摘要
镁合金室温下绝对强度低和塑性变形能力较差是阻碍其广泛应用的瓶颈问题。滑移和孪生是镁合金重要的塑性变形机制，由于室温下只能开启有限的滑移系，所以孪生对塑性变形的协调变得十分重要。孪晶结构镁合金中的退火强化是近年来发现的一个有趣的现象，即，通过预变形和中间退火处理可以让固溶原子偏聚在孪晶界上产生钉扎作用，使孪晶在后续变形过程中的长大受到抑制，从而起到强化作用。然而，孪晶界被钉扎后，合金在后续变形过程中的组织演变规律尚缺乏研究。对孪晶结构镁合金组织演变规律的探讨有助于更深入的理解孪生变形机制，为调控孪生行为提供科学依据。
本课题选取了Nd原子百分比为0.03%（1#）和0.18%（2#）的两种Mg-Nd 合金，研究对比中间退火后孪晶在进一步变形过程中的演变规律。对上述合金挤压棒材先进行预压缩得到预变形样品，然后对一部分样品在200℃下退火6h后做再压缩实验，另一部分样品不进行退火直接做再压缩实验。采用电子背散射技术（EBSD）原位观察研究孪晶的演变，统计分析了孪晶的特征参量，包括孪晶的数量和体积分数、孪晶形核和长大的施密特因子，以及孪晶形核和长大对孪生过程的贡献等，并探讨了中间退火和合金元素对孪晶演变的影响规律。
研究结果表明：
中级工程师论文①对于经过中间退火的1#合金孪晶演变过程，再压缩后孪晶的体积分数增加量为14%，其中形核的贡献
为14%，长大的贡献为86%。再压缩过程中孪生变形由孪晶长大主导。孪晶形核的数量分数为24%，孪晶长大的数量分数为38%。孪晶形核的平均施密特因子（SF）为0.35，孪晶长大的平均SF为0.47。孪晶形核的SF主要分布在0.4-0.3范围内，孪晶形核的SF等级主要是R1和R2等级（R1~R6分别对应于孪晶六个变体中最大~最小的等级），孪晶长大的SF主要分布在0.5-0.4范围内，孪晶长大的SF等级主要是R1和R2等级。
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②对于不经中间退火的1#合金孪晶演变过程，再压缩后孪晶的体积分数增加量为22%，其中形核的贡献为59%，长大的贡献为41%。再压缩过程中孪生变形由孪晶形核主导。孪晶形核的数量分数为22%，孪晶长大的数量分数为39%。孪晶形核的平均SF为0.35，孪晶长大的平均SF为0.40。孪晶形核的SF主要分布在0.5-0.4范围内，孪晶形核的SF等级主要是R1和R2等级，孪晶长大的SF 主要分布在0.5-0.4范围内，孪晶长大的SF等级主要是R1和R2等级。
③对于经过中间退火的2#合金孪晶演变过程，再压缩后孪晶的体积分数增加量为3.7%，其中形核的贡献为35%，长大的贡献为65%。再压缩过程中孪生变
形由孪晶长大主导。孪晶形核的数量分数为36%，孪晶长大的数量分数为32%。孪晶形核的平均SF为0.18，孪晶长大的平均SF为0.26。孪晶形核的SF主要分布在0.1-0范围内，孪晶形核的SF等级主要是R1和R3等级，孪晶长大的SF主要分布在0.5-0.4范围内，孪晶长大的SF等级主要是R1和R2等级。
④对于不经中间退火的2#合金孪晶演变过程，再压缩后孪晶的体积分数增加量为4.1%，其中形核的贡献为51%，长大的贡献为49%。再压缩过程中孪生变形由孪晶形核主导。孪晶形核的数量分数为44%，孪晶长大的数量分数为28%。孪晶形核的平均SF为0.19，孪晶长大的平均SF为0.29。孪晶形核的SF主要分布在0.2-0.1范围内，孪晶形核的SF等级主要是R1和R2等级，孪晶长大的SF 主要分布在0.4-0.3范围内，孪晶长大的SF等级主要是R1和R2等级。
⑤对于不经中间退火的1#和2#合金孪晶演变过程，再压缩过程中孪生变形均由孪晶形核主导。而对于经过中间退火的1#和2#合金孪晶演变过程，孪生变形均由孪晶长大主导。这表明中间退火改变了1#和2#合金再压缩过程中孪生变形的主导机制。
拼魔方的方法⑥对于经过中间退火的孪晶演变过程，再压缩过程中形核对孪生的贡献随着Nd元素含量的增加而增加。而对于不经中间退火的孪晶演变过程，形核对孪生的贡献随着Nd元素含量的增加而降低。
关键词：中间退火，孪晶演变，合金元素，形核，长大讲小红帽的故事
ABSTRACT
我读懂了你作文The bottlenecks that hinder wide application in magnesium alloys are low absolute strength and poor plastic deformation at room temperature. Slip and twinning are important mechanisms of plastic defo
rmation in magnesium alloys. Becau of limited slip systems at room temperature, twinning plays a very important role in the plastic deformation of magnesium alloys. It is found that annealing strengthening in twin-structured magnesium alloys is an interesting phenomenon in recent years, that is, pre-deformation and the intermediate annealing can make the solution atoms gregate in the twin boundaries to produce pinning effects. The pinning effects can hinder the growth of twins in the subquent deformation, resulting in an annealing strengthening. However, the microstructure evolution of the alloys during the subquent deformation is not yet studied when the twin boundaries are pinned.The study on the microstructure evolution of twin-structured magnesium alloys will help us to understand the deformation mechanism of twins better and provide a scientific basis for regulating the twinning behavior.
In this study, two kinds of Mg-Nd alloys with Nd atomic percentage of 0.03% (1#) and 0.18% (2#) were lected, and the evolution of twinning in the process of further deformation was investigated and compared. For the lected extruded bar, the pre-compression experiment was carried out to produce the pre-straining sample, and then a part of the sample was annealed at 200℃for 6h before re-compression. The other part of the sample was not annealed before the re-compression experiment. In the experiment, the evolution process of the twins was traced by electron back-scatte
red diffraction (EBSD) analysis. The characteristic parameters of twins are discusd, such as the number of twins, the volume fraction of twinning, the Schmid factor (SF) of twin nucleation and growth, the contribution of twin nucleation and growth to twin process. The effects of the intermediate annealing and the alloy elements on the twin evolution are also discusd.
The major conclusions are summarized as follows:
猕猴桃不能和什么一起吃①For the twin evolution process of 1# alloy with intermediate annealing, the volume fraction of twins incread by 14% after re-compression, the contribution of nucleation and growth is 14% and 86%, respectively. Twin growth is dominant mechanism during re-compression.The number fraction of twin nucleation and growth
狐狸爸爸is 24% and 38%, respectively. The average Schmid factor (SF) of twin nucleation and growth is 0.35 and 0.47, respectively.The SF of twin nucleation is mainly distributed in the range of 0.4-0.3. The SF rank of twin nucleation is mainly R1 and R2. The SF of twin growth is mainly distributed in the range of 0.5-0.4, and the SF rank of twin growth is mainly R1 and R2.
②For the twin evolution process of 1# alloy without intermediate annealing, the volume fraction of twins incread by 22% after re-compression, the contribution of nucleation and growth is 59% and
41%, respectively. Twin nucleation is dominant mechanism during re-compression. The number fraction of twin nucleation growth is 22% and 39%, respectively. The average Schmid factor (SF) of twin nucleation and growth is 0.35 and 0.40, respectively. The SF of twin nucleation is mainly distributed in the range of 0.5-0.4. The SF rank of twin nucleation is mainly R1 and R2. The SF of twin growth is mainly distributed in the range of 0.5-0.4, and the SF rank of twin growth is mainly R1 and R2.
③For the twin evolution process of 2# alloy with intermediate annealing, the volume fraction of twins incread by 3.7% after re-compression, the contribution of nucleation and growth is 35% and 65%, respectively. Twin growth is dominant mechanism during re-compression.The number fraction of twin nucleation and growth is 36% and 32%, respectively. The average Schmid factor (SF) of twin nucleation and growth is 0.19 and 0.29, respectively. The SF of twin nucleation is mainly distributed in the range of 0.1-0. The SF rank of twin nucleation is mainly R1 and R3. The SF of twin growth is mainly distributed in the range of 0.5-0.4, and the SF rank of twin growth is mainly R1 and R2.
④For the twin evolution process of 2# alloy without intermediate annealing, the volume fraction of twins incread by 4.1% after re-compression, the contribution of nucleation and growth is 60% and 40%, respectively. Twin nucleation is dominant mechanism during re-compression.The number fracti
图解人体摄影on of twin nucleation and growth is 44% and 28%, respectively. The average Schmid factor (SF) of twin nucleation and growth is 0.19 and 0.29, respectively. The SF of twin nucleation is mainly distributed in the range of 0.2-0.1. The SF rank of twin nucleation is mainly R1 and R2. The SF of twin growth is mainly distributed in the range of 0.4-0.3, and the SF rank of twin growth is mainly R1 and R2.
⑤For the twin evolution process of 1# and 2# alloy without intermediate annealing, twin nucleation is dominant mechanism during re-compression. However,
for the twin evolution process of 1# and 2# alloy with intermediate annealing, twin growth is dominant mechanism during re-compression. This shows that intermediate annealing can change the dominant mechanism of twinning precoss during re-compression in 1# and 2# alloy.
⑥For the twin evolution process of 1# and 2# alloy with intermediate annealing, the contribution of nucleation increas with the increa of Nd content during re-compression. However, for the twin evolution process of 1# and 2# alloy without intermediate annealing, the contribution of nucleation decreas with the increa of Nd content during re-compression.
Key words: intermediate annealing, twin evolution, alloying element, nucleation, growth
