长率下降。     

多晶Be室温拉伸变形和断裂行为

通过SEM原位拉伸实验观察室温下多晶Be的变形、裂纹萌生和扩展过程,利用电子背散射衍射(EBSD)标定断裂解理面,结合OM分析孪晶变形,研究多晶Be室温拉伸变形和断裂行为及其机理.结果表明,室温拉伸应力条件下,多晶Be的滑移和孪晶变形均难以发生.滑移带仅在少数取向有利的晶粒中出现,最终孪晶变形晶粒约占晶粒总数的5%.变形过程中存在(0001)基面和{1010}柱面之间的交滑移.多晶Be的微裂纹起源于晶界一侧,发生穿晶扩展后,在另一侧晶界终止,裂纹萌生符合Stroh位错塞积生裂纹理论.因晶界对裂纹强烈的阻碍作用,多晶Be的裂纹长大依靠不同微裂纹之间的汇合,汇合路径有解理台阶和撕裂2种.多晶Be断裂基本解理面为(0001)基面和{1010}柱面,两者均是多晶Be解理萌生和扩展的主要路径.未观察到因孪晶变形诱发的微裂纹.     

一种单晶高温合金的冲击断裂机制研究

模拟了单晶叶片实心和空心结构,制备了单层壁和双层壁的单晶高温合金冲击试样,研究了在600℃、700℃、760℃、800℃、850℃、900℃下的冲击断裂机制。研究结果表明,不同温度单晶高温合金的冲击断裂机制均为类解理断裂,断口上存在解理台阶,滑移带,河流状花样等特征。在试验中温范围内,随着温度增加,冲击断口形貌无明显的变化。裂纹开始沿大解理面扩展,最后断裂区存在大量解理台阶。解理台阶与单晶高温合金中温疲劳断口上的台阶相似,但不同是随着裂纹的扩展,台阶的间距逐渐减小。不同温度冲击断裂试样组织分析表明,γ′相立方状形貌没有变化,靠近断口处存在大量的滑移带。     

TG6钛合金的热稳定性及其在高温下的部分恢复

Thermal Stability of TG6 Titanium Alloy and Its Partial Resumption at High Temperature     

Mechanical properties and fracture behavior of an Nb-Silicide in situ composite

An Nb-Silicide in situ composite with a nominal composition of Nb-16Si-10Ti-10Mo-5Hf (at. %) was fabricated by mechanical alloying followed by hot-pressing sintering. The microstructure consisted of an Nb solid solution, Nb₅Si₃ and a small amount of Nb₃Si. This in-situ composite exhibited good balance of strength between ambient temperature and high temperatures; the ultimate tensile strength was 413 and 496 MPa at room temperature and 1200 °C, respectively. The tensile fracture behavior was dominated by cleavage of the Nbss and Nb₅Si₃ at 1200 °C and lower temperatures. However, the fracture behavior was governed by ductile rupture of Nbss at 1300 °C and higher temperature, which was ascribed to both the increased ductility of Nbss and the decreased interface strength. At 1400 °C and higher temperature, the material exhibited extensive plasticity or superplasticity; the dominant deformation mechanism was grain boundary sliding at 1400 °C and higher temperature.     

Effect of ruthenium on tensile properties of a single crystal Ni-based superalloy

The tensile properties of two single crystal Ni-based superalloys with and without added Ru (0 and 3 wt%) were investigated under a constant strain rate of 3.3×10^?4 /s at 20 °C, 760 °C, 800 °C and 1000 °C, respectively. The deformation mechanisms could be divided into two temperature regimes. From room temperature to 800 °C, the deformation mechanism is caused by the shearing of ?á? particles by anti-phase boundaries (APB) or stacking faults. At 1000 °C, the deformation mechanism is caused by the bypassing of ?á? particles by dislocations. At 20 °C and 800 °C, ?á? particles were sheared by APB. Due to smaller ?á? particles, the yield strength was decreased with addition of 3 wt% Ru. Additionally, work hardening is less pronounced in the alloy without Ru, hence the ultimate tensile strength was not decreased with the addition of 3 wt% Ru. At 760 °C, ?á? particles were sheared by stacking faults. Since the formation of stacking faults was promoted, the yield strength was decreased due to a 3 wt% Ru addition. However, the ultimate tensile strength was significantly increased when 3 wt% Ru was added. This is due to the markedly stronger work hardening caused by large numbers of stacking faults. At 1000 °C, deformation occurred by dislocations bypassing ?á? particles. Due to wider ?? channels, the yield strength was decreased by 3 wt% Ru addition. Moreover, Alloy 3Ru has smaller ?á? particles and a volume fraction as well as less pronounced work hardening, so the ultimate tensile strength was decreased when 3 wt% Ru was added.     

Effects of Pack Rolling Temperature on Microstructure and Mechanical Properties of Powder Metallurgical Ti-45Al-7Nb-0.3W Alloy

Powder metallurgical Ti-45Al-7Nb-0.3W (at.%) alloys were pack rolled at temperatures of 1240°C, 1255°C, 1270°C, and 1285°C. The microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy. The tensile properties were tested at room temperature and 800°C. After rolling, the sheets exhibited duplex microstructures with refined grains. The tensile test results showed the sheet rolled at 1270°C displayed excellent room temperature tensile properties with an ultimate tensile strength (UTS) of 782 MPa and an elongation of 1.95%. When tested at 800°C, all sheets showed UTS of over 600 MPa and elongations of around 50%. The dislocation movements and mechanical twinning played important roles at the initial stage of rolling deformation. However, during the subsequent deformation process, the deformation mechanism should mainly be the result of dynamic recrystallization.     

Joining of alumina via copper/niobium/copper interlayers

Alumina has been joined at 1150°C and 1400°C using multilayer copper/niobium/copper interlayers. Four-point bend strengths are sensitive to processing temperature, bonding pressure, and furnace environment (ambient oxygen partial pressure). Under optimum conditions, joints with reproducibly high room temperature strengths (≈240±20 MPa) can be produced; most failures occur within the ceramic. Joints made with sapphire show that during bonding an initially continuous copper film undergoes a morphological instability, resulting in the formation of isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. For optimized alumina bonds, bend tests at 800–1100°C indicate significant strength is retained; even at the highest test temperature, ceramic failure is observed. Post-bonding anneals at 1000°C in vacuum or in gettered argon were used to assess joint stability and to probe the effect of ambient oxygen partial pressure on joint characteristics. Annealing in vacuum for up to 200 h causes no significant decrease in room temperature bend strength or change in fracture path. With increasing anneal time in a lower oxygen partial pressure environment, the fracture strength decreases only slightly, but the fracture path shifts from the ceramic to the interface.     

The effect of annealing on microstructure and tensile properties of Ti–22Al–25Nb electron beam weld joint

This study investigated the microstructure evolution and tensile properties of Ti–22Al–25Nb EBW joints. The fusion zone of the as-welded joint exhibited a fully B₂ microstructure. Widmanstätten O particles precipitated out of B₂ matrix after annealing and their size increased within a temperature range from 750 °C to 900 °C. In the heat affected zone, there was a transition of microstructure moving away from the fusion zone towards the base material. Strength and elongation of the as-welded sample were significantly improved after annealing, which was attributed to the strengthening effect of O precipitates and the slip transmission between O and B₂ phases. Samples tensile tested at 650 °C all failed within the fusion zone and exhibited intergranular failure instead of transgranular failure at room temperature. The room temperature strength and hardness of the joints decreased with annealing temperature due to the coarsening of O precipitates. At 650 °C, failure occurred by intergranular fracture in the fusion zone and the joint strength of all annealed samples was similar due to similar B₂ grain boundary strength.     

Temperature dependent tensile and fracture behaviors of Mo–10 wt.% Cu composite

Uniaxial tension tests are carried out for the Mo–10 wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25 °C to 725 °C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500 °C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500 °C. Under uniaxial tension at temperatures higher than 650 °C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications.     

纳米银粉体新法制备及其表征

采用均匀沉淀和高温热分解相结合的方法制备了纳米银粉体,分析了粉体形成机理,研究了pH值、烧结温度和时间对粉体粒径和形貌的影响;采用X射线衍射仪、扫描电子显微镜表征了纳米银粉体结构、组成、大小和形貌.结果表明,pH=7、烧结温度300℃、烧结2h的条件下可得到分散性好、颗粒均匀,粒径50 nm的粉体.本制备方法原料易得、成本低、设备及工艺简单,反应副产物易回收且可用作肥料,整个过程满足清洁生产的要求.     

Effect of Welding Parameters on the Microstructure and Strength of Friction Stir Weld Joints in Twin Roll Cast EN AW Al-Mn1Cu Plates

Twin roll cast EN AW Al-Mn1Cu plates were butt welded with the friction stir welding process which employed a non-consumable tool, tilted by 1.5° and 3° with respect to the plate normal, rotated in a clockwise direction at 400 and 800 rpm, while traversing at a fixed rate of 80 mm/min along the weld line. Microstructural observations and microhardness tests were performed on sections perpendicular to the tool traverse direction. Tensile tests were carried out at room temperature on samples cut perpendicular to the weld line. The ultimate tensile strength of the welded EN AW Al-Mn1Cu plates improved with increasing tool rotation speed and decreasing tool tilt angle. This marked improvement in ultimate tensile strength is attributed to the increase in the heat input owing to an increased frictional heat generation. There appears to be a perfect correlation between the ultimate tensile strength and the size of the weld zone. The fracture surfaces of the base plate and the welded plates are distinctly different. The former is dominated by dimples typical of ductile fractures. A vast majority of the intermetallic particles inside the weld zones are too small to generate dimples during a tensile test. The fracture surface of the welded plates is thus characterized by occasional dimples that are elongated in the same direction suggesting a tensile tearing mechanism.