【作者】 尹冬弟；

【导师】 王渠东； CarlJ.Boehlert；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2013， 博士

【摘要】 由于当今对汽车轻量化的强烈需求,研发能够在高温下长期稳定服役的耐热镁合金,使其可以应用于动力系统(powertrain),已经成为现阶段镁合金研究领域的热点和难点。而研发可应用于动力系统核心部件(如发动机活塞等)的高性能耐热镁合金(耐热温度≥300℃)是其中的最前沿。目前国内外针对上述应用温度条件的镁合金的研究还较少,尤其是对稀土镁合金的高温塑性变形、强化以及断裂机制的系统研究则更为欠缺。Mg-11Y-5Gd-2Zn-0.5Zr(WGZ1152,wt.％)合金是由笔者所在课题组新近开发的高性能重力铸造耐热镁合金,前期的研究结果表明其具有应用于300℃及以上温度的潜力。因此,本文以WGZ1152合金为对象,采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、电子背散射衍射(EBSD).图像分析(image analysis)以及位错滑移迹线分析(slip trace analysis)等手段,通过显微硬度、室高温变速率瞬态拉伸实验、高温拉伸蠕变实验、原位(in-situ)瞬态拉伸和原位拉伸蠕变实验,系统研究了该合金：1)在25-400℃(0.33～0.75Tm,Tm为熔点)和1E-4～1E-2s-1应变速率条件下的瞬态拉伸变形和断裂行为；2)在250～340℃(0.58～0.68Tm)不口30～140MPa (0.1-0.6R0.2,R0.2为300℃屈服强度)应力条件下的拉伸蠕变变形和断裂行为。在此基础上探讨了该合金的高温变形、强化以及断裂机制。此外,还成功地进行了镁合金发动机活塞的工业试制和发动机台架试验。时效态WGZ1152-T6合金的室高温力学性能研究表明：1)在25-400℃范围内,WGZ1152-T6合金的抗拉及屈服强度均要显著优于商用耐热镁合金WE54-T6和活塞用耐热铝合金AC8A-T6.其在300℃(0.64Tm)的抗拉和屈服强度分别高于250MPa和225MPa,为室温强度的86%和95%。2)在300℃相同应力条件下,其最小蠕变速率比WE54-T6低两个数量级,比AC8A-T6低一个数量级以上,与应用温度最高的HZ32-T5耐热镁合金相当(由于Th的放射性,此合金已逐渐被淘汰)。WGZ1152-T6合金高温瞬态拉伸实验的研究结果如下：1)变速率拉伸实验结果显示：在250-400℃和1E-4-1E-2s-1应变速率范围内,T6合金的变形本构方程可用双曲正弦函数ε=A[sinh(ασ)]nexp(-Q/RT)描述,应力指数n=7.7±0.7,激活能Q=274±10kJ/mol,n和Q值表明位错交滑移(dislocation cross-slip)为速率控制机制,变形后样品表面出现的波浪形的滑移线(通常由交滑移所致),进一步证实了上述观点。2)通过原位SEM、EBSD和位错滑移迹线分析,定量研究了T6合金在瞬态拉伸过程中位错滑移的激活规律,结果表明：滑移模式从室温下的基面<a>滑移(100%)主导,先是变为250℃下的基面<a>(73%)和棱柱面<a>滑移(16%)共同主导,再逐渐转变为350℃下的基面<a>(67%)和角锥面<c+a>滑移(25%)共同主导；在中等温度(200～250℃),棱柱面<a>滑移在高应变量时更为活跃,而角锥面<c+a>滑移则在高温(≥300℃)和高应变量时更为活跃；上述结果在200-250℃与Mg单晶的临界剪切应力随温度的变化规律高度吻合,但当温度高于250℃时,则与Barnett利用全约束Taylor模型对AZ31合金的模拟结果在一定程度上吻合。3)通过原位SEM研究了T6合金在瞬态拉伸过程中的断裂机制,结果表明：在室温下,试样为穿晶断裂(transgranular fracture)(40%)和沿晶断裂(intergranular fracture)(60%)混合模式,粗化的滑移带是穿晶断裂的主要裂纹萌生位置；在200～350℃,沿晶断裂变为主要断裂模式,明显的裂纹出现于变形的中后期,这些裂纹优先起源于与应力垂直的晶界处或者晶界第二相与α-Mg基体的界面处。铸态、固溶T4态和时效T6态WGZ1152合金拉伸蠕变变形行为的研究结果如下(T=250-325℃,σ=50～140MPa):1)第三阶段主导蠕变(extended tertiary creep)是此合金蠕变的主要特征,这与一些镍基工程合金类似,析出相β’和β的粗化是导致第三阶段主导蠕变的重要原因之一。2)在300℃,较低应力条件下(σ<50MPa),铸态、T4态和T6态合金的最小蠕变速率没有显著区别；而在较高应力条件下(σ≥50MPa), T6态合金的最小蠕变速率略低于铸态合金,而T4态合金的最小蠕变速率最高。3)三种状态的WGZ1152合金的应力指数n介于4.4～6.0之间,这一数值接近于5表明位错蠕变(dislocation process creep)是主要的蠕变机制；平均蠕变激活能Q介于221～266kJ/mol之间,远高于纯Mg的自扩散激活能(135kJ/mol),高的蠕变激活能与非基面滑移和交滑移密切相关,并且交滑移很可能是蠕变的速率控制机制；表面观察结果(暗示交滑移被激活)和滑移迹线分析结果(证实12-25%的非基面滑移被激活)进一步证明了上述观点。4)通过原位SEM、EBSD和位错滑移迹线分析,定量研究了T6合金在拉伸蠕变过程中位错滑移的激活规律,结果表明：在低温高应力下(T=250℃,σ=120MPa),基面<a>滑移占主导(88%),非基面滑移被激活,包括9%的棱柱面<a>和3%的角锥面<c+a>滑移。基面滑移先于非基面滑移出现,随着蠕变时间的增加,非基面滑移的比例逐渐增加；在高温低应力条件下(T=340℃,σ=75MPa),则变为基面<a>(75%)和角锥面<c+a>滑移(16%)共同主导,并且在蠕变的早期阶段,即发现较高比例的非基面滑移。分析表明WGZ1152合金的强化机制包括：1)晶内：垂直于基面、沿三个棱柱面呈三角分布的盘片状β’和β析出相可以有效地阻碍基面位错滑移,而LPSO相则可以有效地阻碍位错的攀移、交滑移和非基面滑移。2)晶界：高硬度(比基体高92～112%)、高体积分数(16～24%)和高热稳定性的晶界X相和共晶相Mg24(GdYZn)5可以有效地钉扎和强化晶界。通过原位SEM研究了T6合金在拉伸蠕变过程中的断裂机制,结果表明：1)在所有测试条件下(T=250-340℃,σ=50～120MPa),蠕变断裂方式均为沿晶断裂,明显的晶界裂纹和蠕变空洞在蠕变中后期(0.4～0.65tr)出现,并且优先起源于与应力垂直的晶界处或者晶界第二相与α-Mg基体的界面处。2)在低温高应力下(T=250℃,σ=120MPa),晶界滑动是裂纹萌生及扩展的主要方式,裂纹边缘平整。3)在高温低应力条件下(T=280-340℃,σ=50～75MPa),孤立的蠕变空洞的长大和合并是晶界微裂纹萌生的主要方式,在微裂纹形成之后,晶界滑移在裂纹扩展过程中起到了重要作用,裂纹边缘呈锯齿状。4)空洞平均直径D与蠕变速率ε满足经验关系D=k·εa,蠕变空洞的长大速率与蠕变速率呈正相关关系,上述关系暗示蠕变空洞的长大机制很可能符合受约束扩散长大模型。在T=250～325℃和σ=50～140MPa下,三种状态的WGZ1152合金的蠕变损伤容限λ位于1.2～2.5之间,最小蠕变速率及断裂时间符合Monkman-Grant关系,表明蠕变空洞和微裂纹在蠕变断裂中会起到重要作用,这与上述原位观察的结果相吻合。本研究进一步完善了镁合金高温力学性能数据库,加深了对复杂体系镁合金高温变形、强化及断裂机制的理解,为新型高性能耐热镁合金的开发和应用提供了理论和实践基础。更多还原

【Abstract】 Due to the strong demand for weight-reduction in automotive industry, research and development of creep-resistant Mg alloys, which can be long-time well served for powertrain applications at elevated temperatures, has progressed considerably in the last decade. Particularly, research and development of the high-performance creep-resistant Mg alloys for the key components of powertrain applications (≥300℃), such as engine piston, are at the forefront. However, research on those Mg alloys, especially for systematic research on the elevated-temperature deformation, strengthening and fracture mechanisms in Mg alloys containing rare-earth elements is limited.Mg-11Y-5Gd-2Zn-0.5Zr (WGZ1152, wt.%) is a gravity-casting high-performance creep-resistant Mg alloy developed by our group recently. Previous work showed that this alloy exhibited potential for elevated-temperature applications (≥300℃). Thus, the present work focused on this alloy. The tensile behavior at25～400℃(0.33-0.75Tm, Tm is the melting point) and strain rate ranges of1E-4-1E-2s-1, as well as the tensile-creep behavior at250～340℃(0.58～0.68Tm) and stress ranges of30～140MPa (0.1-0.6Ro2,R0.2is the yield stress at300℃) were investigated. The important characterization methods and techniques included scanning electron microscope (SEM), transmission electron microscope (TEM), electron backscatter diffraction (EBSD), image analysis, slip trace analysis, and in-situ SEM. Based on above results, the elevated-temperature deformation, strengthening and fracture mechanisms were discussed. What’s more, the industry trials of Mg alloy piston by gravity casting as well as the engine bench test were performed successfully.The room-and elevated-temperature properties of the peak-aged WGZ1152-T6alloy were investigated, and the results showed:1) the tensile strength and yield strength of the WGZ1152-T6alloy were considerably superior to those of WE54-T6(the most successful commercial heat-resistant Mg alloy) and AC8A-T6(the most widely used commercial Al alloy for engine piston) at25～400℃. At300℃(0.64Tm), the tensile strength and yield strength of the T6alloy were above250MPa and225MPa, respectively, which maintained86%and95%of those for room temperature.2) At300℃and the same stress, the minimum creep rate of the WGZ1152-T6alloy was almost two orders of magnitude lower than that for WE54-T6, and was more than one order of magnitude lower than that for AC8A-T6, and was comparable to that of HZ32-T5(the structure Mg alloy has the highest service temperature, but it being phased out because of radioactivity).The results of elevated-temperature tensile deformation and fracture behavior of WGZ1152-T6alloy are as follows:1) The flow behavior of WGZ1152-T6alloy was investigated at250～400℃and at strain rate ranges of1E-4～1E-2s-1, and the results showed:the constitution equation could be described by ε=A[sinh(ασ)]nexp(-Q/RT). The stress exponent n=7.7±0.7and the activation energy of deformation Q=274±10kJ/mol. The values of n and Q indicated that dislocation cross-slip was the rate-controlling mechanism. The observed wavy slip traces, which suggested cross-slip was active, supported the above viewpoint.2) The activities of slip modes during tensile deformation for the T6alloy were investigated quantitatively by in-situ SEM, slip trace analysis, and EBSD. The results showed:the dominate slip modes transited from basal<a> slip (100%) to basal<a> slip (73%) combined with prismatic<a> slip (16%) from25℃to250℃. As the temperature further increased up to350℃, the combination of basal<a> slip (67%) and pyramidal <c+a> slip (25%) became the dominate slip modes; the prismatic<a> slip was more active at higher strains for moderate temperatures (200～250℃), while the pyramidal<c+a> slip was more active at higher strains and temperatures; the above results were consistent with temperature dependence of the critical resolved shear stress (CRSS) of Mg single crystal at200～250℃, but when the temperature was above250℃, they were consistent with the simulation results for AZ31alloy used the full-constraint Taylor model by Barnett to a certain extent.3) The fracture mechanisms during tensile deformation for the T6alloy were investigated by in-situ SEM, and the results showed:the specimen fractured by both transgranular cracking (40%) and intergranular cracking (60%) at25℃; the coarsened slip band was important for the transgranular cracking nucleation; at200～350℃, the dominant fracture mode became intergranular cracking. The onset of obvious cracks was from the middle-to late-deformation stage. The intergranular cracking nucleation site tended to be located at grain boundary which was perpendicular to the load direction and the interface between the a-Mg matrix and the large intermetallic grain boundary phase.The results of tensile-creep deformation behavior of the as-cast, solution treated T4, and peak-aged T6alloys are as follows (T=250-325℃,σ=50～140MPa):1) The alloy exhibited an extended tertiary creep stage, which was similar to Ni-Cr-base superalloys. Such creep characteristic was believed to be associated with the β’and β precipitate coarsening. 2) For300℃condition, at lower stresses (σ<50MPa), there was not a significant difference in the minimum creep rates among the T6, T4and as-cast alloys. At higher stresses (σ≥50MPa), the T6alloy exhibited lower minimum creep rates than the as-cast alloy, while the T4alloy exhibited the highest creep rates.3) The creep stress exponent values were4.4～6.0implying that dislocation process creep was the creep mechanism. The measured average activation energy (221～266kJ/mol) was significantly greater than that for lattice self-diffusion of Mg (135kJ/mol). This was considered to be a result of the activation of non-basal slip and cross-slip, and probably cross-slip was the rate-controlling mechanism. This was consistent with the slip traces analysis which confirmed that12～25%non-basal slip was active and the deformation observations which suggested that cross-slip became more active at higher temperatures.4) The activities of slip modes during tensile-creep for the T6alloy were investigated quantitatively by in-situ SEM, slip trace analysis, and EBSD. The results showed:at low temperature and high stress (T=250℃,σ=120MPa), the dominate slip modes were basal<a> slip (88%), and non-basal slip was active including prismatic<a> slip (9%) and pyramidal<c+a> slip (3%). The basal<a> slip was observed before non-basal slip during creep deformation, and the relative contribution of basal<a> slip decreased with increasing creep time; at high temperature and low stress (T=340℃, σ=75MPa), the dominate slip modes became basal<a> slip (75%) combined with pyramidal<c+a> slip (16%), and high amount of non-basal slip were found in the early stage of creep.The important grain-interior strengthening mechanisms were:the prismatic orientated, plate-shaped, and densely-distributed β’ and β precipitates were most effective obstacles for basal slip, while the long period stacking ordered (LPSO) phases can suppress the non-basal slip, dislocation climb and cross-slip. The grain-boundary X phase and eutectic Mg24(GdYZn)5with high hardness (92～112%higher than matrix), high volume fraction (16～24%), and high thermal stability can pin the grain boundaries and strengthen the boundaries effectively.The fracture mechanisms during tensile-creep for the T6alloy were investigated by in-situ SEM, and the results showed:1) At all the conditions tested (T=250～340℃,σ=50～120MPa), intergranular fracture was the dominant creep fracture mechanism for all the tested conditions. The onset of obvious cracks and creep cavities were from the middle-to late-creep stage (0.4～0.65tr), and they tended to nucleate at grain boundary which was perpendicular to the load direction and the interface between the α-Mg matrix and the large intermetallic grain boundary phase. 2) At low temperature and high stress (T=250℃,σ=120MPa), the crack nucleated and propagated by grain-boundary sliding, and edge of the crack was smooth.3) At high temperature and low stress (T=280～340℃,σ=50～75MPa), the coalescence and growth of isolated cavities and their linkage formed the microcrack, and then grain-boundary sliding played an important role in the propagation of the microcrack. Edge of this kind of crack was serrate.4) The mean cavity diameter D and creep rate followed the empirical relationship D=k·εa, and the growth rate of creep cavity was proportional to creep rate;5) these findings indicated that the growth of creep cavity might be consistent with the constrained diffusional cavity growth mechanism.At T=250～325℃and a=50-140MPa, the creep damage tolerance parameter λ ranged between1.2and2.5. The minimum creep rate and fracture followed the original Monkman-Grant relationship. The λ values and Monkman-Grant relationship indicated that creep cavity and intergranular crack played important role in creep fracture, and these were consistent with the in-situ observations.The present work can contribute to a better understanding for the elevated-temperature deformation, strengthening and fracture mechanisms in complex Mg alloy, and provide both theoretical and practical fundamentals for further research and development of high-performance creep-resistant Mg alloys.更多还原