稀土耐热镁合金微观结构对性能的影响研究

发布时间：2018-08-25 09:41

【摘要】：镁资源和稀土资源在我国储量非常丰富,因此研究开发含稀土的耐热镁合金具备比较明显的优势,汽车采用低成本耐热镁合金材料将是二十一世纪镁工业发展的关键。本论文主要目标为研究开发新型汽车传动耐热镁合金部件,重点研究镁合金材料的性能,而且兼顾材料的成本。重点研究含稀土耐热镁合金微观结构稳定性及其对性能的影响,研究纯稀土元素和富RE混合稀土对Mg-Al基压铸及铸造合金的微观组织、热稳定性、力学性能与腐蚀性能的影响,分析稀土元素在合金中的强化机理及作用机制;研究了挤压态Mg-12Ymm-4Zn(Ymm为富钇混合稀土)合金的微观组织、力学性能及第二相在合金中的强化机理。采用压铸方法制备了Mg-4Al-4RE合金(AE44),其中RE为La,Ce,Pr,Nd混合稀土(La:Ce:Pr:Nd=23:55:6:16,wt.%),研究了该合金微观组织、拉伸力学性能、合金热稳定性和腐蚀性能。结果表明,压力铸造Mg-4Al-4RE合金流动成形性能良好,铸件没有明显缺陷,合金微观组织比较均匀,具有良好的压铸性能;晶粒尺寸约为10 μm,合金中强化相为Al11RE3和Al2RE两种,针状/层片状的强化相Al11RE3主要以密集排列的方式分布在晶粒边界周围。采用全谱拟合Rietveld方法对合金各相进行定量分析,增强相Al11RE3和A12RE含量分别为5.73%和0.36%,合金中Al11RE3相高温条件下不稳定,当合金在400℃加热5000小时后,Al-RE金属间化合物微观结构由针状/层片状转化成短棒状,最后为颗粒状。Al-RE金属间化合物在合金中分布更加松散,不再沿晶粒边界分布。很多Al-RE金属间化合物在测试条件下(400℃加热5000小时)转化为A12RE相。通过定量计算可知,400℃加热5000小时后,Al11RE3和Al2RE含量分别为4.46%和0.96%。该合金从室温到200℃温度范围内具有良好的拉伸性能,抗拉强度在室温下为252MPa,屈服强度为146MPa,延伸率为11.4%;在200℃抗拉强度为116MPa,屈服强度为102MPa,延伸率为25.1%。对压铸AE44合金、400℃加热1000小时和400℃加热5000小时三种合金进行腐蚀行为测试,结果表明,压铸态合金具有较高的耐腐蚀性能。主要是由于合金晶粒边界分布大量的层片状/针状的Al11RE3相作为腐蚀障碍有效阻止腐蚀的进行。热处理后的合金,微观组织及Al-RE金属间化合物分布发生变化导致腐蚀性能下降。采用重力铸造方法制备了 Mg-xAl-yLa(x=4,8;y=2,5,8)合金,对合金进行微观组织、力学性能和腐蚀性能进行研究。AlLa45合金主要由α-Mg和Al11La3相组成。综合比较,AlLa45合金在室温及高温下均具有最佳力学性能和较强的耐蚀性,主要得益于合金大量存在的性能稳定的增强相Al11La3,该相大量聚集在晶界,带来晶界强化。研究了稀土元素Pr对重力铸造Mg-4Al-xPr(x=2,5)合金微观组织、热稳定性、拉伸力学性能和腐蚀性能的影响,添加稀土元素Pr以后合金中的高温不稳定相Mg17Al12被完全抑制,随着Pr含量的增加,合金中两种金属间强化相Al11Pr3和Al2Pr的数量明显增多。Al11Pr3是一种温度敏感的增强相,针状形态的Al11Pr3在400℃加热5000小时会相变分解为颗粒状的Al2Pr相。AlPr45合金在室温及高温下均具有较佳的力学性能。细晶强化和大量第二相在晶界聚集产生的晶界强化及固溶强化是合金保持良好拉伸力学性能的主要原因。研究了富铈混合稀土 RE对重力铸造Mg-4Al-xRE(x=2,5)合金微观组织、热稳定性、力学性能和腐蚀性能的影响,AE42和AE45合金主要由α-Mg和大量Al11RE3相及少量Al2RE相组成,层片状/针状Al11RE3相呈团簇状分布在晶界上。AE4 5合金在400℃高温下进行1000小时和5000小时加热处理后,Al11RE3相发生分解,部分分解为Al2RE相,证明Al11RE3相长时间高温加热的非稳定性。综合比较,AE45合金在室温及高温下均具有最佳力学性能,大量层片状/针状Al11RE3相分布在晶粒边界区域,Al11RE3相层片状近似平行分布的状态使得合金具有较好的综合强度和塑性。研究了挤压态Mg-12Ymm-4Zn合金,合金在300℃时的抗拉强度和屈服强度与室温相差不大,300℃高温条件下分别为314MPa和231MPa,室温时分别为338MPa和278MPa。该挤压态Mg-12Ymm-4Zn合金在高温下具有优异的力学性能主要是由于合金中生成高体积分数的长周期堆垛有序结构相和纳米间距的堆垛层错的共同作用。
[Abstract]:Magnesium and rare earth resources are abundant in our country, so the research and development of heat-resistant magnesium alloys containing rare earth have obvious advantages. Low-cost heat-resistant magnesium alloys for automobiles will be the key to the development of magnesium industry in the 21st century. The effects of rare earth elements and RE-rich mixed rare earth elements on the microstructure, thermal stability, mechanical properties and corrosion properties of Mg-Al based die casting and casting alloys were studied. The microstructure, mechanical properties and second phase strengthening mechanism of extruded Mg-12Ymm-4Zn (Ymm is Yttrium-rich rare earth mixture) alloy were studied. Mg-4Al-4RE alloy (AE44) was prepared by die casting, in which RE is La, Ce, Pr, Nd mixed rare earth (La: Ce: Pr: Nd = 23:55:6:16, wt.) The results show that the die casting Mg-4Al-4RE alloy has good flowing formability, no obvious defects, uniform microstructure and good die casting performance; the grain size is about 10 micron, and the strengthening phases are Al11RE3 and Al2RE, needle-like/layer. Al11RE3 was mainly distributed around the grain boundary in the form of dense arrangement. The contents of Al11RE3 and A12RE were 5.73% and 0.36% respectively. The Al 11RE3 phase in the alloy was unstable at high temperature. When the alloy was heated at 400 C for 5000 hours, the Al RE intermetallic space was determined. Al-RE intermetallics are more loosely distributed in the alloys and no longer distribute along grain boundaries. Many Al-RE intermetallics are transformed into A12RE phase under test conditions (heating at 400 C for 5000 hours). Quantitative calculation shows that after heating at 400 C for 5000 hours, Al-RE intermetallics are more loosely distributed. Al11RE3 and Al2RE contents are 4.46% and 0.96% respectively. The alloy has good tensile properties in the temperature range from room temperature to 200%. The tensile strength is 252 MPa, the yield strength is 146 MPa, and the elongation is 11.4%. The tensile strength is 116 MPa, the yield strength is 102 MPa, and the elongation is 25.1%. For die-cast AE44 alloy, heating at 400 ~1000 is small. The corrosion behavior of three alloys heated at 400 C for 5000 hours shows that as-cast alloys have high corrosion resistance, which is mainly attributed to the large number of lamellar/acicular Al11RE3 phases distributed along the grain boundaries of the alloys as corrosion barriers. Mg-xAl-yLa (x=4,8; y=2,5,8) alloy was prepared by gravity casting method. The microstructure, mechanical properties and corrosion resistance of the alloy were studied. AlLa45 alloy was mainly composed of alpha-Mg and Al11La3 phases. The properties and strong corrosion resistance of Mg-4Al-xPr (x=2,5) alloys are mainly attributed to the presence of a large number of stable strengthening phases, Al 11La 3, which accumulate at grain boundaries and strengthen grain boundaries. The unstable phase Mg17Al12 in the alloy is completely inhibited. With the increase of Pr content, the amount of Al 11Pr 3 and Al 2Pr increases obviously. Al 11Pr 3 is a temperature-sensitive reinforcement phase. The acicular Al 11Pr 3 phase transforms into granular Al 2Pr phase when heated at 400 C for 5000 hours. The fine grain strengthening and the grain boundary strengthening and solid solution strengthening produced by the aggregation of a large number of second phases at grain boundaries are the main reasons for the good tensile properties of the alloy. AE42 and AE4 5 alloys are mainly composed of a-Mg, a large number of Al11RE3 phases and a small amount of Al2RE phases. The lamellar/acicular Al11RE3 phases are distributed in clusters on the grain boundaries. After 1000 hours and 5000 hours of heating treatment at 400 C, the Al11RE3 phase decomposes and partially decomposes into Al2RE phase, which proves that the Al11RE3 phase is heated for a long time at high temperature. The results show that the AE45 alloy has the best mechanical properties at room temperature and high temperature. A large number of lamellar/acicular Al11RE3 phases are distributed in the grain boundary region. The Al11RE3 phase is approximately parallel to the lamellar state, which makes the alloy have better comprehensive strength and plasticity. The tensile strength and yield strength of the extruded Mg-12Ymm-4Zn alloy are similar to those at room temperature, 314MPa and 231MPa at 300 C, 338MPa and 278MPa at room temperature, respectively. The excellent mechanical properties of the extruded Mg-12Ymm-4Zn alloy at high temperatures are mainly due to the formation of Long-period Stacking ordered phases and nano-spacing with high volume fraction. The effect of stacking faults.
【学位授予单位】：哈尔滨工程大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22

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