Mg-RE基稀土镁合金组织、性能与腐蚀机理研究
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摘要
稀土镁合金具有重量轻、比强度高、屏蔽性能好、抗冲击减震性优良,以及较优良的耐热性能,得到了较广泛地研究和应用,成为研究的热点,但其耐腐蚀性较差,工程化应用范围受到了限制,因此提高镁合金的抗腐蚀性能十分重要。本文采用电感耦合等离子直读光谱仪、光学显微镜、带能谱分析的扫描电子显微镜、X射线衍射仪、力学性能试验机、电化学测试系统等分析手段,分别研究了稀土元素Y对Mg-Y系合金在铸态、均匀化处理态、挤压态和时效态的显微组织、力学性能、腐蚀速率、腐蚀形貌、开路电位、极化曲线、阻抗频谱等的变化规律。
铸态合金晶粒尺寸随Y元素含量的增加由1100μm减小到约180μm；均匀化处理后,Mg-(0.25,2.5,5和8)Y中的第二相基本上全部溶入基体中,而Mg-15Y则残存有大量的Mg24Y5相。随着Y元素含量的增加,Mg-Y合金的晶粒明显细化,Mg-0.25Y与Mg-2.5Y的微观组织为单一的α-Mg相,Mg-5Y、Mg-8Y与Mg-15Y的微观组织为α-Mg相和沿晶界呈骨骼状分布的共晶组织Mg24Y5相；随着Y含量的增加,不同状态下合金的抗拉强度增加,仲长率下降。
确定了合金的均匀化处理工艺：(1)525℃均匀化处理时：Mg-0.25Y、Mg-2.5Y、 Mg-5Y、Mg-8Y、Mg-15Y的均匀化时间为：2h、2h、4h、10h和28h；(2)535℃均匀化处理时：Mg-0.25Y、Mg-2.5Y、Mg-5Y、Mg-8Y、Mg-15Y的均匀化时间为：2h、2h、2h、6h和24h,以及Mg-(0.25,2.5,5,8和15)Y合金在250℃峰时效时的时间：4h、6h、10h、12h和16h,此处理工艺可作为制定均匀化或时效处理工艺的依据。
不同状态下的Mg-(0.25,2.5,5,8和15)Y合金在3.5%NaCl溶液中的动电位极化曲线显示五种镁合金的点蚀点电位都比它们各自的自腐蚀电位要负,说明五种镁合金在NaCl溶液中均能自发的产生局部腐蚀。随着Y含量的提高,不同状态下合金的自腐蚀电位和开路电位呈现出先正向移动后负向移动的规律,且以2.5%Y合金的自腐蚀电位和开路电位最正。五种合金极化曲线的形状类似,阳极电流都上升得比较快,且都不对称,阳极分支斜率大于阴极分支斜率,表明阴极过程在腐蚀反应中发挥更重要作用。
不同状态下Mg-(0.25,2.5,5,8和15)Y合金的阻抗频谱都由一个高频容抗环和一个中低频容抗环组成,随着Y含量的增大,高频容抗弧的半径明显增大,当Y含量达到2.5%时达到最大,当Y含量继续增加时,高频容抗弧半径减小,并且Mg-(2.5,5,8和15)Y合金大于Mg-0.25Y合金高频容抗弧半径,这说明合金的耐蚀性随着Y含量的增加先提高后降低。根据电化学反应过程及阻抗分析理论,得出不同状态下Mg-(0.25,2.5,5,8和15)Y合金阻抗频谱曲线的等效电路：一个溶液电阻Rs,一个腐蚀膜阻抗RF，一个电荷传递电阻Rt、一个电感L和一个常相位角元件CPE。通过阻抗谱分析显示Mg-(0.25,2.5,5,8和15)Y合金的腐蚀阻抗Rt(Mg-0.25Y) 不同状态下Mg-(025,2.5,5,8和15)Y合金的腐蚀过样分为三个阶段：腐蚀初期、腐蚀中期、腐蚀后期,合金的腐蚀失重速率递减的顺序为Mg-0.25Y 根据不同状态下Mg-(0.25,2.5,5,8和15)Y合金在3.5%NaCl溶液中的腐蚀行为和腐蚀形貌,建立了Mg-(0.25,2.5,5,8和15)Y合金腐蚀的腐蚀动力学模型模型、腐蚀机理模型和点蚀形成模型：合金中的α-Mg基体与β-Mg24Y5第二相粒子或富含Y的共晶体存在电势差,在溶液中产生电偶腐蚀,β-Mg24Y5第二相稀土粒子或富含Y共晶体是诱发点蚀的根源：Mg-(0.25和2.5)Y合金腐蚀机理模型是均匀腐蚀,Mg-(8和15)Y合金腐蚀机理模型是点蚀,Mg-5Y均匀化处理后腐蚀机制由点蚀转变为均匀腐蚀,在其他状态仍为点蚀。
Mg-Y合金在挤压过程中承受较大的三向压应力,发生动态再结晶,晶粒明显细化,晶粒排列除宏观的带状排列外,微观上有择优取向,而垂直于挤压方向的晶粒则任意排列。纵断面的耐蚀性能优于横断面,性能优于横断面,挤压比提高可以提高合金的耐蚀性能。
Mg-(0.25.2.5,5,8和15)Y合金的主要腐蚀产物是Mg(OH)2,以及少量的Mg2(OH)3Cl.4H20。
Mg-(0.25,2.5,5,8和15)Y合金的耐蚀性能递增顺序为：铸态<热处理态<时效态<挤压态。
Magnesium containing RE alloy with the excellent characteristics of light weight, high strength, shielding performance, excellent impact shock absorption and heat resistance, et al, has been extensively studied and applied. However, its poor corrosion resistance limits the large-scale application in the engineering field, so how to improve the corrosion resistance of magnesium alloy is very important and has drawn great attention at home and abroad. In this paper, inductively coupled plasma direct-reading spectrometer, optical microscopy, scanning electron microscopy with energy spectrum, X-ray diffraction, mechanical testing machine, electrochemical test systems and other analytical tools were adopted to research on the changing rules of Y on the microstructures, mechanical properties, corrosion rates, corrosion morphologies, the open-circuit potentials, polarization curves, et al for the Mg-Y alloys in as-cast, homogenization state, squeezed state and aged state.
The grain size of as-cast Mg-Y alloy reduced to about180μm from1100μm with increasing Y content. After the homogenization treatment, the second phases of Mg-(0.25,2.5,5and8)Y alloys dissolved into substrate substantially, but there were a large number of Mg24Y5phase of Mg-15Y remained on the grain boundaries. The grain size significantly refined with the increase in the content of the Y elements. The microstructures of Mg-0.2Y and Mg-2.5Y were consisted of the single a-Mg phases, and the microstructures of Mg-5Y, Mg-8Y and Mg-15Y had the a-Mg phases and the bone-shaped eutecticsβ-Mg24Y5phases along the grain boundaries. With increasing Y content, the tensile strength and elongation of Mg-Y alloy with different states increased and decreased respectively.
The optimum heat treatment processes were determine through series of experiments:the treatment homogenized at525℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were2h,2h,4h,8h and28h respectively, the treatment homogenized at535℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were2h,2h,2h,6h and24h respectively, the peaking treatment aged at250℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were4h,6h,10h,12h and16h. The treatment processes could act as the basis for drafting the homogenization and aging treatment process.
According to the potentiodynamic polarization curves, the pitting point potentials were positive than the self-corrosion potentials of Mg-(0.25,2.5.5.8and15)Y of different states in3.5%NaCl solution, which stated that the alloys could generate localized corrosion spontaneously. With the increase of the Y content, the corrosion potential and open-circuit potentials of different states alloys moved positively first, then moved positively, and corrosion potential and open-circuit potentials of Mg-2.5Y alloy had the highest values.'The polarization curves of different alloys in the different states had the similar shape, and the anode current rise faster than the cathodic current. The anode and cathode branches of polarization curves were asymmetric. The anode branch slopes were greater than that of the cathode branch, indicating that the cathodic process played a more important role in the corrosion reaction.
The impedance spectroscopies of Mg-(0.25,2.5,5,8and15) Y alloy in different states were consisted of a high-frequency capacitance and a low-frequency capacitance, with the increase of the Y content, the radius of high-frequency capacitive arc increased apparently, and the radius reached the maximum value when the Y content was up to2.5%. but the radius of high-frequency capacitive arc decreased when Y content continued to increase above2.5%, which indicated that the corrosion resistance of the alloy increased first increased and then decreased with increasing of Y content. According to the theory of electrochemical reaction process and impedance analysis, the equivalent circuit of impedance spectrum curve of Mg-(0.25,2.5,5.8and15) Y alloys in different states was consisted of a solution resistance Rs, a corrosion film resistance Rf, a The charge transfer resistance Rt, an inductor L and a constant phase angle element CPE. The analysis of impedance spectroscopy analysis showed that the corrosion resistance decreased progressively as following: Rt (Mg-0.25y)
铸态合金晶粒尺寸随Y元素含量的增加由1100μm减小到约180μm；均匀化处理后,Mg-(0.25,2.5,5和8)Y中的第二相基本上全部溶入基体中,而Mg-15Y则残存有大量的Mg24Y5相。随着Y元素含量的增加,Mg-Y合金的晶粒明显细化,Mg-0.25Y与Mg-2.5Y的微观组织为单一的α-Mg相,Mg-5Y、Mg-8Y与Mg-15Y的微观组织为α-Mg相和沿晶界呈骨骼状分布的共晶组织Mg24Y5相；随着Y含量的增加,不同状态下合金的抗拉强度增加,仲长率下降。
确定了合金的均匀化处理工艺：(1)525℃均匀化处理时：Mg-0.25Y、Mg-2.5Y、 Mg-5Y、Mg-8Y、Mg-15Y的均匀化时间为：2h、2h、4h、10h和28h；(2)535℃均匀化处理时：Mg-0.25Y、Mg-2.5Y、Mg-5Y、Mg-8Y、Mg-15Y的均匀化时间为：2h、2h、2h、6h和24h,以及Mg-(0.25,2.5,5,8和15)Y合金在250℃峰时效时的时间：4h、6h、10h、12h和16h,此处理工艺可作为制定均匀化或时效处理工艺的依据。
不同状态下的Mg-(0.25,2.5,5,8和15)Y合金在3.5%NaCl溶液中的动电位极化曲线显示五种镁合金的点蚀点电位都比它们各自的自腐蚀电位要负,说明五种镁合金在NaCl溶液中均能自发的产生局部腐蚀。随着Y含量的提高,不同状态下合金的自腐蚀电位和开路电位呈现出先正向移动后负向移动的规律,且以2.5%Y合金的自腐蚀电位和开路电位最正。五种合金极化曲线的形状类似,阳极电流都上升得比较快,且都不对称,阳极分支斜率大于阴极分支斜率,表明阴极过程在腐蚀反应中发挥更重要作用。
不同状态下Mg-(0.25,2.5,5,8和15)Y合金的阻抗频谱都由一个高频容抗环和一个中低频容抗环组成,随着Y含量的增大,高频容抗弧的半径明显增大,当Y含量达到2.5%时达到最大,当Y含量继续增加时,高频容抗弧半径减小,并且Mg-(2.5,5,8和15)Y合金大于Mg-0.25Y合金高频容抗弧半径,这说明合金的耐蚀性随着Y含量的增加先提高后降低。根据电化学反应过程及阻抗分析理论,得出不同状态下Mg-(0.25,2.5,5,8和15)Y合金阻抗频谱曲线的等效电路：一个溶液电阻Rs,一个腐蚀膜阻抗RF，一个电荷传递电阻Rt、一个电感L和一个常相位角元件CPE。通过阻抗谱分析显示Mg-(0.25,2.5,5,8和15)Y合金的腐蚀阻抗Rt(Mg-0.25Y)
Mg-Y合金在挤压过程中承受较大的三向压应力,发生动态再结晶,晶粒明显细化,晶粒排列除宏观的带状排列外,微观上有择优取向,而垂直于挤压方向的晶粒则任意排列。纵断面的耐蚀性能优于横断面,性能优于横断面,挤压比提高可以提高合金的耐蚀性能。
Mg-(0.25.2.5,5,8和15)Y合金的主要腐蚀产物是Mg(OH)2,以及少量的Mg2(OH)3Cl.4H20。
Mg-(0.25,2.5,5,8和15)Y合金的耐蚀性能递增顺序为：铸态<热处理态<时效态<挤压态。
Magnesium containing RE alloy with the excellent characteristics of light weight, high strength, shielding performance, excellent impact shock absorption and heat resistance, et al, has been extensively studied and applied. However, its poor corrosion resistance limits the large-scale application in the engineering field, so how to improve the corrosion resistance of magnesium alloy is very important and has drawn great attention at home and abroad. In this paper, inductively coupled plasma direct-reading spectrometer, optical microscopy, scanning electron microscopy with energy spectrum, X-ray diffraction, mechanical testing machine, electrochemical test systems and other analytical tools were adopted to research on the changing rules of Y on the microstructures, mechanical properties, corrosion rates, corrosion morphologies, the open-circuit potentials, polarization curves, et al for the Mg-Y alloys in as-cast, homogenization state, squeezed state and aged state.
The grain size of as-cast Mg-Y alloy reduced to about180μm from1100μm with increasing Y content. After the homogenization treatment, the second phases of Mg-(0.25,2.5,5and8)Y alloys dissolved into substrate substantially, but there were a large number of Mg24Y5phase of Mg-15Y remained on the grain boundaries. The grain size significantly refined with the increase in the content of the Y elements. The microstructures of Mg-0.2Y and Mg-2.5Y were consisted of the single a-Mg phases, and the microstructures of Mg-5Y, Mg-8Y and Mg-15Y had the a-Mg phases and the bone-shaped eutecticsβ-Mg24Y5phases along the grain boundaries. With increasing Y content, the tensile strength and elongation of Mg-Y alloy with different states increased and decreased respectively.
The optimum heat treatment processes were determine through series of experiments:the treatment homogenized at525℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were2h,2h,4h,8h and28h respectively, the treatment homogenized at535℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were2h,2h,2h,6h and24h respectively, the peaking treatment aged at250℃of Mg-0.25Y Mg-2.5Y, Mg-5Y, Mg-8Y, Mg-15Y were4h,6h,10h,12h and16h. The treatment processes could act as the basis for drafting the homogenization and aging treatment process.
According to the potentiodynamic polarization curves, the pitting point potentials were positive than the self-corrosion potentials of Mg-(0.25,2.5.5.8and15)Y of different states in3.5%NaCl solution, which stated that the alloys could generate localized corrosion spontaneously. With the increase of the Y content, the corrosion potential and open-circuit potentials of different states alloys moved positively first, then moved positively, and corrosion potential and open-circuit potentials of Mg-2.5Y alloy had the highest values.'The polarization curves of different alloys in the different states had the similar shape, and the anode current rise faster than the cathodic current. The anode and cathode branches of polarization curves were asymmetric. The anode branch slopes were greater than that of the cathode branch, indicating that the cathodic process played a more important role in the corrosion reaction.
The impedance spectroscopies of Mg-(0.25,2.5,5,8and15) Y alloy in different states were consisted of a high-frequency capacitance and a low-frequency capacitance, with the increase of the Y content, the radius of high-frequency capacitive arc increased apparently, and the radius reached the maximum value when the Y content was up to2.5%. but the radius of high-frequency capacitive arc decreased when Y content continued to increase above2.5%, which indicated that the corrosion resistance of the alloy increased first increased and then decreased with increasing of Y content. According to the theory of electrochemical reaction process and impedance analysis, the equivalent circuit of impedance spectrum curve of Mg-(0.25,2.5,5.8and15) Y alloys in different states was consisted of a solution resistance Rs, a corrosion film resistance Rf, a The charge transfer resistance Rt, an inductor L and a constant phase angle element CPE. The analysis of impedance spectroscopy analysis showed that the corrosion resistance decreased progressively as following: Rt (Mg-0.25y)