Study On Room Temperature Brittleness Of Polycrystalline Beryllium

Beryllium has excellent physical properties, such as low density, high rigidity, strong thermal neutron scattering capability and low coefficient of thermal expansion, etc, and good mechanical properties, so it has many important applications in the fields of weapons, inertial navigation, nuclear energy, infrared optics and high energy physics, and so on. However, room temperature (RT) brittleness of beryllium is its fatal weakness, and thus much attention has been paid to how to understand its mechanism of RT brittleness in order to improve the ductility. Chinese industry has mainly focused on new processing technologies of beryllium so far rather than fundamental research on its brittleness, and then lack understanding of RT beryllium brittleness mechanism, resulting in a series of practical problems in our beryllium industry, such as low yield of beryllium products, difficult to improve beryllium performance, long R&D period of new beryllium materials, and so on.In this work, hot isostatic pressed (HIPed) beryllium has been systematically studied, especially focusing on microstructure factors related to RT ductility of polycrystalline beryllium. The main research contents and achievements are summaried as follows:1. The fracture behavior of polycrystalline beryllium with different elongations under tensile stress, including microcrack initiation, growth and propagation behavior, and also the effects of microstructure defects on the beryllium elongation, have been systematically investigated by using in-situ scanning electron microscope (SEM) tensile test and fractography analysis. The results show that microcracks usually initiate at one grain boundary, then propagate by a transgranular way and terminate at the other side of the grain boundary in polycrystalline beryllium. Crack initiation of polycrystalline beryllium is in accordance with Stroh dislocation pile-up crack theory. Polycrystalline beryllium behaves a cleavage fracture characteristic under tensile stress. Basal cleavage planes of polycrystalline beryllium are (0001) and {1010} planes. Both of them are the main paths of cleavage crack initiation and propagation of polycrystalline beryllium. The growth of microcracks have to depends on different microcracks merging by cleavage steps or tearing way due to a strong blocking effect of grain boundaries on the microcracks propagation. The polycrystalline beryllium has a poor ability resisting the propagation of microcracks, so the limited elongation of polycrystalline beryllium mainly arises from the microcrack nucleating period. Athought microcracks are not preferable to initiate at microstructure defects, microstructure defects lead microcracks to prematurely reach the critical size of crack propagation, which is the main reason responsible for the poor ductility of polycrystalline beryllium. Segregation area of impurity inclusions, loose structure of plate crystals and microholes are equivalent to prefabricated microcracks with certain sizes inside beryllium. The impurity inclusions with large sizes and coarse beryllium grains in local area easily form stress concentration inside beryllium. The thin-film impurity phases along the grain boundaries of beryllium usually lead to the deterioration of grain boundary strength and result in the intergranular cracking.2. The deformation behavior of polycrystalline beryllium has been studied by analyzing twining deformation of polycrystalline beryllium in different yield stage under tensile/compression stresses and in-situ SEM tensile test. The results show that slip and twinning deformation of polycrystalline beryllium are difficult to occur under tensile stress. The slip bands happen only in some grains with a favorable orientation, and finally the twinning deformation grain numbers account for only about 5% of the total grains. There exists the cross slip between (0001) basal plane and{1010} prismatic plane in the tensile deformation process. Polycrystalline beryllium shows good ductility (8=36.30%) under the compression stress. The slip deformation plays a main role for good compression ductility of polycrystalline beryllium, and the twinning deformation mainly happens in the early stage of compression deformation (compression strain ?5.74%). The twinning deformation grain numbers account for about 40?50% of the total grains at the compression strain of 5.74%, and then limited increase in twinning deformation with the compression strain increasing. The twinning deformation of polycrystalline beryllium is not easy to induce the nucleation of microcracks under tensile/compression stresses.3. The morphologies of BeO impurity in polycrystalline beryllium have been systematically analyzed by using transmission electron microscope (TEM), and also the effect of BeO impurity on mechanical properties of polycrystalline beryllium. The results show that the morphology and distribution of BeO impurity are more important factors than the BeO content in controlling mechanical properties of beryllium. When the BeO monodisperse fine particles distribute in the beryllium matrix, they are responsible for the strengthening role of polycrystalline beryllium, and have little side-effect on beryllium ductility. In contrast, with increasing the sizes of BeO impurity particles, the stress concentration increases between BeO particles and beryllium matrix, resulting in obvious brittleness. The critical size of BeO impurity particle balancing the strengthening and brittlness is ?300 nm.4. The grain boundary distribution character of HIPed polycrystalline beryllium has been systematically analyzed by using electron backscatter diffraction (EBSD) technique under SEM, and the micro structure evolution process of beryllium powder sintering body at HIPing as well as its effect on ductility of HIPed beryllium has been studied in detail. The results show that beryllium powder sintering body exists recovery and recrystallization processes at HIPing. Recrystallization grains have specific disorientation relationships with beryllium matrix, and the most disorientation relationship is in the coincidence site lattice (CSL) grain boundaries of beryllium. Beryllium powder sintering body eliminates grain lattice distortion through the recovery and recrystallization, thus decreases dislocation density of HIPed beryllium. The higher HIP temperature is, the more recovery and recrystallization of beryllium powder sintering behaves, and thus the lower dislocation density of HIPed beryllium, finally contributing to a better ductility but a lower strength. Dislocation density has important effect on the ductility of HIPed polycrystalline beryllium.5. The fracture mechanisms of powder HIPed and as-cast Be-Al alloy and its ductility improvement have been studied by analyzing fractograph and metallurgical structure. The results show that microcracks initiate in the Be phase and end in the Al phase in the as-cast Be-Al alloy, which indicates the Be/Al phase interface bonding strength is larger than that of the Be phase in the as-cast Be-Al alloy. In contrast, the microcracks initiate at the Be/Al phase interfaces and the thin area of the Al phase in the powder HIPed Be-Al alloy, which indicates the interface bonding strength is smaller than that of the Be phase in the powder HIPed Be-Al alloy. Behavior of Be-Al alloy crack growth and proragation is very similar to that of polycrystalline beryllium. Grain boundaries take responsibility for resisting crack propagation in polycrystalline beryllium; however the Al phase plays the role in hindering crack propagation in Be-Al alloy. Ductile aluminium phase may relax the stress concentration at the crack tip by the plastic deformation and deliver slip of brittle beryllium phase. In contrast, the grain boundary of polycrystalline beryllium cannot play this role, which is the fundamental reason for the ductility of Be-Al alloy higher than that of beryllium. But overall ductility of Be-Al alloy is controlled by ductility of Be phase.Through the systematical research above, the microstructure factors related to RT ductility of polycrystalline beryllium have been preliminarily understood, which provide much useful information to improve the ductility of polycrystalline beryllium, yield of beryllium products and to develop the Be-Al alloy in our country. Based on these understandings on RT ductility of polycrystalline beryllium obtained in this work, a novel RJY-60 grade beryllium with a micro-yield-strength of >80 MPa has been successfully developed,?20 MPa higher than the present highest micro-yield strength of beryllium (National Defense Patent with an application number of 201318005230.3).