TENSILE CREEP DEFORMATION AND DAMAGE BEHAVIOR IN A NICKEL-BASE SUPERALLOY AT 900℃

The tensile creep deformation and damage evolution in a Ni-base superalloy at 900℃/170MPa were investigated. At the first creep stage, abnormal creep occured due to the resolution of fine particles, and the deformation initiated from grain boundary areas. It is evident that nearly all of the dislocations were in γ matrix channels in form of dislocation pairs and the dislocations were impeded at γ /γ‘ interfaces, thus the dislocation networks developed deformation. At the steady creep stage, impeded dislocations at γ/γ‘ interfaces climbed over γ‘ phases by diffusion-dominant mechanism. At the last creep stage, voids were formed around carbides at grain boundary which leaded to accumulated damage and caused creep rate accelerated. With the dislocation networks being broken, the voids connected and grew into micro-cracks gradually. Finally the cracks propagated along grain boundary area and resulted in failure.     

TENSILE CREEP DEFORMATION AND DAMAGE BEHAVIOR IN A NICKEL-BASE SUPERALLOY AT 900℃

The tensile creep deformation and damage evolution in a Ni-base superalloy at 900℃/170MPa were investigated. At the first creep stage, abnormal creep occured due to the resolution of fine particles, and the deformation initiated from grain boundary areas. It is evident that nearly all of the dislocations were in γ matrix channels in form of dislocation pairs and the dislocations were impeded at γ/γ′ interfaces, thus the dislocation networks developed deformation. At the steady creep stage, impeded dislocations at γ /γ′ interfaces climbed over γ′ phases by diffusion-dominant mechanism.At the last creep stage, voids were formed around carbides at grain boundary which leaded to accumulated damage and caused creep rate accelerated. With the dislocation networks being broken, the voids connected and grew into micro-cracks gradually.Finally the cracks propagated along grain boundary area and resulted in failure.     

动态压缩条件下高纯钛微观组织特征研究

利用光学显微镜(OM)、背散射电子衍射(EBSD)技术及透射电子显微镜(TEM)对高纯钛低-中应变动态压缩变形的微观组织特征进行了研究。结果表明：随着应变量(ε)的增加,晶粒内部通过孪晶与孪晶,孪晶与位错以及位错与位错之间的交互作用逐步细化原始晶粒;变形初期,形变孪生以{11-22}孪晶为主,当ε达到0.2后,{10-12}孪晶转变为主要形变孪生类型,孪生改变了原始晶粒的取向,进一步促进晶粒内部的位错滑移。高纯钛动态压缩变形经历了由位错滑移到形变孪生,再到位错滑移主导的过程,但位错滑移和孪生始终共同作用协调动态压缩变形。     

A NEW MODEL FOR CAVITY NUCLEATION

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Multiple plastic deformation mechanisms of NiTi shape memory alloy based on local canning compression at various temperatures

A Ni-rich NiTi shape memory alloy (SMA), which was in its austenitic state at ambient temperature, was subjected to plastic deformation by means of local canning compression at various temperatures ranging from room temperature to 800 °C. Depending on temperatures, NiTi SMA exhibited multiple plastic deformation mechanisms, such as dislocation slip, deformation twinning, grain boundary slide, grain rotation, dislocation climb and grain boundary migration. Amorphization, dynamic recovery and dynamic recrystallization of NiTi SMA were also observed at various temperatures. Mechanism of localized amorphization, in particular, was investigated based on dislocation slip and deformation twinning. Statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) were found to play an important role in the amorphization of the current NiTi SMA. There appeared a critical dislocation density below which NiTi SMA was unable to amorphize. Accordingly, at a fixed deformation strain, there should be a critical temperature above which amorphous phase would not occur in the NiTi SMA matrix. Furthermore, when NiTi SMA experienced plastic deformation at the critical temperature, amorphization and crystallization would occur simultaneously and compete with each other.     

Slip systems extracted from lattice rotations and dislocation structures

Data on lattice rotations and dislocation structures induced in aluminium by tensile deformation are analysed together in order to extract the active slip systems. The analysis falls in two steps: (i) from the combination of lattice rotation and dislocation structure data, the grain orientation space represented by the stereographic triangle is subdivided into regions with the same active slip systems; and (ii) the active slip systems calculated from the lattice rotations are compared with those known to be active based on the dislocation structure. For the entire stereographic triangle active slip systems which are in good agreement with both lattice rotations and dislocation structures are identified, showing that the grain orientation is the primary factor controlling the slip systems.     

Grain refinement mechanism during equal-channel angular pressing of a low-carbon steel

The grain refinement mechanism during equal-channel angular pressing of a plain low-carbon steel was explored by a careful analysis of the slip systems operating at each pass of repetitive pressing. The steel was subjected to one to eight passes of pressing, in which a single passage yielded an effective strain of ∼1, at 623 K. At the initial stage of pressing, submicrometer-order ferrite grains enclosed by serrated and low-angled boundaries were formed. Transmission electron microscopy examination revealed that these boundaries resulted from interaction between the slip systems that are typical in body-centered cubic structures. Further pressings mainly resulted in rotation of ultrafine subgrains rather than grain refinement, providing the formation of high-angle grain boundaries. Since the serrated boundaries restrict dislocation movement, the rotation of subgrains with the serrated boundaries is more favorable for accommodating further deformation than intragranular strain, and therefore boundaries become high-angled.     

Twinning during hot compression of ZK30 ＋ 0.3Yb magnesium alloy

The twinning process of ZK30＋0.3Yb magnesium alloy was studied. The results show that twinning occurs at the initial stage of deformation, and decreases during further deformation. The original grain is fragmented after small straining. It is investigated that the twinning boundary activates the occurrence of the non-basal slip system due to the stress concentration at the vicinity of twin boundary introduced by the dislocation pile-ups at the vicinity of twinning boundary. The rearrangement of dislocation after dislocation climb introduces new grain boundary. Simultaneously, twinning occurs to form “polygonization” due to the stress concentration relaxation, and the “polygonization” will transform into low angle boundary to refine the original grain under the shear stress with further straining.     

疲劳/蠕变复合作用下GH169合金变形和断裂过程的动态观察

利用高温金相显微镜动态观察了GH169合金在疲劳/蠕变复合作用下的变形和断裂过程。结果表明,疲劳/蠕变复合作用下的变形方式有晶内滑移、孪生和晶界滑动,其失效方式因显微组织而不同。沿晶裂纹源于晶界滑动在三叉点处产生的W型裂纹和晶界局部形变区,其扩展机制为空洞的形核、聚集长大和相互连接;穿晶裂纹源于晶内形变损伤区,其扩展机制为沿滑移面的剪切断裂。     

孔洞对双晶TiAl合金断裂行为影响的声发射响应

本文基于分子动力学方法,对含孔洞的双晶TiAl合金试样进行了单轴拉伸模拟,在纳米尺度下研究了材料变形和断裂过程中的缺陷演化行为及其声发射响应。研究发现：孔洞大小和位置对材料的弹性模量影响较小,屈服强度随孔洞尺寸的增大而降低;进入塑性变形后,孪晶界对孔洞边缘连续发射的位错有阻碍作用,使晶体强度增加;达到屈服应力时,含晶界孔洞的试样更容易产生稳定的位错结构,阻碍其他位错运动,从而提高了晶体强度;通过对拉伸过程中的声发射信号进行分析,发现声发射信号主要来源于晶格振动,并且具有较大的功率值范围和较低的中值频率;位错滑移的声发射信号表现出宽频域的特点,位错增殖和位错塞积的声发射信号表现出低功率的特点;裂纹扩展的声发射信号属于突发型信号,表现为高频率、高功率的特征。     

IN SITU OBSERVATION OF DEFORMATION AND FRACTURE FOR SUPERALLOY GH169 UNDER COMBINED FATIGUE-CREEP ACTION

In Situ observation of deformation and fracture for superalloy GH169 under combinedfatigue-creep action is made by using high temperature metalloscope,it is shown that underthe test conditions the deformution takes place by merely of slipping,twinning and grainhoundary sliding,and the mode of failure depends on the microstructure of specimen.lntergranular cracks arise.from W-type voids produced by the stress concentration at triplepoint which could not be relaxed by the interior deformation of grains and the localdeformathm region along grain bounaries.And the crack propagation mechanism is thenucleation,growth and linkage of carities at the grain boundaries.Transgranular cracks formfrom deformatiom damages within the grain,and its propagation mechanism is shear rupturealong the slip phme.     

Stress relaxation behavior of an Al–Zn–Mg–Cu alloy in simulated age-forming process

The stress relaxation behavior of age-forming for an Al–Zn–Mg–Cu alloy was studied using a designed device that can simulate the age forming process. The mechanism of stress relaxation was also revealed through calculating thermal activation parameters and analyzing the microstructures. The results suggested that the stress relaxation behavior of the Al–Zn–Mg–Cu alloy in the simulated age-forming process can be divided into three stages according to the stress level. The three stages of stress relaxation are: (i) the initial high stress stage, (ii) the subsequent middle stress transition stage and (iii) the last low stress equilibrium stage, respectively. The deformation activation energies are 132 kJ/mol in the initial high stress stage, 119 kJ/mol in the subsequent middle stress transition stage and 91 kJ/mol in the last low stress equilibrium stage, respectively. The analysis of the thermal activation parameters and microstructures revealed that dislocation creep was the dominant deformation mechanism in the initial and subsequent stages of the stress relaxation; whereas diffusion creep is the mechanism in the last stage of the stress relaxation. Additionally, a special threshold stress phenomena was present in the stress relaxation of the age-forming process, which was scribed to the interaction between precipitation and dislocation in the Al–Zn–Mg–Cu alloy     

Deformation mechanisms in nanocrystalline palladium at large strains

The mechanical behaviour and microstructure evolution of nanocrystalline palladium was investigated. Material with an initial grain size ～10 nm was prepared by inert gas condensation. Instrumented high-pressure torsion straining was used to characterize the flow stress during plastic deformation to shear strains up to 300. A change in primary deformation mechanism was induced by stress-induced grain growth. For grain sizes <40 nm, grain boundary mediated processes (shear banding, grain boundary sliding and grain rotation) controlled the deformation, with dislocation slip, twinning, and grain boundary diffusion providing the accommodation. For larger grain sizes, the operative deformation mechanism was dislocation slip.     

亚微米晶Al-3%Mn合金的超塑延展性变形行为(英文)

采用球磨法制备晶粒尺寸为0.3μm的亚微米晶Al-3%Mn(质量分数)合金。Al-3%Mn合金在室温下轧制时,表现为极高的延展性(超过2500%)。采用透射电镜(TEM)观察球磨态和冷轧态的纯铝和Al-3%Mn合金组织;采用X射线衍射对比分析组成,发现连续塑性变形机制包括位错滑移和晶界滑动,同时还有动态回复和再结晶,而动态再结晶是大塑性变形的主要控制机制。     

Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy

By means of surface mechanical attrition treatment, nanometer-sized grains (with an average size of 30 ± 5 nm) were generated in the surface layer of a single-phase AZ91D alloy. Transmission electron microscopy investigations showed that the strain-induced grain refinement process in AZ91D alloy includes three steps. At the initial stage twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets. With increasing strain, double twins and stacking faults form and a number of dislocation slip systems are activated, including basal plane systems, prismatic plane systems and pyramidal plane systems. As a result of the dislocation slip along these systems and of the cross slips, high-density dislocation arrays are formed which further subdivide the twin platelets into subgrains. Obvious evidence of dynamic recrystallization were identified within the high-strain-energy subgrains with a further increase of strain, leading to the formation of nano-sized grains in the surface layer.     

2091铝锂合金超塑变形的晶内断裂

研究了２０９１铝锂合金超塑变形的断裂行为。扫描电镜观察表明,２０９１铝锂合金超塑变形中存在晶内断裂,光学金相观察表明断裂试样存在较活跃的动态再结晶和较多的大轴径比晶粒,透射电镜观察发现断裂试样内部存在较多的晶内位错。研究表明,由于再结晶缩小了大轴径比晶粒的横截面使其在超塑变形的晶粒转动中受到很大的弯曲应力而造成了晶内断裂。     

纳米Ni薄膜在摩擦过程中塑性行为的分子动力学模拟

用分子动力学模拟研究了金刚石压头在Ni晶体薄膜上的摩擦过程和薄膜塑性变形行为的纳观机制.结果表明:在摩擦过程中,穿晶层错和棱形位错环是纳米薄膜结构传递塑性变形的两种载体,纳米薄膜晶界捕获位错阻滞了塑性变形向薄膜晶界下方材料中传播.摩擦过程中易在较薄的薄膜表面和薄膜晶界之间产生穿晶层错,穿晶层错的产生增加了薄膜蓄积塑性变形的能力,从而抑制材料表面摩擦力在黏滑过程中的振荡幅度;在比较厚的薄膜中不易生成穿晶层错,在摩擦过程中位错环依次向体材料发射,并与晶界反应,湮灭于晶界,黏滑动摩擦响应与单晶相似.由于不同厚度薄膜塑性变形产生的位错结构不同,使得在摩擦过程中亚表面微结构的演化亦不同.     

Role of discrete intragranular slip on lattice rotations in polycrystalline Ni: Experimental and micromechanical studies

In this paper, a new micromechanical approach accounting for the discreteness of intragranular slip is used to derive the local misorientations in the case of plastically deformed polycrystalline nickel in uniaxial tension. This intragranular microstructure is characterized in particular single slip grains by atomic force microscopy measurements in the early stage of plastic deformation. The micromechanical modelling accounts for the individual grain size, the spatial distances between active slip bands and the magnitude of slip in bands. The slip bands are modelled using discrete distributions of circular super glide dislocation loops constrained at grain boundaries for a spherical grain boundary embedded in an infinite matrix. In contrast with classic mean field approaches based on Eshelby’s plastic inclusion concept, the present model is able to capture different intragranular behaviours between near grain boundary regions and grain interiors. These theoretical results are quantitatively confirmed by local electron backscatter diffraction measurements regarding intragranular misorientation mapping with respect to a reference point in the centre of the grain.     

Study on Deformation Structure and Texture of Pure Zirconium with Large Grain Size Rolled at Liquid Nitrogen Temperature

主要研究大晶粒退火态纯锆在液氮温度下均匀轧制时的形变组织特征及孪生机制。利用光学显微镜、扫描电镜、电子背散射衍衬（EBSD）、X射线衍射等对不同变形量样品的变形组织和织构进行了研究,重点结合软件对EBSD结果进行组织重构和机理分析。结果表明,液氮温度轧制时大晶粒纯锆中产生的孪晶类型为C1{112}<11>、T1{102}<10>和T2{111}<11>孪晶,其中C1{112}<11>孪晶最容易产生且为主要孪生类型。变形开始时,3种孪晶的数量迅速增多,而小角度晶界含量较少;变形量增大到30%时,小角度晶界含量占优势。变形初始阶段孪生优先于滑移进行, 且孪生变形是最主要的变形方式,当变形量为30%时,孪晶协调的位错滑移成为主要的变形方式。变形过程中织构类型未发生变化,保持基面双峰织构（偏离ND方向±30°左右）,但强度随着变形量的增大呈减小趋势     

拉伸变形对Hastelloy C-276合金组织与力学性能的影响

采用金相(OM)、电子背散射衍射(EBSD)以及拉伸实验等技术手段研究了不同变形量条件下Hastelloy C-276合金薄板的组织演化特征和力学性能。结果表明:变形量小于14%时,位错优先在晶界附近塞积,并产生局部应变集中;变形量在14%~30%范围内,孪晶界附近及晶粒内部产生大量位错,位错滑移引起晶粒内部应变集中增强;变形量由0%增加至30%,晶界应变集中程度因子先增大后减小,变形量为14%时晶界应变集中程度因子最大。利用Ludwigson模型回归拟合了不同变形条件下的真应力-真应变曲线,随变形量的增加,材料的加工硬化程度提高,加工硬化速率减小,发生单滑移向多滑移转变的临界应变减小。