【作者】 刘秋香；

【导师】 张瑞军；

【作者基本信息】 燕山大学 ， 材料物理与化学， 2012， 博士

【摘要】 镁合金作为目前工业应用中最轻的金属结构材料，因其比强度、比刚度高，良好的电磁屏蔽性能及易于加工、回收等优点，被誉为“21世纪绿色金属工程材料”，并广泛用于汽车、通讯、电子、航空航天等领域。众所周知，材料的结构决定材料的性能。研究表明：常压条件下，镁具有密排六方结构（hcp），而在高压条件下还可能以体心立方结构（bcc）、双层密排六方结构（dhcp）和面心立方结构（fcc）存在。从滑移系数量分析，面心立方结构的塑性最好，体心立方结构的次之，密排六方结构的塑性较差。因此，借助高压手段改变镁的晶体结构可以实现提高其塑性之目的。本文的研究工作与成果如下：（1）采用第一性原理理论，分别从高压条件下的能量、电子结构、弹性常数等方面，系统研究了镁的密排六方结构、体心立方结构、双层密排六方结构和面心立方结构在压力条件下的稳定性与相变顺序。结果表明，在0²20GPa范围内，镁的hcp、bcc、dhcp和fcc四种结构的体积随压力的增大逐渐减小，且能量均随着压力的增大而逐渐升高。通过比较镁的四种结构的焓差可知，当P=0GPa时，hcp结构最为稳定，其次是dhcp，再次是fcc，bcc最差。当P=65GPa时，发生hcp→bcc相变，此时bcc最为稳定；当P=130GPa时，dhcp结构较hcp结构能量低，因而可能会产生hcp→dhcp结构转变；当P=190GPa时，fcc结构的能量低于hcp结构，因而发生hcp→fcc结构转变。通过分析镁的hcp、bcc、dhcp和fcc结构在零压力及其相变点处的态密度和弹性常数证实了上述相变发生的可能性。（2）应用第一性原理方法研究了元素铝的添加对于纯镁的密排六方和体心立方结构的稳定性、相变顺序和电子结构的影响。结果表明，在0¹00GPa范围内，Mg，Mg-4.17at.%Al和Mg-8.33at.%Al的hcp和bcc结构的体积随压力的增大逐渐减小，而能量随着压力的增大而逐渐升高。通过比较Mg，Mg-4.17at.%Al和Mg-8.33at.%Al的hcp和bcc结构的焓差可知：对于纯Mg而言，当P=60GPa时，发生hcp→bcc相变；而对于Mg-4.17at.%Al和Mg-8.33at.%Al而言，当压力分别为70GPa和85GPa时，会产生hcp→bcc结构的转变。意味着Al原子的添加延缓了镁的hcp→bcc结构转变，且Al原子的添加量越多，其延缓程度越大。（3）对400℃、24h固溶处理后的AZ91D镁合金进行高压处理，分别研究了保压时间和压力大小对高压处理后的AZ91D镁合金组织及其性能的影响。结果表明：①高压处理明显细化了AZ91D镁合金的晶粒组织。当压力为3GPa、时间为60min时，AZ91D镁合金获得较好的细化效果：晶粒细小且均匀，细化后的晶粒尺寸约为常压处理后晶粒尺寸的1/8¹/10。②高压处理后AZ91D镁合金晶粒内部存在大量的孪晶组织，且孪晶数量与压力和保压时间密切相关。③压力不仅对β-Mg₁₇Al₁₂相的析出具有抑制作用，而且还对β-Mg₁₇Al₁₂相的析出位置有明显影响。④高压处理后的AZ91D镁合金的硬度提高。当保压时间为60min、压力为3GPa时，AZ91D镁合金获得硬度最大值为69.033HV。⑤高压处理后的AZ91D镁合金的耐蚀性得到了提高。当压力为1GPa且时间为60min时，AZ91D镁合金获得最高腐蚀电位为-1.053V。更多还原

【Abstract】 There have been increasing uses of magnesium alloys for light-weight structuralmaterials for automobile, mobile communication, electronics and aerospace industries dueto high specific strength and stiffness, excellent electromagnetic shielding effectivenessand easy recycling capability. Magnesium alloy is being famed for “21century greenmaterials”. It is well known that microstructure plays a critical role in determining theproperties of materials. The results show that the hexagonal-close-packed （hcp） ofmagnesium is stable at the atmospheric pressure, however, magnesium has other structuresunder high pressure, such as body-centered cubic （bcc）, double hexagonal-close-packed（dhcp） and faced-centered cubic （fcc）. The plasticity of the fcc structure is the best, the bccstructure takes the second place, the hcp structure is the poorest, by analysis of the numberof slip systems. Therefore, the change of the structure of magnesium by high pressure canimprove the plasticity of magnesium. The main research works and results are listed asfollows.（1） The structural stability and phase transitions of hcp, bcc, dhcp and fcc structuresof magnesium under high pressure have been studied systematically by using the firstprinciples methods. Meanwhile, the structural stability of magnesium has been studied bythe energy, electronic structure and elastic constants. When the applied pressure is0²20GPa, the volume of four structures of magnesium decreases but the energy of the fourstructures increases with the pressure increasing. By comparing of enthalpy differences forthe bcc, dhcp and fcc structures with the hcp structure, it can be seen that when thepressure is0GPa, the hcp structure of magnesium should be the most stable structure, thebcc structure takes the second place, the third is the fcc structure, the bcc is the last one.When the pressure is up to65GPa, the bcc structure becomes more stable and thetransformation from hcp structure to bcc structure may occur. When the pressure is130GPa, the enthalpy for the dhcp structure of magnesium becomes less than that for the hcpstructure. Therefore, the dhcp structure should be more stable relative to the hcp structureunder such high pressure. So the phase transition between the hcp and dhcp structure ofmagnesium may happen. Similarly, the lower enthalpy for the fcc structure of magnesium at190GPa should also be considered as a more stable structure in comparison to the hcpstructure. Therefore, the hcp structure may change to fcc structure. The calculated resultson the density of states and elastic constants for the four structures of magnesium under0GPa and various phase transformation pressures further evidence the possibility of theabove phase transformation.（2） Under high pressure, the effect of added Al atoms on the structural stability, phasetransition sequence and electronic structure of hcp and bcc structures of magnesium hasinvestigated systematically by using first principles methods.The results show that, when the pressure range is0¹00GPa, the volume for hcp andbcc structures of Mg, Mg-4.17at.%Al and Mg-8.33at.%Al decreases, but the enthalpyfor the three Mg-based materials increases. For Mg, when the pressure is about60GPa,the enthalpy of bcc structure is lower than the hcp structure, indicating that the bccstructure of Mg becomes more stable, and the hcp→bcc transition may take place. Thephase transition pressure under which the hcp→bcc transition may occur for Mg-4.17at.%Al and Mg-8.33at.%Al is about70GPa and85GPa, respectively. It can be concludedthat, as the added Al atoms increase, the phase transition pressure under which thehcp→bcc transition may take place becomes higher, implying that the hcp→bcc transitionof Mg under pressures will be more difficult with increasing Al addition.（3） AZ91D alloys treated by solid solution at673K for24h are aged under highpressure. The effect of holding pressure times and pressure on the microstructures andproperties of AZ91D magnesium alloys are investigated respectively.①High pressure can refine the grain significantly. When the pressure is3GPa andholding pressure time is60min, AZ91D magnesium alloy obtains superior refining effect:the grain size is about1/8¹/10of grains size under ambient pressure treatment.②In the case of holding pressure treatment under high pressure, there occurs a lot oftwining in the grain of AZ91D magnesium alloy, and the number of twining is dependentupon the holding pressure time and the applied pressure.③High pressure inhibits the precipitation of the β-Mg₁₇Al₁₂phase during holdingpressure treatment and affects the precipitated position of the β-Mg₁₇Al₁₂phase.④Holding pressure treatment under high pressure for AZ91D magnesium alloy leads to higher micro-hardness. When the pressure is3GPa and holding pressure time is60min, the micro-hardness reaches the maximum value （69.033HV）.⑤The corrosion resistance of AZ91D magnesium alloys is improved throughholding pressure treatment under high pressure. When the pressure is1GPa and agingtime is60min, the highest corrosion potential is-1.053V.更多还原