【作者】 林丹；

【导师】 王磊；

【作者基本信息】 东北大学 ， 材料学， 2011， 博士

【摘要】 随着人们对能源危机的认识及环保意识的增加,镁合金以其密度低、比强度高、导电导热性好、易回收利用等优点倍受航空、航天、汽车、电子产品等领域的关注。而高性能稀土变形镁合金的研发更是成为近年研究的热点。然而,稀土变形镁合金的性能与用户的要求仍存在一定差距,因此亟需探索提高合金性能的新途径。本文以挤压态GW103、ZK60合金为研究对象,通过多种方法调整合金的组织,研究不同组织特性及其对合金断裂行为的影响,以期为镁合金的工艺优化及安全使用提供理论依据,为新型高性能镁合金的研发提供参考。研究了不同时效处理条件下GW103合金的组织特点及其对拉伸断裂行为的影响。结果表明,473K时效时,析出相尺寸小、数量多,由晶界析出相引发的微裂纹易在晶界处产生并沿着晶粒内的择优取向扩展,合金发生穿晶断裂；523K时效时,晶界析出相尺寸增大,应力集中仅在某些大尺寸晶界析出相处出现,并引发析出相与基体分离而产生微孔,微孔的聚集与长大导致合金的断裂。当孪晶与晶界析出相相交时,微裂纹可在相交界面处产生并沿孪晶与基体界面扩展。时效温度升高,析出相与孪晶的数量减少,滑移变形加剧,合金塑性改善。由此,GW103合金获得强度和塑性均可的时效温度以523K为宜。研究了时效处理对GW103合金低周疲劳行为(LCF)的影响,揭示了时效析出影响疲劳裂纹萌生、扩展的机理。结果表明,±0.20%应变幅条件下,位错滑移需要孪生协调,β’相可抑制LCF裂纹在形变孪晶与基体的界面处萌生并发生穿晶扩展的过程,合金的LCF寿命提高；±0.40%应变幅条件下,滑移变形加剧,晶界析出相加剧了LCF裂纹在晶界的萌生、扩展程度,合金LCF寿命降低。研究发现,塑性变形使GW103合金基体内生成晶体缺陷,为时效析出提供更多的有效形核核心,β’相数量增加,同时促进了晶界、孪晶与基体界面处析出相的生成。析出相数量的增加可有效阻碍拉伸变形过程中的位错滑移而强化合金基体,2%变形合金可实现强度与塑性的良好配合。变形量增大,微裂纹将在晶界析出相与基体界面处产生并沿晶界扩展,再加之断口表面平滑刻面的形成导致合金拉伸性能降低。弹性变形对GW103合金等温析出行为具有诱导作用,析出相形态直接影响拉伸断裂行为。时效过程中施加的弹性变形通过增加基体内的缺陷密度诱发更多β’相析出,而空位等缺陷向晶界的扩散促进了晶界析出相与无析出带的生成。拉伸变形过程中,β’相数量的增加使合金的变形抗力提高,合金强化；而位错在无析出带内的优先滑移加剧了晶界处的应力集中程度,微裂纹在晶界产生并沿晶界扩展,合金发生沿晶断裂。通过控制弹性变形的程度可促进晶界析出相的合并,使析出相间产生空隙、缓解晶界处的应力集中,在提高合金强度的同时亦改善了其塑性。稀土Y、Nd复合添加不但可细化ZK60合金的挤压态晶粒,还可通过增加固溶量、形成弥散第二相使合金的抗拉强度与屈服强度分别提高8.4%与31.2%。Y、Nd复合添加使第二相尺寸减小,拉伸变形过程中,第二相在其脆弱处碎裂,并与基体脱离形成微孔进而导致合金断裂的几率减少,合金达到强度与塑性的良好配合。更多还原

【Abstract】 With the recognition of energy crisis and the awareness of environmental protection, magnesium alloys have been paid great attention in many kinds of fields such as aviation industry, aerospace industry, automobile industry, electronic products etc., because of their low density, high specific strength, high thermal and electrical conductivity, easily recycle and so on. In recent years, the studies have focused on the high performance wrought magnesium alloys contained with rare earth. However, the mechanical properties of wrought magnesium alloys contained with rare earth still have not met the requirements in practical applications. Therefore, new methods for improving the properties of wrought magnesium alloys have to be explored. The as-extruded alloys of GW103and ZK60were employed for the present research. The microstructures of the alloys were modified by several means, and effects of microstructure characteristics on fracture behavior were studied, in order to provide the theory for optimal processes and safe application dates as well as developing high performance magnesium alloys.The microstructure characteristics of GW103alloy under different aging conditions and the effects of them on the fracture behavior were investigated. The results show that there is a great amount of fine precipitations in the alloy aged at473K. Microcracks nucleate at grain boundaries because of the grain boundary precipitations (GBPs), and then propagate trans granular ly along the preferred orientation, which causes the transgranular fracture. For the alloy aged at523K, the size of GBPs enlarges. Stress concentration only emerges at large GBPs, resulting in the nucleating of the microvoids by interface debonding between GBPs and the matrix. Subsequently, the coalescence and growth of microvoids lead to the final fracture. Otherwise, when the twins intersect with GBPs, the microracks nucleate at the interface between GBPs and the matrix, and then propagate along the interface between the twin and the matrix. With the aging temperature increasing, the amount of twins and precipitations decreases and the slip deformation intensifies, thus the ductility of the alloy is improved. Therefore, the suitable aging temperature for GW103alloy which can achieve a good combination of strength and ductility is523K.The effects of aging treatment on low cycle fatigue (LCF) behavior of GW103alloy were studied; the influences of the precipitations on both crack initiation and propagation were discussed. The results show that at the total strain amplitude of±0.20%, the cracks initiate at the interface between deformation twins and the matrix, and then propagate transgranularly since the deformation twins coordinate with the dislocation slipping. Since the initiation and propagation of cracks are hindered by the present of β’ phases, the LCF lives increase. At the total strain amplitude of±0.40%, the slip deformation intensifies and the GBPs promote the initiating and propagating of cracks at grain boundaries, inducing the decrease of the LCF lives.It has been found that the crystal defects in GW103alloy caused by the plastic deformation can provide more nucleating cores for aging precipitation, the increase of the amount of P’phases, and the formation of precipitations at grain boundaries and interfaces between the twins and matrix. Because of the increase of precipitations, the dislocation slipping during deformation process is effectively hindered and the matrix is strengthened, especially for the2%deformed alloy which can achieve a good combination of strength and ductility. When plastic deformation increases, the microcracks nucleate at the interface between GBPs and matrix, and then propagate intergranularly. When combined with the formation of facets on the fracture surface, the tensile properties decrease.Elastic deformation has the inducing effect on precipitation behavior during isothermal aging process of GW103alloy, and the changes of precipitation morphology can directly affect the fracture behavior. With the application of elastic deformation during aging process, the precipitation of β’phases can be induced by increasing the defect density in the matrix, and the formation of GBPs and precipitate-free zones (PFZs) is promoted by the diffusion of vacancy to the grain boundary. During the deformation, the deformation resistance of alloy is improved by the increases of β’numbers, and the alloy is strengthened. But the degree of stress concentration on the grain boundary is intensified with the priority slipping of dislocation in the PFZs, so microcracks nucleate at grain boundaries and then propagate intergranularly, which causes the final fracture. The control of elastic deformation can promote the coalescence of GBPs, and the space between GBPs appears, which can relief the stress concentration at the grain boundaries, and the ductility is improved as well as the strength.With the adding of both Y and Nd, the grain size of as-extruded ZK60alloy is refined. At the same time, the ultimate strength and yield strength increase about8.4%and31.2%, respectively. The improvement of strength is attributed to the increase of solid solution content and the second-phases precipitation. Meanwhile, the probability of second phases to be broken and divorced from the matrix during the deformation is reduced because the second phases in the matrix are refined with the additions of both Y and Nd. Just for this reason, the formation of microvoids which causes the final fracture is hindered. And a good combination of strength and ductility can be achieved.更多还原