【作者】 李廷取；

【导师】 刘勇兵；

【作者基本信息】 吉林大学 ， 材料学， 2011， 博士

【摘要】 镁锂合金是最轻的金属结构材料,它具有密度小、比强度和比刚度高、减震性好、易切削加工等优点,被认为是航天器和汽车轻量化的最佳材料。当镁合金使用温度超过120℃时,由于合金中相的软化导致高温力学性能大幅度下降,这极大地阻碍了镁锂合金大规模地应用。最近几年,国内外对于耐热镁合金的开发给予了高度重视。目前,主要通过降低合金中A1元素的含量,引入高熔点的第二相,减少或抑制低熔点相的形成。已研究和开发的合金体系主要包括：Mg-Al-Sr、Mg-Al-RE和Mg-Al-Ca-Sr系等。因此稀土、铜和锡元素同样可以应用在镁锂合金中,提高合金的耐高温性能。此外,Mg-Zn-Al系合金由于成本相对较低,是一种有发展前途的高温抗蠕变镁合金。该体系合金的力学性能和热裂倾向对成分非常敏感,而且组织中通常存在大量网状的共晶相,导致合金的塑性较差。除少数合金可用于装饰外,关于该体系商业化应用的报道非常少。本论文把RE、Cu和Sn作为合金强化元素添加到Mg-Li-Al-Zn体系合金中,对于已设计优化出的LAZ532-2RE、LAZ532-2Cu和LAZ532-1Sn合金分别进行了热挤压变形加工,研究了合金热挤压变形后的组织和力学性能。挤压态LAZ532-2RE合金在150℃以下,高温拉伸变形机制是孪晶和位错协调的变形机制,当温度升高到200℃左右时,晶粒发生软化,发生的是位错和晶界滑移协调的变形机制,但温度继续升高时,固溶在a-Mg中的Li脱溶析出形成p相,在高温条件下软化的β相变形能力强,它与α相协调发生较大的变形,此时的高温拉伸变形机制是以晶界滑移机制为主要机制,并伴随着脱溶析出的第二相协调变形机制。挤压态LAZ532-2RE合金,在398K、423K和448K温度蠕变测试时,应力指数n值分别为4.25、4.98和6.23,蠕变激活能Qc值在60MPa、80MPa和100MPa的应力条件时分别为104kJ/mol、118kJ/mol和134kJ/mol,其对应的蠕变机制,应力指数在4.25～4.98之间时,蠕变机制是位错攀移型蠕变。当n大于6时,属于受非基面位错运动控制的蠕变,稀土相在高温蠕变过程中阻碍了位错运动,提高了该合金的高温蠕变性能。挤压态LAZ532-2Cu合金在150℃以下时仍然具有较高的强度,当温度达到200℃时强度下降很多,但是在应变速率1×10-2时延伸率高达44%。挤压态LAZ532-2Cu合金,在125℃～175℃温度范围,应力在60MPa-100MPa范围蠕变测试时,应力指数n值分别为3.72、4.8和6.1,蠕变激活能Qc值在60MPa、80MPa和100MPa的应力条件时分别为94.8kJ/mol、123.9kJ/mol和128kJ/mol,其对应的蠕变机制,在125℃～150℃之间时,蠕变机制是位错攀移型蠕变。当温度达到175℃时,属于受非基面位错运动控制的蠕变,铜化合物相在高温蠕变过程中阻碍了位错运动,该合金在蠕变过程中发生了时效强化作用,使得挤压态LAZ532-2Cu合金的蠕变性能与挤压态LAZ532-2RE合金非常接近。挤压态LAZ532-1Sn合金在150℃以下时仍然具有较高的强度,当温度达到200℃时强度下降很多,应变速率1×10-2时延伸率达到75%。挤压态LAZ532-1Sn合金,在125℃～175℃温度范围,应力在60MPa～100MPa范围蠕变测试时,应力指数n值分别为2.5、3.7和5.8,蠕变激活能Qc值在60MPa、80MPa和100MPa的应力条件时分别为104kJ/mol、118kJ/mol和135kJ/mol,其对应的蠕变机制,在应力指数n≈2,蠕变激活能在100kJ/mol左右,为晶界滑移型蠕变。当温度达到150℃时,当应力指数n≈3～4时,为位错滑移型蠕变,当温度继续升高,应力指数n≈6时,此时蠕变激活能Qc=135kJ/mol,为受非基面位错运动控制的蠕变。挤压态LAZ532-2RE合金拉伸曲线中出现了两种塑性失稳现象：第一种小锯齿形波动的失稳现象是由固溶原子与位错相助作用引起的,即是“动态应变时效”机制；第二种大锯齿形波动的失稳现象是由切变变形孪晶诱发的,这种稳态塑性失稳现象的机制是由变形孪晶所造成的“孪晶稳态塑性失稳”机制。挤压态LAZ532-2RE合金常温拉伸出现的异常应变速率敏感现象可以解释为,正应变速率敏感是应变硬化作用引起,负的应变速率敏感是稳态塑性失稳导致的。锂元素的加入降低了镁合金中镁的晶格常数c值,根据X射线步进式扫描计算结果得出LAZ532-2RE合金中a-Mg的晶轴比c/a值从1.624降低到了1.6074,这会导致该合金在常温拉伸时产生较多的变形孪晶,从而大幅度的提高了该合金的延伸率。挤压态LAZ532-2RE合金拉伸变形时产生的切变变形孪晶主要是｛1012｝和｛1011｝孪晶。更多还原

【Abstract】 Mg-Li alloys, as the lightest metallic structural materials, have great potential application in spacecraft and automotive industries due to their low density, high specific strength and specific stiffness, good damping characteristics and excellent machinability etc. However, because of the rapid decrease of strength and poor creep resistance, the application of these alloys is restricted at the temperature higher than 120℃. The deterioration of high temperature mechanical properties in the Mg-Li alloys is attributed to the softening of discontinuous phase. So far, many efforts have been dedicated to improve the high temperature mechanical properties of Mg-Al based alloy, accordingly a series of commercial alloys have been explored, such as Mg-Al-Sn, Mg-Al-RE and Mg-Al-Ca-Sr systems. The aims of these efforts are decrease of Al content and produce the high melting point intermetallics. So rare earth, copper and tin can also be used in Mg-Li alloys, improve alloy high temperature resistant performance. Due to the disadvantage of Mg-Al alloy, the Mg-Zn-Al (ZA) systems alloy with Zn content is proposed as a low-cost, which were considered a promising high temperature creep resistant magnesium alloys. The ZA systems alloy exhibits low ductility due to the formation of a large amount of network phase at the grain boundaries, moreover the alloy is prone to hot tearing, and the die castability is sensitive to the alloy composition. Until now, except for using as decorative material, there is still limited commercial application of this alloy system.The optimized LAZ532-2RE、LAZ532-2Cu and LAZ532-1Sn alloys were extruded at 553K. The diameter of specimens after extrusion was changed from 55mm to 13mm. The microstructure and mechanical properties of the alloys were investigated.The extruded LAZ532-2RE alloy at 150℃, high temperature tensile deformation mechanism is the coordination of twins and dislocation deformation mechanism. When the temperature rises to about 200℃, the grain softening, and the deformation mechanism is dislocations and grain boundary sliding. With temperature increasing, the softening (3 phase precipitates from the a-Mg solid solution, which occurred more in harmony with a large deformation. The high temperature tensile deformation mechanism at this time is the grain boundary sliding mechanism, and accompanied by the precipitation of second phase precipitation. The extruded LAZ532-2RE alloy was performed on creep tests at 398K,423K and 448K, The stress exponent of n varies from 4.25 to 6.23, and the activation energy varies from 104 to 134kJ/mol. There is a transition between dislocations climb dominated creep mechanism and dislocation creep controlled by non-basal planes slip. The stress exponent is between 4.25-4.98, the creep mechanism is dislocation climb creep. When n is greater than 6, are subject to non-basal dislocation motion-controlled creep. At high temperature creep test, the rare earth phase hindered the dislocation movement, improved high temperature creep properties of the alloy.At the temperature 150℃, the extruded LAZ532-2Cu alloy still has higher strength, when the temperature reaches 200℃, the strength decreased a lot, but the elongation up to 44% at the strain rate 1×10-2. Under the temperature of 125℃,150℃and 175℃, the stress of 60Mpa,80Mpa and 100Mpa, the creep test of extruded LAZ532-2Cu alloy was carried out. The stress exponent n values are 3.72,4.8 and 6.1, the creep activation energy Qc values are 94.8kJ/mol,123.9kJ/mol and 128kJ/mol respectively. At the temperature range of 125℃～150℃, the creep mechanism is dislocation climb creep mechanism. When the temperature reaches 175℃, the non-basal plane dislocation motion is dominated creep mechanism. During the creep test, the copper compounds hindered dislocation movement, the alloy occurred strengthening effect of aging. The extruded LAZ532-2Cu alloy creep properties are very close to the extruded LAZ532-2RE alloy.At temperature 150℃, the extruded LAZ532-1Sn alloy has higher strength. When the temperature reaches 200℃, the strength decreased markly, the elongation up to 75% at the strain rate 1×10-2. At temperature range of 125℃～175℃, stress range of 60MPa～100Mpa, the stress exponent n values are 2.5,3.7 and 5.8, the creep activation energy Qc value are 104kJ/mol,118kJ/mol and 135kJ/mol respectively. At the stress exponent n≈2, the creep activation energy around 100kJ/mol, the corresponding creep mechanism is the grain boundary sliding creep. When the temperature reaches 150℃, the stress exponent n≈3～4, the creep mechanism is dislocation climb creep. When the temperature continues to rise, the stress exponent n≈6, the creep activation energy Qc=135kJ/mol, the creep mechanism is the non-basal plane dislocation creep motion control.The extruded LAZ532-2RE alloy, there were two plastic instability phenomena in the tensile curve. The first instability of small fluctuations is the effect of atoms on dislocation, which is "dynamic strain aging" mechanism. The second largest zigzag instability phenomenon is caused by fluctuations in the shear-induced deformation twins. This phenomenon of plastic instability is caused by the deformation twins of the "steady-state twin plastic instability" mechanism. The extruded LAZ532-2RE alloy at room temperature tensile appears abnormal phenomenon can be interpreted as positive strain rate sensitivity is caused by strain hardening, the negative strain rate sensitivity is due to the twin plastic instability.Li elements decrease the lattice constant c value of magnesium. According to X-ray step scanning results, the LAZ532-2RE alloy of a-Mg grain axial ratio c/a values decreased from 1.624 to 1.6074. Therefore, the alloy has a better plastic at room temperature, produce more deformation twins, which greatly enhanced the elongation of the alloy. The{1012} and {1011} deformation twins were observed from the extruded LAZ532-2RE alloy.更多还原