摘要
随着航天航空及汽车工业轻量化、节能化和环保化发展的必然趋势,越来越多的镁合金构件用于承载交变载荷并引起疲劳破坏,这就要求对其疲劳性能进行深入研究。研究镁合金的疲劳行为不仅具有理论价值,而且也具有一定的工程实用价值。本课题选用AZ91D合金为母合金,向其中分别加入不同含量的晶粒细化剂Mg-Al-C、Ce,系统地研究了晶粒细化剂对基体合金的微观组织和低周疲劳性能的影响,以期为AZ91D合金的抗疲劳设计和合理使用提供可靠的理论依据。
力学性能结果表明,晶粒细化剂Mg-Al-C和Ce的添加使AZ91D基体合金的屈服强度、抗拉强度、断面收缩率和弹性模量均得到较大提高。当分别加入1.2%Mg-Al-C和0.9%Ce时,合金的综合力学性能达到最好。加入1.2%Mg-Al-C时,AZ91D合金的σs = 140.34MPa,σb=255.63MPa,ψ=8.60%,E=1.42GPa;加入0.9%Ce时,合金的σs = 136.14MPa,σb=233.29MPa,ψ=7.80%,E=1.38 GPa。两者均较未添加晶粒细化剂的AZ91D合金性能(σs = 103.09MPa,σb=152.13 MPa,ψ=1.30%,E=0.85 GPa)有较大提高,其中添加Mg-Al-C的合金力学性能更好。
显微组织观察表明,晶粒细化剂Mg-Al-C和Ce的添加可以有效地细化AZ91D基体合金的晶粒,改善β-Mg17Al12相的大小和分布,且细化效果与晶粒细化剂添加量有关。当晶粒细化剂Mg-Al-C、Ce的添加量由0.3%增加到1.2%时,随着添加量的加大,AZ91D合金中的β相不断断网破碎、细化和弥散化,树枝晶最终也得以消除。当添加1.2%Mg-Al-C时,合金的平均晶粒尺寸由未添加细化剂的原始尺寸162μm分别降到57μm;添加0.9%Ce后合金平均晶粒尺寸也降至64μm;降幅分别为64.8%和60.5%。在AZ91D基体中分别加入1.2%Mg-Al-C和0.9%Ce时,合金的平均晶粒尺寸达到最小化。
低周疲劳实验结果表明:①晶粒细化剂Mg-Al-C和Ce的添加使AZ91D基体合金的低周疲劳寿命得到大幅度提高。当总应变幅?εt/2为0.2%时,AZ91D+1.2%Mg-Al-C合金、AZ91D+0.9%Ce合金的疲劳寿命由未添加细化剂AZ91D合金的疲劳寿命7694周次分别提高到13615周次和12537周次;当?εt/2为1.2%时,AZ91D+1.2%Mg-Al-C合金、AZ91D+0.9%Ce合金的疲劳寿命从AZ91D基体合金的21周次分别升高到113周次和102周次。晶粒细化剂Mg-Al-C的添加比Ce对合金的疲劳寿命影响更大。②在较大应变幅下,镁合金循环滞后回线上分别出现拐点、拉压不对称和锯齿现象。晶粒细化剂的添加大大减缓了上述现象的发生。③镁合金的循环应力响应行为主要呈现循环应变硬化趋势,应变幅的降低和晶粒细化剂的添加使其呈现循环稳定甚至循环软化,其中应变幅的影响占主要地位。④晶粒细化剂的加入大大增强了镁合金的延性,使镁合金的过渡疲劳寿命高于AZ91D基体合金。⑤镁合金的循环应力-应变行为均表现出循环硬化现象,晶粒细化剂的添加增大了循环强度系数、减小了循环应变硬化指数。⑥镁合金疲劳断口上的三个特征区域并不特别明显,出现了多疲劳源现象,且断口随应变幅的增加越来越凹凸不平。⑦镁合金的循环滞后能与疲劳寿命之间呈线性关系,据此可以预测合金的低周疲劳寿命。总之,在低周疲劳实验中,晶粒细化不仅延长了疲劳裂纹的萌生寿命,也延长了疲劳裂纹的扩展寿命,从而延长了整个疲劳寿命。因此,晶粒越细化,镁合金的低周疲劳寿命越长。
With the inevitable development trend of lightweight, energy-saving and environmental protection of aerospace and automotive industry, more and more magnesium alloy components endure cyclic loading, which results in fatigue fracture, therefore, the fatigue properties of magnesium alloys should be studied in depth. Obviously, the investigation concerning fatigue behaviors of magnesium alloys is of both academic and practical significance. In this investigation, the AZ91D alloy is chosen as the master alloy. Through adding different amounts of the grain refiners Mg-Al-C and Ce, the microstructure and low-cycle fatigue behavior of magnesium alloys with various grain refiner have been researched in order to provide a reliable theoretical foundation for both anti-fatigue design and reasonable application of AZ91D alloys.
The results reveal that the mechanical properties of AZ91D alloy such as yield strength,σs, ultimate tensile strength,σb, reduction of area,ψ, and elastic modulus,E, have been improved greatly after adding the grain refiner Mg-Al-C or Ce. As adding 1.2%Mg-Al-C grain refiner, which is the most proper amount of its addition, into the master alloy AZ91D, whose comprehensive mechanical properties, such asσs equals to 140.34MPa,σb to 255.63 MPa,ψto 8.60%,E to1.42 GPa ; when adding 0.9%Ce (the most proper amount of its addition) into AZ91D, whoseσs equals to 136.14MPa,σb to 233.29 MPa,ψto 7.80%,E to1.38 GPa. Each exceeds that of no refiner–added master alloy, whoseσs equals to 103.09MPa,σb to 152.13 MPa,ψto 1.30%,E to 0.85 GPa, and the alloys adding grain refiner are much better.
Observations of microstructure illustrate that the addition of Mg-Al-C and Ce can effectively refine the grain of AZ91D alloy and improve the morphology ofβ-Mg17Al12 with shape, size and distribution. The grain refinement effect is related to the adding amount. As the addition of Mg-Al-C or Ce grain refiner increases from 0.3% to 1.2%, the reticularβ-Mg17Al12 in the matrix of AZ91D alloy splits into dispersive fine pieces and the dendrite has disappeared. The average grain size of alloy are reduced from 162μm to 57μm which is the minimum, when adding 1.2% Mg-Al-C grain refiner; adding 0.9%Ce, the average grain size falls to the minimum 64μm. Amplitude reduction of both reaches to 64.8%and 60.5%, respectively.
Low-cyclic fatigue tests demonstrate following main points.①the low cycle fatigue life of the AZ91D matrix alloy is enhanced to a great extent with the addition of the grain refiner Mg-Al-C or Ce .When total strain amplitude?εt/2 is 0.2%, fatigue life of AZ91D + 1.2%Mg-Al-C and AZ91D + 0.9%Ce alloy increases to 13615 and 12537cycles, respectively; and that the life of no refiner–added AZ91D alloy is 7694 cycles. When ?εt/2 reaches to 1.2%, the fatigue life of AZ91D alloy is improved from 21cycles up to 113 and 102cycles after adding 1.2%Mg-Al-C and 0.9%Ce, respectively. Therefore, the addition of Mg-Al-C is more effective than that of Ce to the fatigue life of AZ91D alloy.②A t the higher total strain amplitudes, theσ-εhysteresis loops of magnesium alloys exhibit some behaviors such as inflexion, asymmetry of tension and compression and serrated flow deformation, all of which can be reduced by the addition of grain refiner.③The cyclic stress response behavior of magnesium alloys mainly exhibit cyclic strain hardening. The reduction of total strain amplitude and addition of the grain refiner make it show cyclic stability or even cyclic softening, where the impact of total strain amplitudes is more important.④The addition of the grain refiner substantially boosts up the ductility of magnesium alloy, so the transitional fatigue life of magnesium alloy adding grain refiner is much higher than that of the AZ91D matrix alloy.⑤T he cyclic stress-strain behavior of magnesium alloy shows cyclic-hardening; Grain refiner addition increases the cycle intensity factor, meanwhile, reduces the cycle strain hardening index.⑥Fatigue fracture surface of all the magnesium alloys have no three clear regions which characterize fatigue damage, and there exsists several fatigue source zones. As the strain amplitude increases, the fracture surface becomes more rough and concavo-convex.⑦A linear relation exists between the cycle hysteresis energy and fatigue life for all the magnesium alloys. Thus, the low-cycle fatigue life of the magnesium alloys can be predicted by utilizing the cycle hysteresis energy as a material parameter.
In conclusion, in the low-cycle fatigue experiments, with the addition of the grain refiner, both the initiation and propagation life of the fatigue crack are prolonged, so the whole fatigue life is extended. As a result, the finer the grains of magnesium alloy, the longer the low cycle fatigue life of it.
引文
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