High Energy Ball Milling Of Pure Al Nanocrystalline Materials, And Performance

Nanocrystalline materials ( nc ) have attracted much research interest due to their peculiar structures and excellent properties. However, traditional techniques have limitations.They are difficult to obtain inexpensive and three-dimensional large-size nc materials with compact and steady structure and cleanly interface. The properties of nc materials fabricated by different traditional techniques are very different. In order to reveal the intrinsic properties of nc materials, especially mechanical properties, nc materials need to have as much as possible high density and less structural defects. Therefore, the development of techniques being able to be used to fabricate nc materials which have cleanly interface, no pores, and can factually reflect their intrinsic structures and properties is the key. In resent years, high energy ball milling technique have been founded to be able to fabricate nc materials, attracted much research interest due to simple equipment and easy manipulation.In the experiment, using the pure aluminium powders for raw materials, pure Al nc materials were successfully fabricated via high energy ball milling technique. The effect of ball milling techniques on pure Al nc materials was studied by changing milling time and ball-to-powder weight ratio (BPR). Meanwhile, the effect of process control agent (PCA) on preparation and properties of pure Al nc materials was discussed by introducing oleic acid during milling process. Structures of as-fabricated pure Al nc materials were analyzed by X-ray diffraction (XRD) and Scanning electron microscope (SEM) etc. Their mechanical properties were evaluated via micro-hardness and room temperature tensile tests. Furthermore, the relationship between average grain size and micro-hardness was discussed, and plastic deformation mechanism of as-fabricated pure Al nc materials was probed. Finally, the thermal stability of as-fabricated pure Al nc materials also was studied by annealing at different temperature.The results show that high performance pure Al nc materials are able to be fabricated by controlling ball milling technique. During the ball milling process, the average grain size of samples initially reduces, subsequently increases and finally reduces with increasing ofmilling time. When BPR is 10:1, the average grain size of as-fabricated samples after 3h, 5h, 8h and 14h of milling are 59nm, 49nm, 55nm and 47nm, respectively. With prolonging of milling time, the micro-hardness of as-fabricated samples initially raise, then fall and again raise, finally keep stabilizing. Compare to annealing Al, the micro-hardness of all as-fabricated pure Al nc materials are higher, the micro-hardness of sample obtained after 5h of milling is about 430MPa which is about as two and half times as annealing Al. When process control agent is introduced, the micro-hardness of as-fabricated samples is about as five times as annealing Al, and grain size of sample reduces to 33nm. As-fabricated samples occur recrystallization and grains growth after annealing at 75℃, thus micro-hardness of samples recedes to 33%～48% of initialization. When BPR is higher, efficiency of milling is also higher, which promotes the samples to achieve stabilization. Moreover, higher BRP makes the variety of samples' micro-hardness slower, the structure of as-fabricated samples is more compact, grain is more well-distributed.Specific elongations of all as-fabricated samples are less 5% due to presence of shrinkage cavities and microcracks which promotes the germination of shearing deformation. And these shrinkage cavities and microcracks consisting in interior of samples lower the plastic property. The SEM images of fracture reveal a lot of dimples, indicating fracture of samples belong to tenacity fracture. However, strain-hardening doesn't occur during room temperature tensile tests process. Yield strength and breaking strength of as-fabricated samples achieve to 198MPa and 217MPa, respectively. The relation between yield strength and grain size conforms with Hall-Petch formula.