The influence of the processing route on the fragmentation of (Nb,Ti)C stringers and its role on mechanical properties and hydrogen embrittlement of nickel based alloy 718

The nickel-base superalloy 718 is a precipitation hardened alloy widely used in the nuclear fuel assembly of pressurized water reactors (PWR). However, the alloy can experience failure due to hydrogen embrittlement (HE). The processing route can influence the microstructure of the material and, therefore, the HE degree. In particular, the size and distribution of the (Nb,Ti)C particles can be affected by the processing. In this regard, the objective of this work was to analyze the influence of cold and hot deformation processing routes on the development of the microstructure, and the consequences on mechanical properties and hydrogen embrittlement. Tensile samples were hydrogenated through gaseous charging and compared to non-hydrogenated samples. Characterization was performed via scanning and transmission electron microscopies, as well as electron backscattered diffraction. The processing was effective to promote significant variations in average grain size and length fraction of special Σ3ⁿ boundaries, as well as reduction of average (Nb,Ti)C particle size, being these changes more intense for the cold-rolled route. For the mechanical properties, on one side, the cold-rolled route presented the highest increase in ductility for non-hydrogenated samples, while, on the other side, had the highest degree of embrittlement under hydrogen. This dual behavior was attributed to the interaction of hydrogen with the (Nb,Ti)C particles and stringers and its ensuing influence on the fracture processes.     

Phase Field Modelling Allotropic Transformation of Solid Solution

Based on multiphase field conception and integrated with the idea of vectorvalued phase field, a phase field model for typical allotropic transformation of solid solution is proposed. The model takes the non-uniform distribution of grain boundaries of parent phase and crystal orientation into account in proper way, as being illustrated by the simulation of austenite to ferrite transformation in low carbon steel. It is found that the misorientation dependent grain boundary mobility shows strong influence on the formation of ferrite morphology comparing with the weak effect exerted by misorientation dependent grain boundary energy. The evolution of various types of grain boundaries are quantitatively characterized in terms of its respective grain boundary energy dissipation. The simulated ferrite fraction agrees well with the expectation from phase diagram, which verifies this model.     

热轧温度对436不锈钢热轧织构的影响

利用EBSD技术研究了不同热轧温度下436不锈钢热轧织构的变化规律和显微组织特征.试验中选取不同的热轧初轧和终轧温度:分别在1 080和1 050℃开轧,850和800℃终轧.结果表明:较低温下开轧、终轧晶粒内部的缺陷较多,表现为较大的累积取向差,而较高温度下开轧、终轧其累积取向差明显低于低温热变形下的样品;在较高温条...     

表面纳米化是否生成纳米晶

喷丸或滚压和摩擦磨损的表面具有相同本质,都是冷加工导致组织细化。20世纪七八十年代从摩擦学角度对表面变形组织进行了系统研究,所得结论已成为材料学教材中的重要内容。21世纪初纳米风带动表面纳米化(SMAT)研究,忽视已有成果,认为变形细化的组织就是纳米晶。早期研究已经明确,变形后组织细化的尺度对性能影响不大,关键因素是其错角,SMAT却仍然强调纳米化尺度越小越好;摩擦学研究认为要获得纳米晶则表层转动要达到一定值,但此种转动将破坏连续性导致表层材料剥落。当SMAT认识到错角是决定性能的关键,并通过超强滚压处理获得大错角纳米晶时,材料表面已布满裂纹。重点讨论了表面形变结构的本质和特征,指出用动态再结晶机理解释纳米晶在位错胞壁生长的观点是错误的,并从胞壁和晶界对位错阻力不同的角度说明胞和晶在Hall-Petch关系中的差异。     

Boundary characteristics in Heavily Deformed Metals

The potential of creating nanostructured metals by plastic deformation to very high strains is currently the subject of intensive research. An important part of this research concerns evolution of the characteristics of deformation induced boundaries, in particular boundary spacing and boundary misorientation. The aim of this paper is to give an overview of the present understanding of the relations between these characteristics, the microscopic deformation mechanisms and the macroscopic deformation mode.     

Temperature dependence of microstructure and texture in cold drawn aluminum wire

The effect of strain and drawing temperature on the evolution of microstructure and fiber textures of aluminum wires drawn at room temperature and cryogenic temperature was investigated by TEM and EBSD observations. The results show that low angle boundaries frequency increases and high angle boundaries frequency decreases with strain increasing when the strain is low. At high strain, most of grain and dislocation boundaries are parallel to the drawn direction and low angle boundaries frequency decreases and high angle boundaries frequency increases with strain increasing. The decrease of deformation temperature leads to microstructure finer and low angle boundaries frequency increasing. Texture analysis indicates that volume fraction of complex texture component decreases with strain increasing and a mixture of 〈111〉 and 〈100〉 fiber texture forms at high strain. 〈111〉 is stable at low strains but 〈100〉 becomes stable at high strain. The decrease of temperature can enhance the stability of 〈111〉 orientation at high strain.     

Grain subdivision and recrystallization in oligocrystalline tantalum during cold swaging and subsequent annealing

A coarse-grained ingot of high-purity tantalum was deformed by swaging at room temperature to a strain of 1.28. During annealing at 900°C for 30 min two neighboring grains were observed to behave quite differently. Electron backscattering diffraction (EBSD) results show noticeable differences in terms of the misorientations developed in both grains. The grain developing larger misorientations recrystallized much more readily than the other. The result is interpreted in terms of the differences in grain subdivision into strongly misoriented regions.     

大马士革铜互连线织构的研究

采用EBSD研究了不同线宽和退火前后Cu互连线的织构和晶界特征分布.Cu互连线均具有多重织构,其中(111)织构强度最高.沉积态样品在室温下发生了自退火现象,并出现了一些异常长大的晶粒.随高宽比降低和退火处理,Cu互连线晶粒尺寸变大,(111)织构得到加强,而具有较低应变程度的织构与(111)织构强度的比例下降.沉积态样品出现了(111)<112>和(111)<231>织构组分.退火后,出现了(111)<110>组分,而且(111)<112>和(111)<231>组分得到增强.Cu互连线以大角度晶界为主,其中具有55°～60°错配角的晶界和∑3晶界比例最高,35°～40°的错配角和∑9晶界次之.随高宽比增加和退火处理,∑3晶界比例逐渐升高,∑9晶界比例下降.     

大马士革铜互连线织构的研究

采用EBSD研究了不同线宽和退火前后Cu互连线的织构和晶界特征分布.Cu互连线均具有多重织构,其中(111)织构强度最高.沉积态样品在室温下发生了自退火现象,并出现了一些异常长大的晶粒.随高宽比降低和退火处理,Cu互连线晶粒尺寸变大,(111)织构得到加强,而具有较低应变程度的织构与(111)织构强度的比例下降.沉积态样品出现了(111)<112>和(111)<231>织构组分.退火后,出现了(111)<110>组分,而且(111)<112>和(111)<231>组分得到增强.Cu互连线以大角度晶界为主,其中具有55°～60°错配角的晶界和∑3晶界比例最高,35°～40°的错配角和∑9晶界次之.随高宽比增加和退火处理,∑3晶界比例逐渐升高,∑9晶界比例下降.     

Evolution of microstructure and texture in an ultrafine-grained Al6082 alloy during severe plastic deformation

The evolution of microstructure and texture in Al6082 precipitation-hardened alloy during equal-channel angular pressing (ECAP) was studied. It was found that although the dislocation density and the subgrain size saturated after 1 pass, the size of grains bounded by high angle boundaries reached its minimum value only after 4 passes. Furthermore, the grain orientation distribution changes between 4 and 8 passes, indicating the development of grain boundary structure even after the saturation of the parameters of the microstructure. As a result of this evolution, the initial texture of the commercial alloy was diminished after 8 passes and the grain orientation distribution became to be close to random case.