【作者】 石宁；

【导师】 曹占义； 王立民；

【作者基本信息】 吉林大学 ， 材料学， 2008， 硕士

【摘要】 镁合金以其密度小、比强度高的优异性能而广泛地应用于汽车、电子、航空航天等领域。但是,由于镁合金的力学性能较差,很难作为高温环境下的结构部件,因此,常通过变形工艺改善镁合金的综合性能。本文以Mg-Al-Zn系列合金为基础,通过添加富铈稀土(MM),设计的合金为:Mg-3Al-0.5Mn-1MM、Mg-3Al-0.5Zn-0.5Mn-1MM、Mg-2Al-0.5Mn-1MM、Mg-2Al-0.5Zn-0.5Mn-1MM、Mg-2Al-0.5Zn-0.5Mn-0.5MM。采用电阻炉加热和钢模浇注制备合金,选取Mg-3Al-0.5Zn-0.5Mn-1MM和Mg-2Al-0.5Zn-0.5Mn-1MM合金进行轧制和挤压变形,并对变形后合金分别进行了退火和时效处理。分别对铸态、变形态及热处理态合金进行显微组织观察和力学性能测试。其中显微组织观察采用合金相组成、金相分析、扫描分析等分析手段;力学性能的测试主要为合金的显微硬度和拉伸性能。结果表明,添加MM提高了合金结晶过程中的形核率,明显细化铸态组织。铸态合金中添加MM和锌显著提高了合金的强度和伸长率。析出稀土相主要为针状的Al4MM(MM为Ce,La)和颗粒状的Al10Ce2Mn7。合金在轧制及挤压变形过程中发生了动态再结晶,得到较小的等轴晶粒,稀土相破碎并均匀弥散地分布在晶内及晶界上。在轧制后退火过程中,出现晶粒的回复再结晶,晶粒长大。挤压后时效强化效果不明显,随时效时间延长晶粒尺寸变化不大。断口分析表明,铸态合金断口属于准解理断裂,韧性特征明显,轧制变形后合金退火处理后,断裂由解理脆断向准解理断裂转变,挤压变形后的合金属于准解理断裂。添加少量的富铈稀土使基体合金的强度和塑性均得到提高,析出相强化作用明显。更多还原

【Abstract】 Magnesium alloys, as the lightest structural material in practical application, has achieved rapid development since coming out, attributing to its low density, high specific strength and specific rigidity, excellent damping capacity and electromagnetic shielding capacity, as well as recuperability. However, magnesium alloys are limited to be used at elevated temperature due to its poor mechanical properties. Mg-Al-Zn alloy system wrought magnesium alloys were reported to be the best candidate in practical use till now. Thus, it’s of great significance to research and develop a high Mechanical Properties magnesium alloy, serving for industry.In this dissertation, new magnesium alloys Mg-3Al-0.5Mn-1MM、Mg-3Al-0.5Zn-0.5Mn-1MM、Mg-2Al-0.5Mn-1MM、Mg-2Al-0.5Zn-0.5Mn-1MM、Mg-2Al-0.5Zn-0.5Mn-0.5MM are achieved by adding mischmetal(MM) on the basis of Mg-Al-Zn alloys are melted in electric resistance furnace and poured into steel mould, then select # 2 and # 4 alloys to rolling, extrusion deformation, heat treatment by annealing and aging treatment. This article studies effect of MM on solidification and microstructure, deformation capacities of as-cast alloys, mechanics property and fracture behavior of rolled and extruded alloy.The following are main studying content:(1) Selected alloy consisting of: Mg-2-3Al-0.5Mn-0~0.5Zn-0.5~1.0MM(2) Preparation of alloys, research on organization, structure, the composition of the precipitates, shape and distribution of as-cast, as-rolled, as-extruded and heat treatment states alloys.(3) Mechanical properties of the alloy, tensile fracture morphology observation and fracture mechanism analysis of the alloy for different states.(4) Mechanism of Rare Earth (MM) on the structure and properties of alloys, proposed alloy design. After research above, reach conclusions as fellow:As-cast alloys is composed ofα-Mg, pole-like Al4MM(MM is Ce、La) and grain-like Al10Ce2Mn7, In the Mg-3Al alloy system there are a small number of bone-likeβ-phase precipitates Mg17Al12.α-Mg crystal refines obviously and get coarse grain. Pole-like precipitates major segregate around the grain boundary as discontinuous eutectic structure, and grain-like precipitates are mainly distributed in the grain. Most of Zn dissolved into Mg matrix. The addition of MM, increased alloy solid-liquid phase temperature difference is conducive to the formation of segregation and composition undercooling.,MM enhanced crystallization rate in the process of nucleation, refine microstructure of as-cast alloy. And significantly enhanced mechanical properties of the alloy. The Mg-3Al-0.5Zn-0.5Mn-1MM and Mg-2Al-0.5Zn-0.5Mn-1MM alloy have outstanding performance particularly.In the process of rolling and extrusion deformation dynamic recrystallization phenomenon can be observed, grain size decreases, precipitates broken and uniformly distribute on the grain and the grain boundary. In the annealing process after rolling, recovery and recrystallization phenomenon can be observed, and then grain grows. In the aging process after extrusion, there is little change on grain size. Aging strengthening is not obvious.After the deformation, the flow stress of the alloy increase, thereby enabling alloy internal dislocation density increased significantly enhanced the strength of the alloy. In addition, at the room temperature, Al4MM and Al10Ce2Mn7 have higher hardness, under high temperature are stable phase, the second phase particles in the matrix surrounding the will bring uneven deformation, For the coordination uneven deformation in the second phase particles around Cypriot plot large dislocation, broken Al4MM and Al10Ce2Mn7 which distribute in the matrix will produce more dislocations, and further enhance the strength of alloy. MM phase can also be effective to inhibit grain growth in the process of crystallization, improve the mechanical properties of heat treatment alloy.After rolling, the max Vickers-hardness of Mg-3Al-0.5Zn-0.5Mn-1Ce(MM) alloy increases from 67HV to 92HV, the max ultimate tensile strength (UTS) and yield tensile strength (YTS) of rolled state alloys at room temperature are 301MPa and 228MPa, respectively, Annealed alloy strength decreased slightly, but a clear enhancement of elongation, reach 21%. For the Mg-2Al-0.5Zn-0.5Mn-1Ce(MM) alloy, the max Vickers-hardness increases from 63HV to 76HV after rolling, The UTS and YTS of rolled state alloys are 290MPa and 202MPa at room temperature, respectively, and the elongation after anneal treatment is 14%. Extruded alloy with higher tensile strength and elongation, but not rolling yield strength alloy state, only 100 MPa at room temperature.Fracture analysis showed that as-cast alloy fracture surface belong to quasi-cleavage fracture, exhibits obvious toughness characteristics. As to as-rolled alloys, its plasticity decreases, and fracture mechanism changes from mix fracture of intergranular fracture and cleavage fracture to quasi-cleavage fracture after annealed. A little number of MM added can increase the strength and plastic deformation of the matrix alloy, the second phase significantly enhanced role.更多还原