Mg-Gd-Y-Zn系合金的微观组织与高温性能研究
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摘要
Mg-Gd系耐热合金可以作为新型高强、轻质结构材料应用于较高温度场合，在高温下具有优良的性能，是一种极具发展潜力的新型镁合金材料，但是对其的研究依然很不充分。本文以三种典型的Mg-Gd合金为对象，研究了它们的热处理工艺，微观组织，力学性能，透射电镜原位拉伸时的断裂特征，高温下的氧化行为等方面的内容，考察了高温循环应变下疲劳微观裂纹的萌生和扩展，合金在高温和循环应力双重作用下微观组织的演变规律等，并探讨了合金在高温下具有良好抗氧化能力的微观机制。
（1）Mg-1.0Gd-0.50Zn合金
Mg-1.0Gd-0.50Zn合金存在明显的时效硬化效应，在峰值状态下的强化相主要是平行于α-Mg的_{0001}基面γ″。合金在175℃下的拉伸实验中，γ″不能对孪生切变形成阻碍，{1012}变形孪晶可以切过γ″相，导致γ″相在孪晶界面处发生偏转。为了考查γ″相对裂纹扩展的影响，使用透射电镜原位拉伸装置观察了裂纹对γ″相的作用。实验证实只有几个纳米厚度的γ″相不能对裂纹的扩展产生明显的阻碍作用。
γ″相平行于α-Mg的{0001}基面，其可以作为一个标志物来确定透射电镜原位拉伸实验中局部的拉伸应力、裂纹扩展方向与晶体位向之间的关系。在透射电镜原位拉伸实验中，当裂纹扩展垂直于γ″相（即裂纹沿着[0001]α-Mg方向扩展）时，由于Schmid因子接近于0，基面位错滑移难以激活。随着应变的增加，裂纹尖端局部应力超过了Mg原子之间的结合力，镁的棱柱面沿着[0001]的方向一层一层的撕裂开，从而使裂纹尖端的弹性能转变为表面自由能而得以释放。当裂纹沿着非[0001]方向扩展时，位错发射导致裂纹前方的材料逐渐变薄。当位错发射耗尽后，在裂纹的尖端和位错塞积区之间会出现无位错区（DFZ：dislocation-free zone），随着应变的增加，在无位错区生成只有几个纳米大小的微观孔洞。最终，主裂纹合并了萌生于无位错区的微观裂纹，从而向前扩展。这一实验结果说明，对于延性和脆性两种断裂模式，裂纹前端的材料具有完全不同的减薄机制。
（2）Mg-2.0Gd-0.60Y合金
Mg-2.0Gd-0.60Y合金具有明显的时效硬化效应，时效至峰值时合金的强化相主要是β′。时效态合金在室温下具有较高的抗拉强度和屈服强度。
Mg-2.0Gd-0.60Y合金在300℃下的低循环应变实验中，循环软化是其主要特征。Mg-2.0Gd-0.60Y合金在低的总应变幅下，晶界处往往出现小的孔洞，这可能是合金在高温下长期加载导致的疲劳－蠕变效应所致。在高应变幅下，疲劳小裂纹往往起源于镁基体的滑移带，并且沿晶界扩展。
Mg-2.0Gd-0.60Y合金在较低的循环应变幅下，在疲劳断口附近可以观察到大量的变形孪晶，位于镁基体里面的析出相比较粗大，呈板条状，密度较低，而存在于孪晶里面的析出相则要细小的多，呈圆盘状，体积分数较高。这可能是孪生切变使沉淀相处于热力学不稳定状态，从而在高温下的持续服役中逐渐转变成为尺寸较小的平衡相。
在合金的高温氧化实验中，合金在高温下发生了选择性氧化，氧化膜主要由(Gd0.18Y1.82)O3组成。结构致密、完整的稀土氧化膜提高了合金在高温下的抗氧化能力。将合金放入箱式炉中于730℃的熔融状态下保持60分钟而不燃烧，说明合金在熔融状态下具有较好的阻燃能力。
（3）Mg-2.1Gd-1.1Y-0.82Zn合金
Mg-2.1Gd-1.1Y-0.82Zn合金在500℃的固溶处理中会生成沿晶界广泛分布的长周期有序相，时效至峰值后，其强化相主要是β′和β1，也可以观察到少量沿α-Mg的{0001}基面分布的γ″相。此合金在室温下的抗拉强度和屈服强度不及Mg-2.0Gd-0.60Y合金，但是随着温度的升高合金的强度下降却较少，在300℃的高温下它的抗拉强度和屈服强度都超过了Mg-2.0Gd-0.60Y合金。
为了考查长周期有序相对裂纹扩展的影响，使用透射电镜原位拉伸技术研究了裂纹和长周期有序相的相互作用。当裂纹扩展至长周期有序相附近时，裂纹尖端倾向于发生钝化。微观裂纹前方的长周期有序相在持续加大的应变中逐渐变薄，最终被裂纹所穿透，长周期有序相的存在并不能完全阻止裂纹的向前扩展，但是会对裂纹的扩展过程产生影响。
Mg-2.1Gd-1.1Y-0.82Zn合金在300℃下的低循环应变实验中，循环软化是其主要特征。疲劳微观裂纹往往萌生于长周期有序相和镁基体的界面处，并且在持续的循环加载中与其他的小裂纹合并，试样表面的裂纹一般沿镁的滑移面扩展。当疲劳裂纹扩展至晶界处存在的体积较大的长周期有序相处时，往往发生钝化或者出现微观孔洞。这说明，晶界处的体积较大的长周期有序相能够影响疲劳裂纹的扩展过程。在高温低周疲劳的早期，合金的主要强化相为β′和β1，随着时间的延长，其主要强化相变为β′，β1，γ″和γ′相。
利用扫描电镜、X射线衍射、电子探针等技术研究了Mg-2.1Gd-1.1Y-0.82Zn合金高温下氧化膜的成分、结构和相组成。Mg-2.1Gd-1.1Y-0.82Zn合金氧化膜的主要相组成为(Gd0.18Y1.82)O3和ZnO等。在高温下生成的致密完整的稀土氧化膜有利于改善合金的耐蚀性。将高温氧化温度升高到730℃，合金在熔融状态下保温2个小时而不燃烧，说明其具有比Mg-2.0Gd-0.60Y合金更好的阻燃效果，在火灾等极端条件下有着更高的安全性。
Mg-Gd based heat-resistant alloys, which show excellent performance at hightemperatures, can be employed as a new type of high-strength, lightweight structuralmaterials in high temperature occasions. It is a potential candidate for theheat-resistant magnesium alloy material, but the research is still far from adequate. Inthis paper, the heat treatment process, microstructure, mechanical properties, tensilefracture characteristics in in-situ transmission electron microscopy (TEM), as well ashigh temperature oxidation behavior were investigated for three typical Mg-Gd alloys.Under the strain-controlled low cycle fatigue experiment at high temperature, both theinitiation and propagation of fatigue microcrack, and the evolution of themicrostructure of the Mg-Gd-Y alloy were explored. In a high-temperature heattreatment, Mg-Gd-Y alloys exhibit preferred antioxidant ability. In order tounderstand the antioxidant, the microstructre and phase composition of scale werestudied using of scanning electron microscopy, electron microprobe, XRD etc.
(1) Mg-1.0Gd-0.50Zn alloy
Mg-1.0Gd-0.50Zn alloy shows notable hardening effect during the artificialaging. At the peak ageing, precipitate γ″is parallel to the {0001}α-Mgbasal plane.Under the tensile test at175°C, the γ″does not impede the the twin shear. Thedeformation twinning pass through the γ″phase, and the γ″phase at the interface ofthe twins deflect about4.2°along the shearing direction.
In order to examine the effect of γ″phase on the crack propagation, the in-situtransmission electron microscopy observations were carried out. The results show thatthe γ″phase, just a few nanometers thickness, can not significantly hinder crackgrowth.
The γ″phase, which is parallel to the basal plane of the α-Mg, can be used as amarker to determine the relationship of local tensile stresses, the crack propagationdirection and crystal orientation. The in-situ TEM strain experiments were carried outto study the fracture behaviors of the alloy. When the crack propagatedperpendicularly to γ″phase (i.e. the crack extent along the [0001]α), the basaldislocations slips were difficult to operation for the Schmid factor was nearly equal tozero. As the local stress at the tip of crack exceeded the cohesion strength, theprismatic planes split layer-by-layer along the [0001]αto release the elastic energy. When the crack extended along the random orientation (non-[0001] direction),dislocations emissions led the foil near the tip of crack to thinning gradually. As thestress increased, the micro-voids initiated in the dislocation-free zone (DFZ). Finally,the main crack merged the void, and then propagated forward. In general, the resultsof in-situ TEM strain experiments show that the material of the crack front exhibt aquite different thinning manner for ductility and brittle fracture.
(2) Mg-2.0Gd-0.60Y alloy
The Mg-2.0Gd-0.60Y alloy show a significant age-hardening during artificialaging at225℃. In peak aging, the precipiate β′phases play a major role instrenthening materials.
The strain controlled low cycle fatigue tests were carried out for aMg-2.0Gd-0.60Y alloy at573K. The cyclic softening responses were observed underthe diverse total strain amplitudes at573K. For high strain amplitudes, the alloyshowed a stronger cyclic softening response than that at low strain amplitudes of0.3%and0.45%. At the lower strain amplitudes, the grain boundaries are considered as theprincipal microcracks resource because of the creep-fatigue cavity inititation at thegrain boundaries. At the higher strain amplitudes, the microcracks, which initiate atthe slip bands, propagate along the grain boundaries.
Under the low cyclic loading, abundant twins appeared near the crack. Densedislocations slip leaves the precipitates in a thermodynamically unstable condition.Then those precipitates disintegrate to form a small equilibrium phase duringuninterruptedly cyclic loading at573K. At the precipitation-free zone, dislocationscells, dislocations nets as well as small angle boundaries appeared extensively. Due tothe dense dislocations pile up at the boundaries, strip-shaped grains of the dynamicrecrystallization distribute along the grain boundaries.
The selective oxidation was detected under the high temperature for theMg-2.0Gd-0.60Y alloy. According to the XRD and the electron probe microanalysis(EMPA), the scales were believed mainly composed of (Gd0.18Y1.82)O3. The dense richrare-earth scale on the surface can reduce the oxygen inward diffusion, and thusimprove the oxidation resistance of the substrate at high temperatures. As the ingot ofalloy is placed in a box furnace at730℃, the alloy show a good flame retardant.
(3) Mg-2.1Gd-1.1Y-0.82Zn alloy
The long-period stacking ordered phase (LPSO) are widely distributed along thegrain boundary after the solution treatment at500°C in the Mg-2.1Gd-1.1Y-0.82Znalloy. At peak aging, the priciapitate β′and β1is believed as the principal strengthening phase. Moreover, a small amount γ″phase, which exists along the α-Mg{0001}, can also be observed. The tensile and yield strength of this alloy at roomtemperature is less than Mg-2.0Gd-0.60Y alloy is, but its strength at300°C exceedMg-2.0Gd-0.60Y alloy.
In order to survey the effect of LPSO on the crack propagation, the in-situtransmission electron microscopy observation was carried out. The experiment resultsindicate that the crack prefer to bluntness near the LPSO. With the tension strainincreaseing, the LPSO is gradually thinning, and eventually penetrated by the crack.
The low cycle fatigue tests were carried out for a Mg-2.1Gd-1.1Y-0.82Zn alloy at573K. The cyclic softening responses were observed under the diverse total strainamplitudes at573K. In general, the fatigue microcracks initiate at the interfacebetween the the long-period ordered phase and the magnesium matrix, and thenpropagate along the slip bands of Mg. When the fatigue crack extends to the LPSOnear grain boundary, either the crack bluntness or a cavity formation are observed.This shows that the massive LPSO can affect the fatigue crack growth process.
The scale of Mg-2.1Gd-1.1Y-0.82Zn alloy was investigated employing scanningelectron microscopy, X-ray diffraction, electron probe microanalysis. The dense oxidefilms are believed to reduce the oxygen inward diffusion, and thus improve theoxidation resistance of the alloy at high temperatures. When the ingot of alloy is set ina box furnace at730℃, the alloy did not burn after melted, which show high safety atfire hazard.
（1）Mg-1.0Gd-0.50Zn合金
Mg-1.0Gd-0.50Zn合金存在明显的时效硬化效应，在峰值状态下的强化相主要是平行于α-Mg的_{0001}基面γ″。合金在175℃下的拉伸实验中，γ″不能对孪生切变形成阻碍，{1012}变形孪晶可以切过γ″相，导致γ″相在孪晶界面处发生偏转。为了考查γ″相对裂纹扩展的影响，使用透射电镜原位拉伸装置观察了裂纹对γ″相的作用。实验证实只有几个纳米厚度的γ″相不能对裂纹的扩展产生明显的阻碍作用。
γ″相平行于α-Mg的{0001}基面，其可以作为一个标志物来确定透射电镜原位拉伸实验中局部的拉伸应力、裂纹扩展方向与晶体位向之间的关系。在透射电镜原位拉伸实验中，当裂纹扩展垂直于γ″相（即裂纹沿着[0001]α-Mg方向扩展）时，由于Schmid因子接近于0，基面位错滑移难以激活。随着应变的增加，裂纹尖端局部应力超过了Mg原子之间的结合力，镁的棱柱面沿着[0001]的方向一层一层的撕裂开，从而使裂纹尖端的弹性能转变为表面自由能而得以释放。当裂纹沿着非[0001]方向扩展时，位错发射导致裂纹前方的材料逐渐变薄。当位错发射耗尽后，在裂纹的尖端和位错塞积区之间会出现无位错区（DFZ：dislocation-free zone），随着应变的增加，在无位错区生成只有几个纳米大小的微观孔洞。最终，主裂纹合并了萌生于无位错区的微观裂纹，从而向前扩展。这一实验结果说明，对于延性和脆性两种断裂模式，裂纹前端的材料具有完全不同的减薄机制。
（2）Mg-2.0Gd-0.60Y合金
Mg-2.0Gd-0.60Y合金具有明显的时效硬化效应，时效至峰值时合金的强化相主要是β′。时效态合金在室温下具有较高的抗拉强度和屈服强度。
Mg-2.0Gd-0.60Y合金在300℃下的低循环应变实验中，循环软化是其主要特征。Mg-2.0Gd-0.60Y合金在低的总应变幅下，晶界处往往出现小的孔洞，这可能是合金在高温下长期加载导致的疲劳－蠕变效应所致。在高应变幅下，疲劳小裂纹往往起源于镁基体的滑移带，并且沿晶界扩展。
Mg-2.0Gd-0.60Y合金在较低的循环应变幅下，在疲劳断口附近可以观察到大量的变形孪晶，位于镁基体里面的析出相比较粗大，呈板条状，密度较低，而存在于孪晶里面的析出相则要细小的多，呈圆盘状，体积分数较高。这可能是孪生切变使沉淀相处于热力学不稳定状态，从而在高温下的持续服役中逐渐转变成为尺寸较小的平衡相。
在合金的高温氧化实验中，合金在高温下发生了选择性氧化，氧化膜主要由(Gd0.18Y1.82)O3组成。结构致密、完整的稀土氧化膜提高了合金在高温下的抗氧化能力。将合金放入箱式炉中于730℃的熔融状态下保持60分钟而不燃烧，说明合金在熔融状态下具有较好的阻燃能力。
（3）Mg-2.1Gd-1.1Y-0.82Zn合金
Mg-2.1Gd-1.1Y-0.82Zn合金在500℃的固溶处理中会生成沿晶界广泛分布的长周期有序相，时效至峰值后，其强化相主要是β′和β1，也可以观察到少量沿α-Mg的{0001}基面分布的γ″相。此合金在室温下的抗拉强度和屈服强度不及Mg-2.0Gd-0.60Y合金，但是随着温度的升高合金的强度下降却较少，在300℃的高温下它的抗拉强度和屈服强度都超过了Mg-2.0Gd-0.60Y合金。
为了考查长周期有序相对裂纹扩展的影响，使用透射电镜原位拉伸技术研究了裂纹和长周期有序相的相互作用。当裂纹扩展至长周期有序相附近时，裂纹尖端倾向于发生钝化。微观裂纹前方的长周期有序相在持续加大的应变中逐渐变薄，最终被裂纹所穿透，长周期有序相的存在并不能完全阻止裂纹的向前扩展，但是会对裂纹的扩展过程产生影响。
Mg-2.1Gd-1.1Y-0.82Zn合金在300℃下的低循环应变实验中，循环软化是其主要特征。疲劳微观裂纹往往萌生于长周期有序相和镁基体的界面处，并且在持续的循环加载中与其他的小裂纹合并，试样表面的裂纹一般沿镁的滑移面扩展。当疲劳裂纹扩展至晶界处存在的体积较大的长周期有序相处时，往往发生钝化或者出现微观孔洞。这说明，晶界处的体积较大的长周期有序相能够影响疲劳裂纹的扩展过程。在高温低周疲劳的早期，合金的主要强化相为β′和β1，随着时间的延长，其主要强化相变为β′，β1，γ″和γ′相。
利用扫描电镜、X射线衍射、电子探针等技术研究了Mg-2.1Gd-1.1Y-0.82Zn合金高温下氧化膜的成分、结构和相组成。Mg-2.1Gd-1.1Y-0.82Zn合金氧化膜的主要相组成为(Gd0.18Y1.82)O3和ZnO等。在高温下生成的致密完整的稀土氧化膜有利于改善合金的耐蚀性。将高温氧化温度升高到730℃，合金在熔融状态下保温2个小时而不燃烧，说明其具有比Mg-2.0Gd-0.60Y合金更好的阻燃效果，在火灾等极端条件下有着更高的安全性。
Mg-Gd based heat-resistant alloys, which show excellent performance at hightemperatures, can be employed as a new type of high-strength, lightweight structuralmaterials in high temperature occasions. It is a potential candidate for theheat-resistant magnesium alloy material, but the research is still far from adequate. Inthis paper, the heat treatment process, microstructure, mechanical properties, tensilefracture characteristics in in-situ transmission electron microscopy (TEM), as well ashigh temperature oxidation behavior were investigated for three typical Mg-Gd alloys.Under the strain-controlled low cycle fatigue experiment at high temperature, both theinitiation and propagation of fatigue microcrack, and the evolution of themicrostructure of the Mg-Gd-Y alloy were explored. In a high-temperature heattreatment, Mg-Gd-Y alloys exhibit preferred antioxidant ability. In order tounderstand the antioxidant, the microstructre and phase composition of scale werestudied using of scanning electron microscopy, electron microprobe, XRD etc.
(1) Mg-1.0Gd-0.50Zn alloy
Mg-1.0Gd-0.50Zn alloy shows notable hardening effect during the artificialaging. At the peak ageing, precipitate γ″is parallel to the {0001}α-Mgbasal plane.Under the tensile test at175°C, the γ″does not impede the the twin shear. Thedeformation twinning pass through the γ″phase, and the γ″phase at the interface ofthe twins deflect about4.2°along the shearing direction.
In order to examine the effect of γ″phase on the crack propagation, the in-situtransmission electron microscopy observations were carried out. The results show thatthe γ″phase, just a few nanometers thickness, can not significantly hinder crackgrowth.
The γ″phase, which is parallel to the basal plane of the α-Mg, can be used as amarker to determine the relationship of local tensile stresses, the crack propagationdirection and crystal orientation. The in-situ TEM strain experiments were carried outto study the fracture behaviors of the alloy. When the crack propagatedperpendicularly to γ″phase (i.e. the crack extent along the [0001]α), the basaldislocations slips were difficult to operation for the Schmid factor was nearly equal tozero. As the local stress at the tip of crack exceeded the cohesion strength, theprismatic planes split layer-by-layer along the [0001]αto release the elastic energy. When the crack extended along the random orientation (non-[0001] direction),dislocations emissions led the foil near the tip of crack to thinning gradually. As thestress increased, the micro-voids initiated in the dislocation-free zone (DFZ). Finally,the main crack merged the void, and then propagated forward. In general, the resultsof in-situ TEM strain experiments show that the material of the crack front exhibt aquite different thinning manner for ductility and brittle fracture.
(2) Mg-2.0Gd-0.60Y alloy
The Mg-2.0Gd-0.60Y alloy show a significant age-hardening during artificialaging at225℃. In peak aging, the precipiate β′phases play a major role instrenthening materials.
The strain controlled low cycle fatigue tests were carried out for aMg-2.0Gd-0.60Y alloy at573K. The cyclic softening responses were observed underthe diverse total strain amplitudes at573K. For high strain amplitudes, the alloyshowed a stronger cyclic softening response than that at low strain amplitudes of0.3%and0.45%. At the lower strain amplitudes, the grain boundaries are considered as theprincipal microcracks resource because of the creep-fatigue cavity inititation at thegrain boundaries. At the higher strain amplitudes, the microcracks, which initiate atthe slip bands, propagate along the grain boundaries.
Under the low cyclic loading, abundant twins appeared near the crack. Densedislocations slip leaves the precipitates in a thermodynamically unstable condition.Then those precipitates disintegrate to form a small equilibrium phase duringuninterruptedly cyclic loading at573K. At the precipitation-free zone, dislocationscells, dislocations nets as well as small angle boundaries appeared extensively. Due tothe dense dislocations pile up at the boundaries, strip-shaped grains of the dynamicrecrystallization distribute along the grain boundaries.
The selective oxidation was detected under the high temperature for theMg-2.0Gd-0.60Y alloy. According to the XRD and the electron probe microanalysis(EMPA), the scales were believed mainly composed of (Gd0.18Y1.82)O3. The dense richrare-earth scale on the surface can reduce the oxygen inward diffusion, and thusimprove the oxidation resistance of the substrate at high temperatures. As the ingot ofalloy is placed in a box furnace at730℃, the alloy show a good flame retardant.
(3) Mg-2.1Gd-1.1Y-0.82Zn alloy
The long-period stacking ordered phase (LPSO) are widely distributed along thegrain boundary after the solution treatment at500°C in the Mg-2.1Gd-1.1Y-0.82Znalloy. At peak aging, the priciapitate β′and β1is believed as the principal strengthening phase. Moreover, a small amount γ″phase, which exists along the α-Mg{0001}, can also be observed. The tensile and yield strength of this alloy at roomtemperature is less than Mg-2.0Gd-0.60Y alloy is, but its strength at300°C exceedMg-2.0Gd-0.60Y alloy.
In order to survey the effect of LPSO on the crack propagation, the in-situtransmission electron microscopy observation was carried out. The experiment resultsindicate that the crack prefer to bluntness near the LPSO. With the tension strainincreaseing, the LPSO is gradually thinning, and eventually penetrated by the crack.
The low cycle fatigue tests were carried out for a Mg-2.1Gd-1.1Y-0.82Zn alloy at573K. The cyclic softening responses were observed under the diverse total strainamplitudes at573K. In general, the fatigue microcracks initiate at the interfacebetween the the long-period ordered phase and the magnesium matrix, and thenpropagate along the slip bands of Mg. When the fatigue crack extends to the LPSOnear grain boundary, either the crack bluntness or a cavity formation are observed.This shows that the massive LPSO can affect the fatigue crack growth process.
The scale of Mg-2.1Gd-1.1Y-0.82Zn alloy was investigated employing scanningelectron microscopy, X-ray diffraction, electron probe microanalysis. The dense oxidefilms are believed to reduce the oxygen inward diffusion, and thus improve theoxidation resistance of the alloy at high temperatures. When the ingot of alloy is set ina box furnace at730℃, the alloy did not burn after melted, which show high safety atfire hazard.
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