【作者】 朱素琴；

【导师】 严红革；

【作者基本信息】 湖南大学 ， 材料加工工程， 2012， 博士

【摘要】 镁合金板材具有密度低、比强度和比刚度高、导热性好、阻尼减震性和电磁屏蔽性能高、易回收利用等一系列优点,在交通运输车辆、航空航天、3C电子产品等领域具有极其重要的应用价值和广阔的应用前景。特别是超细晶镁合金板材,不仅强度高,而且塑性、韧性也非常好,属于高强高塑性轻质化材料,并可表现出低温或高应变速率超塑性,因此其应用价值更大。国内外虽然在超细晶镁合金制备技术及相关基础理论方面开展了一些探索性研究,但一直未能取得突破。本论文在前期探索性实验研究结果的基础上,提出采用中高应变速率轧制工艺制备超细晶镁合金板材的方法；并通过提高轧制应变速率来强化孪生和动态再结晶在轧制变形过程中的作用,以此来改善镁合金的塑性变形能力；同时,通过孪晶内发生的强烈动态再结晶来细化晶粒组织,获得超细晶板材。论文选取常见的轧制牌号AZ31和高合金含量的ZK60为研究对象,探索了采用中高应变速率轧制镁合金的工艺可行性并探讨了镁合金的变形原理；通过对两种合金进行热轧模拟实验,研究了镁合金在轧制变形过程中的流变行为及微观组织演变规律。深入研究了轧制应变速率和轧制温度对镁合金板材微观组织、晶粒取向和力学性能的影响,重点探讨了变形过程中的晶粒细化机制。另外还对中高应变速率轧制过程中镁合金中合金相的应变诱导析出行为进行了研究,并重点探讨这种析出行为对轧制变形行为的影响。论文得到主要结论如下：(1)镁合金在中高应变速率下的轧制成形性能是由其变形机制决定的。镁合金在中高应变速率轧制变形过程中的变形机制与传统的低应变速率小变形量轧制过程的有很大不同。常规轧制工艺中,镁合金的变形机制以位错滑移为主导,而在中高应变速率下轧制过程中,孪生机制占主导,同时发生强烈的动态再结晶,孪生和和动态再结晶为变形过程中先后出现的两个最重要的变形行为。孪生、动态再结晶和裂纹的萌生和扩展均为轧制过程中消耗变形储能的方式,孪生和动态再结晶的形核和裂纹萌生之间形成了竞争关系。当应变速率足够大时,孪生和动态再结晶的形核速率快到足以抑制裂纹萌生,轧制过程得以顺利进行,这是镁合金中高应变速率轧制过程具备可行性的最根本原因。(2)无论在低应变速率(0.01s-1-0.1s-1)还是在中高应变速率(10s-1-50s-1)下,ZK60和AZ31合金在轧制变形过程中均表现出类似的硬化—软化—硬化流变行为。但在中高应变速率下,流变曲线的变化更为急剧,且变形前期流变曲线出现了明显的应力波动现象。在低应变速率变形条件下,前期应变硬化过程是由位错增殖所主导,随后的软化过程则由晶界动态再结晶的形成和扩展所主导,变形后期的再次应变硬化则是由动态再结晶所引起的晶粒细化作用所造成的。在中高应变速率变形条件下,前期变形由高密度的变形孪生所主导,随后应力的急剧下降是由孪晶上高密度的动态再结晶形核所引起的,而随后再次的应变硬化也是由于细晶强化所造成的。(3)采用中高应变速率单道次轧制工艺成功地制备出了高强度高塑性的超细晶板材。在温度为300℃,道次应变量80%,平均轧制应变速率为9.1s-1时轧制所得ZK60板材的平均晶粒尺寸为0.5μm,屈服强度为307MPa,抗拉强度达371MPa,断后伸长率达28%；AZ31板材的平均晶粒尺寸为3μ.m,屈服强度为238MPa,抗拉强度达317MPa,断后伸长率为29%。而常规轧制AZ31板材的平均晶粒尺寸为18μm,屈服强度222MPa,抗拉强度262MPa,延伸率为17%。中高应变速率轧制工艺是一种短流程高效率制备超细晶镁合金板材的有效方法。(4)采用中高应变速率轧制工艺可以拓宽镁合金的轧制温度范围。本研究在9.1s-1的应变速率下,在250℃-400℃温度范围内对两种镁合金进行轧制,均获得了综合力学性能优异的细晶镁合金板材,突破了常规轧制中加工窗口狭窄的局限性。随着应变速率的增加,板材的晶粒得到进一步细化,ZK60合金在16.8s-1的应变速率下轧制时晶粒被细化到了纳米晶。(5)镁合金在中高应变速率轧制过程中通过强烈的动态再结晶形成均匀超细晶组织。在此过程中,材料内发生了一系列的组织转变。在轧制变形初期,合金中形成了高密度的变形孪晶片层,并且孪晶片层中产生了高密度的位错,位错随即发生重排和动态回复；通过动态回复,孪晶片层内先后形成多边形结构、胞状结构和亚晶,而这些亚晶则成为连续动态再结晶形核核心。核心通过旋转和长大,最终形成细小均匀低位错密度的大角动态再结晶晶粒。在进一步的变形中,这些连续动态再结晶的晶界上会发生非连续动态再结晶形核,从而抑制晶粒长大。(6)与常规轧制板材相比,采用中高应变速率轧制的板材虽仍具有{0002}基面织构,但其强度明显减弱,特别是当应变速率增加时,织构密度出现双峰分布特征,这和孪晶类型和密度的增加,以及动态再结晶形核密度的增加有关。(7)在中高应变速率轧制变形过程中,ZK60和AZ31合金中发生了纳米第二相的应变诱导析出,其中ZK60中的析出相分布更密集,其成分也更复杂。第二相析出过程和轧制变形行为之间相互影响,并最终影响板材的微观组织。第二相析出主要影响变形过程中位错的产生、运动和重排,动态再结晶形核和晶粒的长大过程。第二相分布密度不同是造成ZK60板材和AZ31板材动态再结晶晶粒尺寸和再结晶比例差别的主要原因。更多还原

【Abstract】 Magnesium alloy sheets have many outstanding advantages, such as low density, high specific strength and stiffness, good thermal conductivity, superior damping and electromagnetic shielding properties, and good recycling potential, which make them of great application importance and potentials in the fields such as transport vehicles, aerospace,3C products. Especially, the unltafine grained magnesium sheets present excellent strength and high ductility and toughness except for the lightness; in addition, they show low temperature and high strain rate superplasticity. Therefore, the unltafine grained magnesium sheets have wider application. Some domestic and overseas researches have done some exploratory studies on the fabrication of ultrafine grained magnesium sheets and the associated fundamental theories. However, they have been unable to achieve a breakthrough.Based on the results of the preliminary exploratory experiements of this study, the present dissertation explores a novel methods called medium-high strain rate rolling to produce ultrafine grained magnesium sheets. In order to improve the formability of magnesium alloys, the effects of twinning and dynamic recrystallization (DRX) on the rolling process are strengthened by increasing the rolling strain rate. The ultrafine grained microstucuture is obtained via the extensive DRX in twins.Two series of magnesium alloys were chose in this study, which are AZ31and ZK60. AZ31is the most common magnesium alloy for rolling and ZK60is a high-alloy content sery. The feasibility of the MHSRR process and the deformation principle of magnesium alloys were explored. The thermal rolling simulation experiments were conducted upon the two alloys, and the flow behaviors and micro structural evolution of magnesium alloys during rolling process were studied. The effects of rolling strain rate and rolling temperature on the microstructure, grain orientation and mechanical properties of the magnesium sheets were discussed and the grain refinement mechanisms were focused. The strain-induced precipitation of magnesium alloys during MHSRR was observed and the influence of the precipitation on the rolling deformation behaviors was concentrated.The main conclusions are listed below (1) The feasibility of MHSRR of magnesium alloys depends on the deformation mechanism. The deformation mechanism during MHSRR are of great difference from that of conventional rolling with low strain rate and small strain. During conventional rolling, dislocation sliding dominates the deformation. However, during MHSRR, twinning dominates the deformation and extensive DRX occurs. Twinning and DRX are the most important deformation behaviors successively taking place during the deformation. Since twinning, DRX and the initiation and propagation of cracks are all the ways consuming strain energy during rolling, twinning and DRX compete with cracking. When the deformation strain rate is high enough, the initiation of twinning and DRX is rapid enough to prohibit cracking and guarantee the rolling process, which is the basic reason for the feasibility of MHSRR on magnesium alloys.(2) During the thermal simulation experiments, both at low strain rates (0.01-0.1s"1) and medium-high strain rates (10-50s-1), ZK60and AZ31alloys showed similar overall flow behaviour, i.e., hardening-softening-hardening. But at medium-high strain rates, the change of the flow curves was sharper and the flow curves fluctuated at the early stages. At low strain rate, the strain hardening at early stages was dominated by dislocation multiplication, and the subsequent softening was controlled by the nucleation and extension of DRX at grain boundaries, and the hardening at the later stages was induced by the grain refinement caused by DRX. At medium-high strain rate, the early-stage hardening was dominated by high-density deformation twinning, the subsequent sharp stress drop was due to the high-density DRX nucleation in twins, and the later-stage hardening was also resulted from the grain refinement.(3) The ultrafine grained magnesium sheets with high strength and high ductility were successfully produced by on-pass MHSRR. Rolled at300℃, with a strain-per-pass80%and a strain rate of9.1s-1, the average grain size of the ZK60sheet was0.5μm. The yield strength (YS) was307MPa and the ultimate tensile strength (UTS) reached371MPa and the elongation to failure was28%; the average grain size of AZ31sheet was3μm with the YS of238MPa, UTS317of MPa and elongation to failure of29%. However, the average grain size of the conventionally rolled AZ31sheet was18μm. The YS and UTS were222MPa and262MPa, respectively, and the elongation to failure was17%. The MHSRR is an efficient method to produce ultrafine grained magnesium sheets with a high efficiency and shortened process.(4) The rolling temperature range of magnesium were broadened by MHSRR. Rolled at250-400℃with a strain rate of9.1s-1, the fine grained sheets with superior mechanical properties were obtained in both the two alloys, which breaks the limitation of the narrow process window for conventional rolling. With the increase in strain rate, the grains were futher refined. In the ZK60sheets produced at16.8s-1, grains were refined to nano size.(5) MHSRR produced homogeneous ultrafine grained microstructure via extensive DRX. During MHSRR, a series of microstructural evolution occurred in the materials. First, very high-density deformation twinning took place and high-intensity dislocations formed in these twin lamellae. Subsequently, the dislocations rearranged and dynamic recovery (DRV) occured. The DRV successively formed polygons, cells and subgrains in the twin lamellae. The subgrains turned into the nuclei of continuous DRX. These nuclei transformed to the homogeneous fine DRX grains with low dislocation density and high misorientation by rotation and growth. With further deformation, the nucleation of discontinuous DRX took place at the grain boundaries of the continuous DRX grains, which prohibits the grain growth.(6) Compared with conventionally rolled sheets, the sheets fabricated by MHSRR still presented{0002}basal texture, but the intensity reduced remarkably. Especially with the increase in strain rate, the texture intensity of the sheets split to two peaks. The weakening of the basal texture results from the increase in both of the types and intensity of twins and the increase in the density of DRX nucleation.(7) During MHSRR, the strain-induced precipitation for the nano-sized second phase occurred in both of the ZK60and AZ31alloys. However, the distribution of the precipitates in ZK60alloy was much denser than AZ31alloy, and the composition of the precipitates was more complex. The precipitation processs and various deformation behaviors have mutual effects with each other, and thus influenced the final microstructure of the sheets. The main effects of precipitation were on the dislocation formation, movement and rearrangement, the DRX nucleation and grain growth. The difference of DRX grain size and DRX ratio between ZK60and AZ31sheets was attributed to the difference of the distribution density of the precipitates.更多还原