Al-Zr-RE(YbY)合金中L12结构相的时效析出机制与作用.pdf

申请上海交通大学博士学位论文 Al-Zr-RE(Yb/Y)合金合金中中 L12结构相的结构相的时效析出机制时效析出机制与与作用作用 作作 者者：： 张永志张永志 导导 师：师： 孙宝德孙宝德 教授教授 副副 导导 师：师： 高海燕高海燕 副研究员副研究员 专专 业业：： 材料加工工程材料加工工程 学学 科科：： 材料科学与工程材料科学与工程 中国中国•上海上海 上海交通大学材料科学与工程学院上海交通大学材料科学与工程学院 二〇一二〇一五五年年一一月月 Dissertation Submitted to Shanghai Jiao Tong University for Ph. D. Degree THE PRECIPITATION MECHANISM AND EFFECTS OF L12-PHASES IN AL-ZR-RE (YB/Y) ALLOYS Candidate: Yongzhi Zhang Advisor: Prof. Baode Sun Assistant Advisor: Associate Prof. Haiyan Gao Specialty: Materials Processing Engineering Subject: Materials Science and Engineering School of Materials Science and Engineering Shanghai Jiao Tong University People’s Republic of China January, 2015 上海交通大学上海交通大学 学位论文原创性声明学位论文原创性声明 本人郑重声明：所呈交的学位论文，是本人在导师的指导下，独立进行研究工作所取得的成果。除文中已经注明引用的内容外，本论文不包含任何其他个人或集体已经发表或撰写过的作品成果。对本文的研究做出重要贡献的个人和集体，均已在文中以明确方式标明。本人完全意识到本声明的法律结果由本人承担。 学位论文作者签名： 日期： 年 月 日 上海交通大学上海交通大学 学位论文版权使用授权书学位论文版权使用授权书 本学位论文作者完全了解学校有关保留、使用学位论文的规定，同意学校保留并向国家有关部门或机构送交论文的复印件和电子版，允许论文被查阅和借阅。本人授权上海交通大学可以将本学位论文的全部或部分内容编入有关数据库进行检索，可以采用影印、缩印或扫描等复制手段保存和汇编本学位论文。 保密保密□，在 年解密后适用本授权书。 本学位论文属于 不保密不保密□。 （请在以上方框内打“√√” ） 学位论文作者签名： 指导教师签名： 日期： 年 月 日 日期： 年 月 日 附件四上海交通大学学位论文原创性声明本人郑重声明:所呈交的学位论文,是本人在导师的指导下,独立进行研究工作所取得的成果。除文中已经注明引用的内容外,本论文不包含任何其他个人或集体己经发表或撰写过的作品成果。对本文的研究做出重要贡献的个人和集体,均已在文中以明确方式标明。本人完全意识到本声明的法律结果由本人承担。学位论文作者签名:钊廴卸吒L日期:丬丬5年∫月丨5日附件五上海交通大学学位论文版权使用授权书本学位论文作者完全了解学校有关保留、 使用学位论文的规定,同意学校保留并向国家有关部门或机构送交论文的复印件和 电子版,允许论文被查阅和借阅。本人授权上海交通大学可以将本学位论文的全部或部分内容编入有关数据库进行检索,可以采用影印、缩印或扫描等复制手段保存和汇编本学位论文。保密□,在 年解密后适用本授权书。 本学位论文属于谣不保密Ⅸ(请在以上方框内打“√”)日期:氵刃∫ 歹年∫月15日 日期:z ρ丨5年 丨月| 5日学位论文作者签名:锥私 红擀绷锦多刂J} '〈{ } ·!∶∶)㈤、上海交通大学博士学位论文 摘要 I Al-Zr-RE(Yb/Y)合金合金中中 L12结构结构相的时效析出机制与作用相的时效析出机制与作用 摘摘 要要 铝合金具有密度低、比强度高、容易加工、导电导热性能好等优点，广泛应用于交通运输、航空航天与电力工业等领域。随着航空航天等领域的发展，提高铝合金的高温性能成为了材料领域备受关注的课题。Al-Zr 合金时效过程中析出 L12结构 Al3Zr 相，析出相在高温下动力学稳定，具有析出强化和抑制再结晶的作用，在发展耐热铝合金方面展现了良好的应用前景。然而， Zr 在 α-Al 中缓慢的扩散速率使Al3Zr 析出迟缓， 限制了析出相的体积分数及合金性能的提高。 此外，Zr 在凝固过程中的微观偏析导致时效后 Al3Zr 在基体中分布不均匀，析出相贫化区对合金的强度和耐热性不利。因此，提高 Al-Zr 合金性能的关键在于如何加速 Al3Zr 析出动力学过程并改善析出相在合金中的分布，减小或消除析出相贫化区。 本文采用稀土元素 RE(Yb 或 Y)作为 Al-Zr 合金微合金化元素， 通过促进析出相的形核析出， 形成弥散分布的 L12结构析出相 Al3(Zr,RE)，以增强合金强度和耐热性能，为开发新型 L12相强化耐热铝合金提供理论指导。 采用光学显微镜(OM)、 扫描电镜(SEM)、 透射电镜(TEM)、高分辨电镜(HREM)和三维原子探针(3DAP)等结构分析手段以及维氏显微硬度和电导率测量等测试手段，结合第一性原理计算研究了Al-Zr-RE(Yb/Y)合金中 L12结构相的时效析出机理，分析了析出相对合金强度与再结晶行为的影响规律。主要研究内容和结论如下： (1) Al-Zr-Yb 合 金 中 Al3(Zr,Yb) 的 析 出 机 制 与 演 变 规 律 。上海交通大学博士学位论文 摘要 II Al-0.08Zr-0.03Yb(原子分数 at%，下同)中时效初期形成的 Al3Yb 促进Zr 的时效析出，形成纳米级核心－外壳结构 Al3(Zr,Yb)复合相，合金在等时时效过程中表现了双峰时效强化行为。 Al3Yb 在 100℃与 150℃之间析出，使合金在 300℃达到第一个时效强化峰。随着时效温度的提高， 在 Al3Yb 外围形成了成分约为 Al3(Zr0.6Yb0.4)的富 Zr 外壳， 使合金在 475℃达到第二个时效强化峰，在高温下保持良好的析出强化效果。 Zr 和 Yb 元素在 Al3(Zr,Yb)析出相形核与长大过程存在协同效应。一方面，Yb 促进 Zr 的析出。时效初期形成的 Al3Yb 析出相作为异质形核核心，促进 Al-Zr 固溶体的分解。另一方面，Zr 减缓了 Al3Yb 的析出和粗化。在时效初期，固溶体中的 Zr 对 Yb 的吸引作用延缓了Al3Yb 析出相的形核和长大；随着时效进行，大量 Zr 元素偏聚于Al3Yb/α-Al 界面形成富 Zr 外壳， 有效阻碍了 Al3Yb 核心的粗化并增大了析出相的临界共格转变半径。 (2) Al-Zr-Y 合 金 中 Al3(Zr,Y) 的 析 出 机 制 与 演 变 规 律 。Al-0.08Zr-0.025Y 时效初期不单独形成 L12结构 Al3Y 析出相，α-Al 晶格中 Y 通过与 2NN 位置 Zr 的相互吸引作用促进 L12结构 Al3(Zr,Y)的形核， Al3(Zr,Y)析出相不具有核心－外壳结构。 Al-0.08Zr-0.025Y 合金在 500℃等温时效 1h 后即形成 Al3(Zr,Y)析出相，而 Al-0.08Zr 合金在时效 3h 后才生成 Al3Zr 析出相， 这是由于 Y 加入 Al-Zr 体系后有利于增加 Zr-Y 原子对周围的空位浓度， 降低 Zr 在 Al 中的扩散激活能， 加速 Zr 的扩散过程，从而促进 Zr 的析出并加速了 Al3(Zr,Y)的析出动力学过程。 (3) Al-Zr-RE(Yb/Y)合金的析出强化和再结晶行为。在 400℃等温时效时， Al-0.08Zr 合金中 Al3Zr 析出缓慢， 析出强化不明显； 添加 0.025% 上海交通大学博士学位论文 摘要 III Y 和 0.05% Y 的 Al-0.08Zr 合金硬化速率加快， 并在高温下保持稳定；添加 0.03% Yb 的 Al-0.08Zr 合金析出强化速率最快，时效 10h 后即达到双峰时效强化的第二个硬度峰， 延长时效时间至 750h 硬度基本保持不变， 具有明显的析出强化和高温稳定性。 添加 0.01% Yb 使 Al-0.08Zr时效峰值硬度提高约 16%，远高于添加 0.01% Y 的 3%，这是由于与Al3Zr 和 Al3(Zr,Y)相比，Al3(Zr,Yb)半径最小，数量密度最大，在高温下有效强化了 Al-Zr-Yb 合金。 添加 0.025% Y 和 0.03% Yb 均提高了 Al-0.08Zr 合金的再结晶温度，而添加 0.05% Y 则降低了 Al-0.08Zr 合金的再结晶温度。Y 含量不同的 Al-0.08Zr 合金再结晶温度主要取决于第二相的尺寸与分布。在 Al-0.08Zr 中添加 0.025% Y 生成了细小弥散的纳米尺度 Al3(Zr,Y)析出相，减小了析出相贫化区，对位错和亚晶界的钉扎作用增强，从而提高再结晶温度； Y 含量达到 0.05%时尽管析出相分布同样得到改善，然而大量微米尺度共晶 Al3Y 成为再结晶晶粒的优先形核区域，因此降低了再结晶温度。 关键词：铝合金，Al3Zr，显微组织，力学性能，时效析出，稀土上海交通大学博士学位论文 ABSTRACT IV THE PRECIPITATION MECHANISM AND EFFECTS OF L12-PHASES IN AL-ZR-RE(Yb/Y) ALLOYS ABSTRACT Aluminum alloys have been playing an important role in transportation, aerospace, electric-utility industry and other fields due to low density, high specific strength, good machining properties and excellent electrical and thermal conductivity. With the development of aerospace industry, the improvement of mechanical properties at elevated temperature was a continuing goal in materials research. The Al-Zr system exhibits particular promise for developing high temperature aluminum alloys. During aging, supersaturated Al-Zr solid solutions form cubic L12-structured Al3Zr precipitates, which are kinetically stable at elevated temperature. The Al3Zr precipitates are effective in the precipitation strengthening and inhibition of recrystallization. However, the precipitation kinetics of Al3Zr precipitates is very slow due to slow diffusivity of Zr in α-Al, thereby limiting the volume fraction of precipitates and mechanical properties of alloys. The Al3Zr precipitates are heterogeneously distributed after aging owing to microsegregation of Zr solute during solidification. The precipitates free regions have a deleterious effect on the strength and recrystallization resistance. Therefore, extensive investigations have aimed to accelerate the precipitation kinetics and to improve distribution of Al3Zr precipitates. In this thesis, the rare earths Yb or Y were added into Al-Zr as microalloy element, the precipitation kinetics and distribution of Al3(Zr,RE) precipitates was improved to develop high temperature 上海交通大学博士学位论文 ABSTRACT V aluminum alloys with L12-phases strengthener. The mechanisms of nucleation and growth of Al3(Zr,RE) precipitates, as well as the effects of Al3(Zr,RE) precipitates on the strength and recrystallization resistance of Al-Zr alloys, were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution electron microscopy (HREM), three dimensional atom probe (3DAP), Vickers microhardness, electrical conductivity measurement and first-principles method. The main results can be summarized as follows: (1) The nucleation and precipitation mechanism of Al3(Zr,Yb) in Al-Zr-Yb alloy. The Al3Yb precipitates form in early stage of aging and stimulate the precipitation of Zr, resulting in the formation of Al3(Zr,Yb) composite precipitates. Al-0.08Zr-0.03Yb alloy shows double-peak age strengthening behavior during isochronal aging. The precipitation of Al3Yb (L12) commences between 100 and 150℃, leading to first peak hardness at 300℃. With the increase of aging temperature, Zr begins to segregate at the α-Al/Al3Yb interface, forming a Zr-rich shell with composition of Al3(Zr0.6Yb0.4) surrounding a Al3Yb core, which results in second peak hardness at 475℃. The Al-Zr-Yb alloy maintains significant precipitation strengthening at elevated temperature. A synergetic effect of Zr and Yb on the precipitation evolution can be concluded in Al-Zr-Yb alloy. On the one hand, Yb stimulates the precipitation of Zr. The Al3Yb precipitates act as heterogeneous nucleation sites and stimulate the decomposition of Al-Zr solid solutions. On the other hand, Zr decreases the precipitation and coarsening kinetics of Al3Yb, which can be divided into two stages. In the early stage of aging, the nucleation and growth of Al3Yb is retarded due to the attractive interaction between Yb and Zr in solid solution. As the aging temperature increases, with the segregation of Zr at the α-Al/Al3Yb interface, core-shell structure of Al3(Zr,Yb) forms. The Zr-enriched shell retards the 上海交通大学博士学位论文 ABSTRACT VI coarsening of Al3Yb core effectively and increases the critical coherency transition radius of precipitate. (2) The nucleation and precipitation mechanism of Al3(Zr,Y) in Al-Zr-Y alloy. The L12-Al3Y precipitates, as well as the core-shell structure of Al3(Zr,Y), was not observed in Al-Zr-Y alloys. First-principles calculations illustrate that the binding of Y-Zr at 2NN distance in α-Al lattice is attractive, which stimulates the nucleation of Al3(Zr,Y) with L12 structure. The Al3(Zr,Y) precipitates were observed in Al-0.08Zr-0.025Y aged isothermally at 500℃ for 1h, while the Al3Zr precipitates were not observed in Al-0.08Zr until the aging time is longer than 3h. It is attributable that the addition of Y to Al-Zr system will increase the local vacancy concentration close to a Zr-Y dimer and decrease the diffusion activation energy of Zr, which increases the diffusivity of Zr in Al. Thus the addition of Y significantly accelerates the precipitation kinetics of Al3Zr in Al-Zr-Y alloys. (3) The precipitation strengthening and recrystallization resistance of Al-Zr-RE(Yb/Y) alloys. During isothermal aging at 400℃, the precipitation of Al3Zr proceeds slowly in Al-0.08Zr, resulting in slight precipitation strengthening. The Al-0.08Zr-0.025Y and Al-0.08Zr-0.05Y alloys show similar hardening curve during aging. The rate of strengthening in Al-Zr-Y is faster than that in Al-Zr. The Al-Zr-Y alloy illustrates significant thermal stability. Al-0.08Zr-0.03Yb exhibits the fastest rate of strengthening. Al-0.08Zr-0.03Yb achieves second peak hardness after aging for 10h and maintains the peak hardness after aging for 750h. Al-0.08Zr-0.03Yb shows significant precipitation strengthening and thermal stability. The additions of 0.01% Yb increase the peak hardness of aged Al-0.08Zr by 16%, which is significantly larger than increase of 3% for additions of 0.01% Y. The Al3(Zr,Yb) precipitates act as effective strengtheners in Al-0.08Zr-0.03Yb alloy at elevated temperature, 上海交通大学博士学位论文 ABSTRACT VII which is attributable that Al3(Zr,Yb) precipitates are of smallest radius and highest number density as compared with Al3Zr and Al3(Zr,Y). The additions of 0.025% Y and 0.03% Yb both increase the recrystallization temperature of Al-0.08Zr, respectively, while excess additions of 0.05% Y decrease the recrystallization temperature of Al-0.08Zr. The recrystallization temperature of Al-0.08Zr alloys with different Y additions is mainly influenced by the size and distribution of second-phase particles. Compared with Al3Zr in binary alloy, the Al3(Zr,Y) precipitates with higher number density and smaller radius in Al-0.08Zr-0.025Y alloy are favor to attain a large drag force on dislocations and subgrain boundaries (Zener drag). Moreover, the precipitates free regions in Al-0.08Zr-0.025Y are much narrower than that in Al-0.08Zr, therefore the recrystallization resistance is substantially improved in Al-0.08Zr-0.025Y alloys. As for Al-0.08Zr-0.05Y, the Zener drags should be closed to that in Al-0.08Zr-0.025Y owing to the similar distribution of Al3(Zr,Y) precipitates. However, the number density and radius of eutectic Al3Y phases in Al-0.08Zr-0.05Y are both larger than that in Al-0.08Zr-0.025Y. The micron-sized eutectic Al3Y phases promote the recrystallization by particle simulated nucleation (PSN), which results in degradation of recrystallization resistance in Al-0.08Zr-0.05Y. KEY WORDS: Aluminum alloy, Al3Zr, Microstructure, Mechanical property, Precipitation, Rare earth 上海交通大学博士学位论文 目录 VIII 目目 录录 摘摘 要要 ................................................................................................................................ I ABSTRACT.................................................................................................................. IV 主要符号说明主要符号说明 ............................................................................................................... XI 第一章第一章 文献综述文献综述 ......................................................................................................... 1 1.1 引言 ................................................................................................................... 1 1.2 耐热铝合金的研究进展 ................................................................................... 1 1.2.1 耐热铝合金的发展现状 ......................................................................... 1 1.2.2 耐热铝合金元素的选择准则 ................................................................. 2 1.3 Al-Zr 合金中 Al3Zr 的时效析出特性与作用 ................................................... 5 1.3.1 Al-Zr 合金凝固特性 ................................................................................ 5 1.3.2 Al3Zr 的形态、结构与高温稳定性 ........................................................ 6 1.3.3 L12结构 Al3Zr 对合金性能的影响 ......................................................... 9 1.4 提高 Al-Zr 合金性能的方法 .......................................................................... 10 1.4.1 热处理工艺优化 ................................................................................... 11 1.4.2 多元合金化 ........................................................................................... 12 1.5 本文研究意义、目的与内容 ......................................................................... 20 1.5.1 研究意义和目的 ................................................................................... 20 1.5.2 研究内容 ............................................................................................... 20 参 考 文 献 .......................................................................................................... 21 第二章第二章 材料制备与实验方法材料制备与实验方法 ................................................................................... 30 2.1 引言 ................................................................................................................. 30 2.2 合金成分设计 .................................................................................................. 30 2.2.1 Al-Yb、Al-Y 合金体系 ......................................................................... 31 2.2.2 合金成分选择 ....................................................................................... 33 2.3 材料制备 ......................................................................................................... 33 2.3.1 合金熔炼工艺 ....................................................................................... 33 2.3.2 合金热处理 ........................................................................................... 35 2.3.3 再结晶温度测量 ................................................................................... 36 2.4 组织观察与结构表征 ...................................................................................... 37 2.4.1 金相观察 ............................................................................................... 37 2.4.2 扫描电镜分析 ...........................................................