Kinetics and microstructural evolution during recrystallization of a Ni_3Al-based single crystal superalloy

The recrystallization kinetics and microstructural evolution of a Ni3Al-based single crystal superalloy were presented, especially the different recrystallization behaviors between the dendrite arm and the interdendritic region. The single crystal alloy was deformed by grit blasting. A succeeding annealing under inert atmosphere at 1280 ℃ for different time led to the formation of recrystallized grains close to the grit blasting surface. It was found that the recrystallization depth and velocity in the dendrite arm were respectively deeper and faster than those in the interdendritic region where the Y-NiMo phase existed. The recrystallization process in the interdendritic region was significantly inhibited by the Y-NiMo precipitates. However, the pinning effect gradually weakened with the annealing time due to the dissolution of the Y-NiMo phase, and the recrystallization depth in the dendrite arm was deeper than that in the interdendritic region.

Surface recrystallization of a Ni_3Al based single crystal superalloy at different annealing temperatures and blasting pressure

The effects of annealing temperature and grit blasting pressure on the recrystallization behavior of a Ni3Al based single crystal superalloy were studied in this work. The results show that the precipitation of the Y-NiMo phase occurs at 900 and 1000 °C, which precedes recrystallization. The initial recrystallization temperature was between 1000 and 1100 °C. Cellular recrystallization was formed at 1100 and 1200 °C, which consisted of large columnar γ′ and fine γ + γ′. The dendrite arm closed to the interdendritic region may act as nucleation sites during initial recrystallization by a particle simulated nucleation mechanism at 1280 °C. The size of the grains first turned large and then became small upon the pressure while the recrystallization depth increased all the time.

两种取向Ni_(3)Al基单晶高温合金的组织和拉伸性能

为了研究晶体取向对Ni_(3)Al基单晶高温合金组织和力学性能的影响,采用扫描电镜和拉伸试验机研究了[001]和[111]两种取向合金的显微组织和拉伸性能.结果表明:经过热处理后,在单晶棒横截面内[001]取向合金γ′相为立方形,[111]取向合金γ′相为三角形或六角形;[001]和[111]取向合金存在屈服强度随温度升高而升高的反常屈服现象;[001]与[111]取向合金均呈现明显加工硬化现象,两种取向合金的屈服强度峰值范围分别为600~800℃与400~600℃;两种取向合金均表现为韧性断裂.

Impact of replacement of Re by W on dislocation slip mediated creeps of γ'-Ni_(3)Al phases

The anomalous flow behavior of γ'-Ni_(3)Al phases at high temperature is closely related to the cross-slip of 1/2<110>{111}super-partial dislocations.Generalized stacking fault energy curves(i.e.,Γ-surfaces)along the lowest energy path can provide a great deal of information on the nucleation and movement of dislocations.With the first-principles calculation,the interplay between Re and W,Mo,Ta,Ti doped at preferential sites and their synergetic influence on Γ-surfaces and ideal shear strength(τ_(max))in γ'-Ni_(3)Al phases are investigated.Similar to single Re-addition,the Suzuki segregation of W at stacking faults is demonstrated to enable to impede the movement of 1/6<112>{111} Shockley partial dislocations and promote the cross-slip of 1/2<110>{111}super-partial dislocations.With the replacement of a part of Re by W,a decreased γ_(APB)^(111)/γ_(APB)^(001) indicates that the anomalous flow behavior of γ'phases at high temperature is not as excellent as the double Re-addition,but an increasedτmax means that the creep rupture strength of Ni-based single crystal superalloys can be benefited from this replacement to some extent,especially in the co-segregation of Re and W at Al−Al sites.As the interaction between X1_(Al) and X2_(Al) point defects is characterized by an correlation energy function ΔE^(X1_(Al)+X2_(Al))(d),it is found that both strong attraction and strong repulsion are unfavarable for the improvement of yield strengths of γ'phase.

用锡盘抛光 α-Al_2O_3 单晶的初步实验研究

概述了抛光α－Ａｌ２Ｏ３单晶的意义、介绍了用锡盘抛光α－Ａｌ２Ｏ３单晶的实验装置、描述了抛光实验过程、研究了抛光时间和抛光液酸碱度对工件表面粗糙度的影响。通过改变抛光时间及抛光液ｐＨ值，可使α－Ａｌ２Ｏ３单晶工件表面粗糙度优于０．４ｎｍｒｍｓ。

Recrystallization of Ni_3 Al Base Single Crystal Alloy IC6SX with Different Surface Mechanical Processes

The recrystallization behavior of Ni 3 Al base single crystal alloy IC6SX with different mechanical processes has been studied.The specimens of the alloy were treated by surface cleaning and cold working during the blades manufacture,and then heat treated in the temperature range of 800-1260℃ for 1 h to 4 h.The microstrcture of the specimens were examined by optical and scaning electron microscopy,and the room temperature tensile property,stress rupture property under the test condition of 1100℃/130 MPa,and the thermal cycle fatigue resistance under 1100-20℃ of the specimens with and without recrystallization were carried out.The experimental results showed that the initial recrystallization nucleation temperatures of the alloy by dry grit blasting,wet grit blasting,indentation,shot blasting and burnishing were 900-950℃/1 h,1000-1100℃/1 h,1200-1220℃/4 h,1220-1240℃/1 h and 1220-1240 C/1 h,respectively and the primal recrystallization completing temperatures of the alloy by those mechanical processes mentioned above were 1200-1210℃/1 h,1100-1200℃/1 h,1220-1240℃/4 h,1240-1260℃/1 h and 1240-1260℃/1 h,respectively.The results also indicated that the deformation amount of the specimens by burnishing and dry grit blasting was bigger than that by wet grit blasting and shot blasting according to their recrystallization temperature and the microhardness profile below the surface of specimens.It has been found that the recrystallization had no evident effect on mechanical properties of the alloy.

Orientation dependency of shear stress-strain curves in B2-ordered Fe_3Al single crystals deformed in tension at room temperature

B2-ordered Fe3Al single crystals with various orientations were deformed in tension at room temperature in vacuum. The shape of shear stress-strain curves and work hardening rates were found to be strongly dependent on the orientation. The formation of the five different work hardening stages were considered to be related to the number of operative slip systems, the effect of secondary slip systems and the dissociation of the twofold superdislocation. Stage I is an easy glide stage corresponding to single slip. Stage II, with high hardening rate, often corresponds to the existence of conjugate slip systems. Stage III, with relatively low hardening rate, corresponds to the weaker hardening of secondary slip systems. Stage IV, with the highest hardening rate, is not only related to multiple slip but also the dissociation of twofold superdislocations and the moving of superpartials with an antiphase boundary (APB) trap. Stages V, with a negative hardening rate, may be caused by the cross slip of single dissociated superdislocation. The number of stages and the work hardening rate of the same stage were also found to change significantly, when the tensile orientation lies in different orientation regions.

单晶Ni_(3)Al裂纹扩展行为的分子动力学模拟

基于嵌入原子势的分子动力学模拟分析了单晶Ni_(3)Al的裂纹扩展机理和显微组织演化。首先,通过设置初始裂纹并对拉伸速率恒定时的单晶Ni_(3)Al模型进行单轴拉伸,模拟了裂纹尖端的形状和微观结构的变化,从模拟结果可以观察到在裂纹尖端产生了位错堆积现象,扩展后的裂纹方向与位错线成45°和135°夹角。其次,分别讨论了温度对5×10^(-11)s时裂纹尖端应力集中现象、屈服点的抗拉强度以及裂纹扩展速率的影响。研究结果表明:当温度从300 K升温至1300 K时,5×10^(-11)s时裂纹尖端的应力值以线性的形式降低;其屈服点抗拉强度的降低趋势也是线性的,但其应变值增加;高温可有效地减缓裂纹扩展速率。

Pulling growth technique towards rare earth single crystals

Pulling growth technique serves as a popular method to grow congruent melting single crystals with multiscale sizes ranging from micrometers to centimeters.In order to obtain high quality single crystals,the crystal constituents would be arranged at the lattice sites by precisely controlling the crystal growth process.Growing interface is the position where the phase transition of crystal constituents occurs during pulling growth process.The precise control of energy at the growing interface becomes a key technique in pulling growth.In this work,we review some recent advances of pulling technique towards rare earth single crystal growth.In Czochralski pulling growth,the optimized growth parameters were designed for rare earth ions doped Y_3Al_5O_(12)and Ce:(Lu_(1-x)Y_x)_2Si O_5on the basis of anisotropic chemical bonding and isotropic mass transfer calculations at the growing interface.The fast growth of high quality rare earth single crystals is realized by controlling crystallization thermodynamics and kinetics in different size zones.On the other hand,the micro pulling down technique can be used for high throughput screening novel rare earth optical crystals.The growth interface control is realized by improving the crucible bottom and temperature field,which favors the growth of rare earth crystal fibers.The rare earth laser crystal fiber can serve as another kind of laser gain medium between conventional bulk single crystal and glass fiber.The future work on pulling technique might focus on the mass production of rare earth single crystals with extreme size and with the size near that of devices.