【作者】 石章智；

【导师】 张文征；

【作者基本信息】 清华大学 ， 材料科学与工程， 2011， 博士

【摘要】 本文以相变晶体学为主要指导原则设计了Mg-7.5Sn-2.2Mn（wt.%）合金（TM72），基于对现有Mg合金中析出相晶体学特征的系统调研提出了2条晶体学判据，推测在Mg-Sn-Mn合金中可能有沿Mg的[0001]_α方向择优生长的β-Mn析出相，这种形貌的析出相有利于阻碍位错的基面滑移。对TM72合金时效组织的透射电镜（TEM）研究证实β-Mn是沿[0001]_α生长的棒状相。基于对Mg合金成分的系统调研，在Mg-Sn-Zn-Mn合金中加入合金元素Ag，制备了一种新合金，成分是Mg-7Sn-3Zn-1Ag-0.2Mn（TZQM7310）。该合金的时效强化效果明显优于不含Ag的TZM730合金。TZQM7310合金在200C时效，早期析出Mg₅₁Zn₂₀相，该相在时效峰值转变为MgZn₂相，过时效阶段析出Mg₅₄Ag₁₇相。Ag的加入改变了合金的时效析出序列，促进了析出相的形核和弥散分布，从而提高了合金的性能。使用TEM对TM72，TZM730，TZQM7310和Mg-6Zn-5Al（wt.%）合金的时效组织进行了系统研究，精确测量了14种新的OR的析出相的晶体学形貌，其中包括Mg2Sn析出相的7种新的OR，MgZn₂析出相的1种新的OR，β-Mn析出相的1种OR，Mg₅₄Ag₁₇析出相的2种OR和τ-Mg₃₂（Al, Zn）₄₉析出相的3种OR。其中，首次对Mg合金中β-Mn、Mg₅₄Ag₁₇和τ-Mg₃₂（Al, Zn）₄₉析出相的晶体学形貌（包括它们与Mg基体的OR和它们的刻面取向）进行了研究。使用g理论和奇异面理论解释了本文观察到的14种新OR中的9种OR对应的析出相的晶体学形貌。在对OR10和OR11型Mg2Sn析出相的研究工作中，推导了刻面满足g//g//g条件时，两相OR的一般解，以及两相中立方相的理想晶格常数解。在对近OR1型Mg2Sn析出相的研究工作中，发展了一种用二次衍射较精确测量OR的小偏角的方法。本文所研究的Mg合金中的析出相的晶格常数与Mg的晶格常数差别明显，它们与Mg构成的相变系统是大晶格错配系统。前人报导的OR5型Mg2Sn析出相和本文观察到的第一类τ相皆具有晶体学对称的无理主刻面。该种晶体学形貌特征用已有的模型无法解释，本文发展了二次强制重位点阵（二次CCSL）模型，合理解释了上述析出相的形貌。更多还原

【Abstract】 Guided by crystallography of phase transformation, a novel Mg-Sn-based alloywith high Mn content, i.e., Mg-7.5Sn-2.2Mn （wt.%） alloy （TM72）, has been designed.According to a comprehensive survey, two crystallographic criteria are proposed topredict precipitates that are likely to grow along [0001]_αof Mg matrix. β-Mn issupposed to be a potential candidate. The existence of rod-like β-Mn precipitates along[0001]_αis confirmed by transmission electron microscopy （TEM） investigation on theaged alloy.Another novel Mg-Sn-based alloy has been designed by micro-alloying Ag inMg-Sn-Zn-Mn alloy. The resulted alloy is Mg-7Sn-3Zn-1Ag-0.2Mn （TZQM7310）. Theage-hardening effect of TZQM7310is much higher than that of TZM730. As toTZQM7310alloy aged at200C, Mg₅₁Zn₂₀forms at the early aging stage andtransforms to MgZn₂at the aging peak. In the overaging period, metastable Mg₅₄Ag₁₇phase is identified. The element Ag changes the precipitation sequence of the alloy andresults a more uniform dispersion density of the precipitates.Crystallography of the precipitates in aged TM72, TZM730, TZQM7310andMg-6Zn-5Al （ZA65） alloys has been quantitatively studied by TEM. Fourteen neworientation relationships （ORs） of different precipitates in Mg alloy have been identified,including7,1,1,2and3ORs for precipitates of Mg2Sn, MgZn₂, β-Mn, Mg₅₄Ag₁₇andτ-Mg₃₂（Al, Zn）₄₉, respectively. The ORs of the latter three precipitates are firstinvestigated. Approach based on analysis of g vectors and singularity of interfacialstructures has been employed to interpret interfacial facets of the precipitatescorresponding to9of the14new ORs. The interpretation of the facets of OR10andOR11type Mg2Sn precipitates lead to general solutions of OR and lattice parameters ofcubic phase under the condition of their major facets normal to g//g//g. Moreover, amore precise method has been developed to measure the angular deviation in the OR,and it was used to identify near OR1type Mg2Sn precipitates.A new concept of secondary constrained coincidence site lattice （secondary CCSL）and the associated model have been proposed. Its applications provided consistentinterpretation of the morphology and the ORs of the Type τ precipitates and the OR5 type Mg2Sn precipitates.更多还原