首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到10条相似文献,搜索用时 78 毫秒
1.
High-temperature compression testing combined with high-resolution electron backscatter diffraction (EBSD) analysis has been used to study microstructural-scale straining processes that occur during high-temperature deformation of a powder-consolidated nickel-base superalloy, René 88DT (GE Aviation, Evendale, OH). Orientation imaging has been employed to study grain-level straining and strain storage at temperatures between 1323 K (1050 °C) and 1241 K (968 °C) for strain rates between 0.1/s and 0.00032/s at nominal strain levels between 0.1 and 0.7. Two distinct deformation mechanisms were observed. At strain rates below 0.01/s, superplastic deformation dominates, while power-law creep occurs during high rate compression. Stored strain and evolution of the grain structure during deformation are dependent on strain rate during compression. At low strain rates in the superplastic regime, low levels of stored strain and some grain growth are observed. At high strain rates, dynamic recrystallization occurs along with higher levels of stored strain within selected grains, particularly those at the high end of the grain size distribution. A constitutive model for superplastic deformation was employed to predict the temperature and strain rate dependence of the transition from superplastic to power law deformation. The transition in rate sensitivity was consistent with the transition in stored strain measured by EBSD. Superplasticity-enhanced grain growth is observed and the implications for the transition in deformation mechanisms are discussed.  相似文献   

2.
3.
This study is concerned with the effects of microstructural modification on superplastic deformation characteristics of a rapidly solidified (RS) Al-3Li-1Cu-0.5Mg-0.5Zr (wt pct) alloy. This Al-Li alloy has a very fine grain structure desirable for improved superplasticity. The results of superplastic deformation indicated that the alloy exhibited a high superplastic ductility, e.g., elongation of approximately 800 pct, when deformed at temperatures above 500 °C and at the strain rates of 10−2/s to 10−1/s. Such a high strain rate is quite advantageous for the practical superplastic forming application of the alloy. Stress-strain rate curves were obtained by performing a series of load relaxation tests in the temperature range from 460 °C to 520 °C in order to examine the superplastic deformation behavior and to establish its mechanisms. The stress-strain rate curves could be separated into two parts according to their respective physical mechanisms, i.e., grain matrix deformation and grain boundary sliding, as was proposed in a new superplasticity theory based on internal deformation variables. The microstructural evolution during superplastic deformation was also analyzed by using transmission electron microscopy. During superplastic deformation, grains were kept fine and changed into equiaxed ones due to the presence of fine secondary phase particles and the continuous recrystallization due to the development of subgrains. Consequently, the rapidly solidified (RS) alloy showed much improved superplasticity compared to the conventional ingot cast 8090 alloy.  相似文献   

4.
对预变形后的Al-25%Si合金进行半固态等温处理,研究了半固态等温处理工艺参数对合金组织的影响。利用光学显微镜观察组织,Image Pro Plus金相分析软件来测定初生硅平均直径和形状因子。结果表明,挤压变形后初生硅相的尺寸有明显的细化,但形态较差,而通过半固态等温处理后初生硅的颗粒得到明显的球化,但其平均直径有所增加。  相似文献   

5.
The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 μm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.  相似文献   

6.
 Microstructures and critical phase-transformation temperature of boron-nickel added Nb-treated high strength low alloy (HSLA) H-beams cooled at different cooling rate, with different deformation were investigated. Continuous cooling transformation (CCT) diagram of this new type of steel was obtained by using Gleeble 1500 thermomechanical simulator. Microstructures and hardness, especially micro-hardness of the experimental steel were investigated by optical microscopy (OM), scanning electron microscope (SEM), Rockwell and Vickers hardness tests. Phase analysis was also studied by X-ray diffraction (XRD). The results indicated that with increase of cooling rate, microstructures of continuous cooled specimens gradually transformed from polygonal ferrite and pearlite, grain boundary ferrite and bainite, bainite and martensite to single martensite. The CCT diagram revealed that slow cooling was needed to avoid austenite-bainite transformation to ensure toughness of this steel. By plastic deformation of 40%, austenite-ferrite transformation temperature increased by 46 ℃, due to deformation induced ferrite transformation during continuous cooling, but Rockwell hardness has little change.  相似文献   

7.
新型PMS塑料模具钢超塑性成形及强化   总被引:2,自引:1,他引:1  
利用PMS镜面塑料模具钢进行恒温超塑性试验,利用试验参数对话机键模进行超塑性精密成形,并对其组织结构、宏观强度及其强化机制进行了研究分析。指出,由于超塑性变形导致PMS钢的位错强化、沉淀强化和细晶强化,使其在超塑性精密成形的同时能有效地提高强度。  相似文献   

8.
It has been shown that the application of hydrostatic gas pressures during superplastic deformation of fine grained 7475 Al can entirely prevent the intergranular cavitation normally encountered at atmospheric pressure. A critical ratio of hydrostatic pressure to flow stress may be defined for each superplastic forming condition above which virtually no cavitation occurs. In superplastic deformation conditions where intergranular cavitation plays a significant part in final tensile rupture, the superplastic ductility may be improved by the application of hydrostatic pressures. Similarly, detrimental effects of large superplastic strains on service properties may be reduced or eliminated by the application of suitable hydrostatic pressures during superplastic forming. In this case, superplastically formed material may have the same design allowables as conventional 7475 Al sheet. Formerly with the Rockwell International Science Center, Thousand Oaks, CA.  相似文献   

9.
Although Ti-6A1-4V displays extensive superplasticity at 1200 K, lower superplastic forming temperatures are desirable. A study was conducted with the goal of modifying the composition of the Ti-6A1-4V alloy to lower the optimum superplastic forming temperature. Computer modeling results and previous experimental data suggested that additions to Ti-6A1-4V of beta-stabilizing elements which have high diffusivity in the beta-phase would permit lower superplastic forming temperatures. A series of modified alloys with 2 wt pct additions of Fe, Co, and Ni was prepared for experimental evaluation. The modified alloys achieved desirable microstructures for superplasticity at 1088 K,i.e., the grain size was approximately 5 μm and roughly equal volume fractions of the alpha- and beta-phases were present at the deformation temperature. The superplastic properties of the modified alloys were measured at 1088 K and 1144 K. The modified alloys produced values of flow stress, strain rate sensitivity, and total elongation at 1088 K approaching those of the base Ti-6A1-4V alloy at its standard superplastic forming temperature of 1200 K. In addition to lowering the superplastic forming temperature, the β-stabilizing additions also increased room temperature strength levels above those normally found for Ti-6A1-4V. Based on the room temperature and elevated temperature tensile properties, addition of selected beta-stabilizing elements to Ti-6A1-4V simultaneously raises resistance to deformation at room temperature and lowers resistance to deformation at elevated temperatures. This reversal in behavior is explained by considering the effect of beta-stabilizer additions on the deformation mechanisms at room temperature and at elevated temperatures.  相似文献   

10.
The deformation of AA8090 Al-Li-Mg-Cu-Zr alloy at elevated temperature and slow strain rates has been investigated in uniaxial tension. Under suitable conditions, this material exhibited a high strain-rate sensitivity of the flow stress and was superplastic. This superplastic behavior was obtained in material with an initially elongated grain structure combined with a distribution of similarly oriented grains and low-angle grain boundaries that was not conducive to boundary sliding. Observations of the development of microstructure and of the crystallographic preferred orientation indicated that no significant rigid body translation and little rotation of grain interiors occurred up to strains of about 0.4 and that the probability of relative translation of grain interiors up to strains of at least 1 was low. The changes of structure observed could be accounted for by a combination of grain growth and grain rotation. The consequence of these observations on the grain switching and grain boundary sliding mechanisms generally assumed to operate during superplastic deformation is discussed, with the conclusion that those mechanisms may not be wholly appropriate for explaining high rate sensitivity in this material over the range of strain rates investigated.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号