【作者】 常海；

【导师】 吴昆；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2011， 博士

【摘要】 本文针对镁合金板材的低弹性模量、低强度、较差的耐蚀性等问题,采用累积叠轧技术（ARB）在不同工艺下制备了纯Mg板材,纯Mg/AA5052以及纯Mg/AA1050多层复合板材。采用光学显微镜、扫描电镜、透射电镜等方法,研究了ARB板材的显微组织演变和界面结构;采用中子衍射,EBSD等方法研究了ARB板材中的宏观,微观织构演变规律;并进而探讨了ARB板材的力学性能与组织演变的关系。对ARB纯Mg板材的研究表明:纯Mg板材晶粒细化主要发生在初始循环中,晶粒并不能随着ARB循环次数的提高而逐步细化,后续ARB循环只能使组织更加均匀。动态再结晶,ARB循环间对板材的重新加热保温是导致ARB过程中纯Mg晶粒细化不明显的主要原因。ARB纯Mg板材织构类型仍然为典型的Mg合金轧制织构,织构强度基本不随ARB循环次数的提高而发生明显改变;结合界面的引入并不能有效改变复合板材织构类型。板材强度基本不随ARB循环次数的提高而发生明显改变。后续变形仍然是改善ARB纯Mg板材界面结合强度的必需手段。高温下制备的ARB纯Mg/AA5052复合板材中Mg层晶粒尺寸基本不变,Al层晶粒有所细化。ARB过程当中,Al层厚度和Mg层的厚度随着应变量的增加而逐渐降低,界面结合良好。纯Mg/AA5052复合板材中Mg层织构类型为轧制织构,而Al层织构为变形织构组份为主导,剪切以及再结晶织构并存的混合织构类型。三次循环后复合板中Mg层基面织构强度降低以及Al层中变形织构组份强度下降。复合板材的强度随着ARB循环次数的上升而逐渐上升,而经过三次ARB循环的多层复合板材延轧向强度出现明显下降。Mg/Al界面处大量开裂的金属间化合物以及Al层延轧向断开是导致复合板材强度下降的主要原因。纯Mg/AA1050复合板材在室温ARB过程当中,两组元保持了良好的变形协调性,各金属层保持了较好的连续性,三次循环后复合板材中没有出现明显的“波浪”状结构。复合板材中Mg层晶粒尺寸基本不随应变量的升高发生明显变化,Al层晶粒随着ARB循环次数的上升而逐步细化。三次循环后,Mg/Al界面处形成了厚度为150nm的Mg17Al12层。Mg和Mg17Al12之间存在一种确定的晶体学位相关系: [（1|-）11]_Mg17Al12//[01（1|-）0]_Mg（110）//（1|-）11]_Mg17Al12,两者之间以半共格的原子匹配方式结合,结合强度较高,而Al和Mg17Al12之间是否存在位相关系并不明显。随着ARB循环次数的上升,Mg层基面织构强度逐渐上升,Al层逐渐形成以轧制织构为主导,变形织构组份并存的混合织构类型。纯Mg/AA1050复合板材强度随着ARB循环次数的上升而逐渐上升,延伸率则逐渐下降。经过两次ARB循环后,复合板材更多的表现出来单一均匀材料的特性。更多还原

【Abstract】 In order to improve the strength, elastic modulus and corrosion resisitance of Mg sheet, accumulative roll bonding （ARB） was utilized to fabricate ultra-fine grained pure Mg sheets, pure Mg/AA5052 and pure Mg/AA1050 laminated composites. Optical microscope, scanning electronic microscope （SEM） and transmission electronic microscope （TEM） were employed to observe microstructure and interface structure of ARBed sheets. The global texture evolution and local texture of the ARBed sheets were measured by neutron diffraction and EBSD. Then, the relationship between microstructure and mechanical properties of these ARBed sheets was investigated.For the ARBed pure Mg sheets, the microstructure was refined due to the large strain imposed on the sheet after 1 ARB cycle. During the following ARB cycles, the homogeneity of the microstructure was improved while the average grain size didn’t change obviously. The unnoticeable refining effect during the subsequent ARB processing can be attributed to the interval reheating and dynamic recrystallization. ARBed Mg sheet exhibited a typical rolling texture whose intensity almost kept stable during the whole ARB processing. The EBSD measurement indicated that ARB was not an effective method in grain refinement and texture modification for the pure Mg sheet. Moreover, the following rolling was still required to improve the bonding quality of the ARBed pure Mg sheet.The pure Mg/AA5052 laminated composite was processed by ARB at 400?C up to 3 cycles successfully. Necking and rupture of Al layers took place at the final cycle because of the difference of the flow properties between the Mg layer and Al layer, and then introduced a waviness structure into the Mg/AA5052 laminated composite Mg grains were not refined during ARB process due to high deformation temperature and interval annealing while the grain size of Al layer was decreased with increasing of ARB cycle. The texture type of Mg layer was typical rolling texture and the dominant texture of Al layer wasβfiber rolling texture with scatter around the ideal orientations. The significant waviness structure caused by the shear band reduced both the intensity and the sharpness of the rolling texture of both Mg and Al layer. Strengths of the Mg/AA5052 laminated composite were enhanced significantly after sandwich preparation and then increased slightly with the increasing of the ARB cycles till the second ARB cycle. However, strengths along the rolling direction decreased dramatically after the third ARB cycle. Obvious cracking of the coarse Mg/Al intermetallic compound and rupture of the Al layers led to the dramatic decreasing of yield strength along the rolling direction after the final cycle.According to the results above, it was necessary to reduce ARB temperature and avoid formation of the Mg/Al intermetallic compounds in the Mg/Al laminated composite. Thus, pure Mg/AA1050 laminated composite was processed by ARB at ambient temperature. The two constituent layers kept continous during ARB and no waviness structure could be observed in the Mg/AA1050 laminated composite. As similar as the Mg/AA5052 laminated sheet, the Mg grains in the Mg/AA1050 laminated composite were not refined during ARB process due to the dynamic recrystallization while the grain size of Al layer decreased with increasing of ARB cycle. Mg17Al12 phase with thickness of 150nm formed at Mg/Al interface after 3 cycles. There existed a definite orientation relationship between Mg17Al12 and Mg, that is [（1|-）11]_Mg17Al12//[01（1|-）0]_Mg（110）//（1|-）11]_Mg17Al12, with good lattice matching between the two phases at the interface. The texture type of Mg layer was typical rolling texture. The major components of the Al layer were characterized as a combination texture type including evidentβfiber texture and Rotated Cube shear component. Strengths of the Mg/AA1050 laminated composite increased gradually with the increasing of the ARB cycles. After 2 cycles, the laminated composite behaved like other single phase materials.更多还原