铸件尺寸对镍基单晶高温合金力学性能的影响

浇注了两种不同尺寸的镍基单晶高温合金铸件试样,研究了试样尺寸对显微组织的影响,利用扫描电镜分析了合金的断口形貌和断裂机制.结果表明,浇注试样尺寸增大,合金的枝晶间距和热处理后γ′相的尺寸增大.合金在760、980℃下的拉伸性能以及980℃和250MPa条件下的持久性能均有不同程度的降低.合金760℃的拉伸断裂为类解理断裂,980℃的拉伸断裂以及980℃和250MPa条件下的应力断裂都为韧窝断裂.     

取向偏离度对镍基第三代单晶高温合金DD9在760～1100℃横向拉伸性能的影响（英文）

研究[100]、[120]和[110]取向的镍基第三代单晶高温合金DD9在760～1100℃范围内的拉伸性能。采用OM、SEM和TEM观察显微组织与断口形貌。结果表明,[100]取向试样在760℃和850℃的抗拉强度高于[210]和[110]取向试样,而在980℃以上,三种取向试样的抗拉强度接近。[100]、[120]和[110]取向试样在760℃与980℃的断裂机制相同,而在1100℃条件下,[100]与[120]取向试样断口为韧窝断裂,而[110]取向试样断口为类解理断裂与韧窝断裂共存。在760℃条件下,仅在[100]取向试样中发现明显层错,而在1100℃条件下三种取向试样中位错组态相似。[100]、[120]和[110]取向试样拉伸变形过程中可开动的潜在滑移系数量不同是造成横向拉伸性能各向异性的主要原因。     

DD9单晶高温合金拉伸性能各向异性

采用OM,SEM和TEM研究了[001],[011]和[111]取向第三代单晶高温合金DD9组织,在拉伸试验机上测试了3种取向760和1100℃下的拉伸性能.结果表明,在垂直于晶体生长方向的截面上,[001],[011]和[111]取向DD9合金铸态枝晶形貌、热处理态γ'相形状不同;随着温度的升高,合金的抗拉强度与屈服强度降低,各向异性减弱;除1100℃下[001]取向屈服强度略低于[011]取向,[001]取向DD9合金抗拉强度与屈服强度分别高于[011]和[111]取向合金;[001],[011]和[111]取向DD9合金760℃下拉伸断口呈类解理特征,1100℃下断口为韧窝断裂特征;760℃下DD9拉伸试样在基体通道内含有浓密的位错,[001]取向在γ'相内出现了层错,1100℃下[001]与[111]取向在基体通道内和γ'相内累积了大量浓密的位错网,[011]取向出现了大量的形变孪晶带.     

一种单晶高温合金的持久断裂行为

研究了一种单晶高温合金在不同试验温度和加载应力下的持久性能、失效机制和组织演变特征。结果表明:合金标准热处理后获得立方化较好的γ'相组织,在不同试验条件下都具有良好的持久性能。在相同加载应力下,随着试验温度升高,合金的持久寿命降低。760℃/800 MPa条件下的持久断裂为类解理断裂,980℃/250 MPa、1070℃/140 MPa、1100℃/140 MPa和1120℃/140 MPa的持久断裂为韧窝断裂,而850℃/550 MPa下的持久断裂为混合断裂机制。在760℃/800 MPa下持久断裂后,γ'相形状不变,但γ相通道沿垂直应力方向变大,沿平行应力方向变小。其它持久条件下都形成了筏排结构,随着温度的增加其厚度变大。在760℃/800 MPa、850℃/550 MPa和980℃/250 MPa条件下,没有发现析出TCP相,而在1070℃/140 MPa、1100℃/140 MPa和1120℃/140 MPa条件下,有少量针状的TCP相沿一定方向析出,主要含有Re、W、Mo等元素。     

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。     

高氮奥氏体钢的冲击断口形貌研究

通过采用金相及扫描电子显微镜对高氮奥氏体Fe-20Mn-19Cr-0.5C-0.6N钢不同冲击温度下的断口形貌进行观察,并结合冲击韧性研究了材料在不同温度下的冲击断口形貌变化规律。研究结果表明:在20~-50℃冲击时,断面纤维率超过88%,断口放射区形貌由全韧窝态转变为以粗大撕裂棱和二次解理组成的准解理混合断裂形貌,断口纤维区和剪切唇均由韧窝组成。在-60~-196℃冲击时,冲击断面纤维率逐步降至0%,断口放射区形貌以细小撕裂棱和一次解理为主的混合断裂形貌,随着冲击温度的降低,断口纤维区和剪切唇由拉长韧窝为主转变为以等轴细小且平坦韧窝为主的形貌。     

淬火态TC4合金热循环硬化行为及断口特征

在７７－６２３Ｋ对淬火态ＴＣ４合金进行了热循环试验，测定了合金热循环后的硬度，并与６２３Ｋ时效合金的硬度进行了对比；还观察了拉伸试样断口形貌。结果表明：热应力能加速亚稳相的分解，使合金中的位错密度升高，层错和形变孪晶的数量增多；循环温度场作用下淬火态合金拉伸试样断口由韧窝型断裂向韧窝和撕裂棱混合型断裂转变。     

7D04-T7451铝合金高温性能及组织变化特征

通过高温拉伸、SEM、TEM等方法对7D04-T7451铝合金厚板在不同温度下的组织、性能变化及断裂行为进行试验研究。结果表明,随拉伸温度升高,合金的强度降低,伸长率随温度增加而逐渐增加;在较低温度拉伸时,出现了二次析出的现象;当温度超过150℃后,晶内析出相发生了粗化;随温度升高,合金断口逐渐由以位错滑移为主的层片状沿晶断裂转变为以基体/析出相滑脱为主的韧窝断口。     

M35高速钢热塑性行为研究

利用Gleeble-1500热模拟试验机进行热拉伸试验,研究了变形温度在950～1150 ℃范围内,变形速率为0.1 s-1、1 s-1时M35高速钢热塑性行为及断裂机理。结果表明：M35高速钢在试验条件下具有优异的高温塑性,峰值应力随变形温度升高线性下降,随应变速率增加相应升高。热拉伸过程中断裂机制都为韧性断裂,变形温度低于1100 ℃时断口呈韧窝状,随着温度升高韧窝直径变大、深度增加;变形温度高于1100 ℃时断口呈沿晶断裂。高温拉伸过程中,碳化物的大小、分布对M35高速钢的热塑性存在明显影响。     

M35高速钢的热塑性行为

利用Gleeble-1500热模拟试验机进行热拉伸试验,研究了变形温度在950~1150℃范围内,变形速率为0.1 s~(-1)、1 s~(-1)时M35高速钢热塑性行为及断裂机理。结果表明:M35高速钢在试验条件下具有优异的高温塑性,峰值应力随变形温度升高线性下降,随应变速率增加相应升高。热拉伸过程中断裂机制都为韧性断裂,变形温度低于1100℃时断口呈韧窝状,随着温度升高韧窝直径变大、深度增加;变形温度高于1100℃时断口呈沿晶断裂。高温拉伸过程中,碳化物的大小、分布对M35高速钢的热塑性有明显影响。     

High-temperature low-cycle fatigue behavior of superalloy MA 760

The high-temperature cyclic behavior of oxide dispersion-strengthened superalloy MA 760 was studied at different temperatures and specimen orientations. The cyclic hardening curves at different strain am-plitudes were determined at 650, 900, and 950 °C, and the stress-strain response of the material was sub-stantiated by detailed fractographical scanning electron microscopy (SEM) and microstructuralJoptical microscopy, STEM) observations. During cycling, MA 760 did not exhibit marked hardening or soften-ing, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the material. The “brittle” nature of deformation behavior of MA 760, especially at lower test tempera-tures, as well as oxidation-induced secondary cracking along grain boundaries, was clearly revealed by microscopical studies of the fracture surfaces. At 650 °C, evidence of dislocation cutting of γ was ob-served in transmission micrographs, but at higher temperatures, dislocation climb and by-passing of γ particles were found to become more prominent features. At all test temperatures, dislocation climb over yttria particles, as well as the departure side pinning effect of dislocations at nonshearable particles, was frequently observed.     

镍基单晶高温合金在不同条件下的蠕变性能和组织演化(英文)

研究[001]取向的镍基单晶高温合金在不同测试条件下的蠕变性能,采用扫描电镜和透射电镜研究合金蠕变断裂后的γ′相、TCP相和位错组织演化特征。结果表明:合金具有良好的蠕变性能,蠕变曲线显示出两种不同的蠕变变形特征。在(760°C,600 MPa)、(850°C,550 MPa)条件下,蠕变第一阶段较长;在(980°C,250 MPa)、(1070°C,140 MPa)和(1100°C,120 MPa)条件下,蠕变第一阶段很短。蠕变断裂后,在(760°C,600 MPa)条件下γ′相形态变化不大;在(850°C,550 MPa)条件下γ′相已经合并长大;在(980°C,250 MPa)条件下基体γ被γ′相包围;在(1070°C,140 MPa)条件下基体γ不再连续;在(1100°C,120 MPa)条件下基体γ厚度进一步增加。在(760°C,600MPa)、(850°C,550 MPa)和(980°C,250 MPa)条件下合金无TCP相析出,而在(1070°C,140 MPa)和(1100°C,120MPa)条件下有针状TCP相析出。在低温高应力下,变形特征为位错包括层错的剪切机制;在高温低应力下为位错绕过机制,并在γ/γ′相界面形成位错网。     

Hydrogen embrittlement of a Ti-strengthened 250 grade maraging steel

Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 °C (800 °F) underaged specimens are very sensitive to strain rate; the 482 °C (900 °F) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 °C (1100 °F) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni₃Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.     

MAR-M247合金高温断裂机制的原位研究

利用原位高温拉伸台在扫描电镜中研究了镍基铸造高温合金MAR-M247在室温、400 ℃与760 ℃拉伸过程中的动态组织演变和断裂机制。原位测试结果表明,在室温到760 ℃范围内,MAR-M247合金的屈服强度与抗拉强度随温度的升高略有下降,拉伸塑性略有提高。室温原位拉伸过程中,并没有出现滑移带;400 ℃与760 ℃的原位拉伸,只在样品断口附近存在少量的滑移带。随拉伸温度的提高,合金的断裂机制并无明显变化,均表现为韧性穿晶断裂。合金的微裂纹主要来源于变形过程中碳化物的破裂,晶内与晶界都存在因碳化物破裂而形成的微裂纹。     

Primary creep deformation behaviors related with lamellar interface in TiAl alloy

Constant tensile stress creep tests under the condition of 760~816°C/172~276 MPa in an air environment are conducted, and the microstructural evolution during primary creep deformation at the creep condition of 816°C/172 MPa was observed by transmission electron microscopy (TEM) for the lamellar structured Ti-45. 5Al-2Cr-2.6Nb-0.17W-0.lB-0.2C-0.15Si (at.%) alloy. The amount of creep strain deformed during primary creep stage is considered to be the summation of the strains occurred by gliding of initial dislocations and of newly generated dislocations. Creep rate controlling process within the primary stage seems to be shifting from the initial dislocation climb controlled to the generation of the new dislocations by the phase transformation of 2 to as creep strain increases.     

Effect of Fatigue Behavior on Microstructural Features in a Cast Al-12Si-CuNiMg Alloy Under High Cycle Fatigue Loading

High cycle fatigue tests of a cast Al-12Si-CuNiMg alloy are carried out under different stress amplitudes at room temperature. The scanning and transmission electron microscopy observations are used to examine the fracture surfaces and dislocation structures of the tested material, respectively. The results show that the fatigue damage originates from the microstructural defects, and the fracture surface morphology is typical quasi-cleavage fracture. With the increasing strain amplitude, the material fatigue life obviously decreases; however, the dislocation density increases significantly, which leads to the formation of the dislocation walls and cells. Under the cycle loading, the eutectic Si phase and the secondary particles undergo fracture. The pinning effect of the precipitates on the dislocations becomes obvious, indicating that the Al-12Si-CuNiMg alloy has the cyclic hardening characteristic.     

AZ91D+xNd压铸镁合金拉伸和疲劳断裂模式

压铸AZ91D镁合金中添加稀土Nd后,生成了新的稀土化合物Al11Nd3相,合金显微组织得到细化,力学性能得到提高,但是当Nd的添加量达到1.5%时,合金的力学性能又有所下降。AZ91D xNd合金拉伸断口具有准解理、撕裂棱及韧窝等韧性特征,但其断裂方式仍属于脆性断裂。含Nd的AZ91D合金疲劳裂纹萌生于试样的表面或亚表面下的孔隙或夹杂处,疲劳裂纹扩展区为平面状断面,含1.0%Nd合金的疲劳辉纹最为清晰,疲劳断口主要表现为脆性与韧性的混合断裂方式,局部存在准解理台阶与河流花样以及穿晶型和沿晶型二次裂纹。     

Microstructure/mechanical property relationship in a DS Cast Ni3Al-base alloy

A Ni-Al-Mo directionally solidified (DS) casting γ-base superalloy, with the chemical composition (wt%) 7.5 to 8.5% Ni, 10 to 14% Al, Mo ≤0.15% B, has been developed for advanced gas turbine blades and vanes. The mechanical properties of this alloy have been determined by tensile tests at room temperature and in the temperature range 700 to 1000 °C and by stress-rupture tests in the temperature range 760 to 1100 °C. The microstructures of the as-cast and homogenized specimens and of specimens after creep de-formation at 1000 to 1100 °C have been examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and optical microscopy techniques. The results show that this alloy has a high yield strength from room temperature up to 1100 °C, excellent creep resistance at temperatures up to 1100 °C, as well as a lower density and higher melting point than currently available nickel superal-loys. The microstructural observations and analysis indicate that the superior mechanical properties of this alloy may be attributed to solid solution hardening by the large molybdenum addition, second-phase strengthening by y phase and other minor phases that precipitate in various temperature ranges, the for-mation of a γ raft structure during creep, and to the existence of high-density misfit dislocation networks at γ / γ interface areas due to a high value of γ / γ misfit.     

α-Al2O3 sapphire and rubies deformed by dual basal slip at intermediate temperatures (900–1300 °C): II. Dissociation and stacking faults

Sapphire and rubies (undoped and Cr-doped α-Al₂O₃ single crystals) have been deformed in compression at temperatures lower than those previously used in studies of dislocations in the basal slip plane. Above 1400 °C, several features associating stacking faults out of the basal planes and partial dislocations (dissociation and faulted dipoles) have been observed in previous transmission electron microscopy investigations. The formation of these features involves climb controlled by atomic diffusion. Properties of climb-dissociated dislocations are discussed in relation to dislocation dynamics. Transmission electron microscopy examination of dislocation structures at lower deformation temperatures (1000–1100 °C) shows that similar features are formed but that they often imply cross-slip. A new mechanism for the formation of faulted dipole by glide is presented and an explanation for the 30° Peierls valley orientation is proposed. The presence of chromium has a small influence on stacking fault energies on planes perpendicular to the basal plane.     

Relationship between tensile and primary creep properties of near γ-TiAl intermetallics

This paper presents the tensile and primary creep behaviour of fully lamellar microstructures for three near γ-TiAl compositions: binary TiAl, ternary TiAl+W and TiAl+NbMnWMoSi. Reducing the lamellar interface spacing or aging to precipitate β along interfaces increases the 760 °C tensile strength. Primary creep at stresses from 140 to 276 MPa and 760 °C is characterized in terms of the instantaneous strain occurring during creep loading and a subsequent primary transient. The presence of interface β precipitates reduces the instantaneous strain and increases the time to 0.5% creep strain. Reducing lamellar interface spacing does not influence the instantaneous strain, but decreases the strain rate during the primary transient. Correlating the creep response with deformation structures after primary creep suggests that the instantaneous strain depends on the mobility of interface dislocations and the primary transient depends on dislocation mobility in both the lamellar interfaces and γ lamellae.