【作者】 周杨；

【导师】 许春香；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2011， 硕士

【摘要】 在金属资源日益匮乏的今天,加速镁合金材料相关的科学技术研究是实现可持续性发展的重要措施之一。然而由于镁铝基合金的主要强化相相Mg17Al12的熔点比较低,在高温条件下易软化,不能有效钉扎晶界和抑制高温晶界移动。因此,提高镁合金的高温性能成为国内外材料研究的热点。本文针对Mg-Al基镁合金,通过碱土元素Ca以及Ca、Nd复合变质的方法,研究了Ca及Ca、Nd元素对合金组织及力学性能的影响。本文采用金属型普通铸造法制备出Mg-(3%～9%)Al-xCa及Mg-6%Al-1Ca-xNd实验合金。利用了光学金相显微镜、SEM扫描电镜、EDS能量散射光谱、x射线衍射分析仪、布氏硬度计、电子万能试验机等手段,对添加Ca及Ca、Nd复合变质的镁铝基合金铸态显微组织及力学性能进行研究和分析。结果表明：(1)合金元素Ca的加入,对Mg-(3%～9%)Al合金的组织形貌和相组成都产生了较大的影响。当Al/Ca之比不大于6时,Ca的加入明显细化了基体组织,粗大的β-Mg17Al12相的枝晶尺寸和二次枝晶间距明显变小,数量也减少,并有部分β-Mg17Al12相成短条状与颗粒状。当Ca含量进一步增多时,合金中产生Al2Ca相,Al2Ca相的形貌从细骨骼状逐渐转变成网格状,β-Mg17Al12相的数量继续下降,当Al/Ca比超过3时,Al2Ca相以连续网状沿晶界分布,此时β-Mg17Al12相已经完全消失。(2)不同含量Ca的加入对实验合金硬度主要体现在：实验合金的硬度都是随着Ca含量的增加而上升的,三种合金的最高硬度分别是54.7HB、56.4HB、61.8HB。在Al2Ca相成为合金中主要第二相时,硬度上升最明显,造成这一现象的主要原因是由于合金组织的细化和高硬度第二相的析出。而随着Al含量的提高,Ca对合金硬度的提升程度较小(3)适量Ca的加入可以提高合金的高温性能。实验合金的高温拉伸性能随着Ca含量的提升呈先上升后下降的趋势,试验表明添加适量的Ca对合金高温性能有益,由于耐热脆硬相Al2Ca的析出,A、B组试验合金的高温性能都在Ca含量1 wt%时达到最高值,而随着Ca含量的进一步增加,高温抗拉强度开始下降,但均高于基体合金。合金的高温屈服强度随着Ca的含量的增加而上升,说明Ca能够提升合金的高温抗变形能力。无论是室温拉伸还是高温拉伸,Ca的加入都使合金的塑性下降,Ca的加入提高了合金的脆性。(4)Mg-6Al-1Ca合金中加入合金元素Nd后,组织得到明显细化,第二相形貌也发生很大变化。当Nd含量为0.3%wt时,组织中未能发现含有Nd的化合物,与基体合金相比,半连续的网状Al2Ca相已经消失,组织形貌得到一定细化。当Nd含量达到0.6wt%时,组织中生成了细针状的Al11Nd3相,Al2Ca以细骨骼状存在,β-Mg17Al12相的形貌更加细化,基本以颗粒状存在,并有微量的Ca固溶。此时合金中第二相主要由Al11Nd3、Al2Ca和β-Mg17Al12相组成。当Nd含量为0.9wt%时,组织中Al11Nd3相从细针状转变成短棒状,而β-Mg17Al12相和Al2Ca相的形貌粗化。(5)添加适量的Nd能够提高合金的室温和高温拉伸性能,随合金元素Nd含量的增加,试验合金的抗拉强度和伸长率都呈先增加后降低趋势。当Nd含量为0.6wt%时,合金的室温和高温抗拉强达到最高值,分别为191MPa和173MPa,比基体分别提高了17.1%和19.3%,随后抗拉强度开始下降,但均高于基体合金。合金的塑性也得到有效改善,室温和高温的伸长率分别为6.6%和9.6%,相比于基体合金提高了34.6%和23.0%。Nd的添加抑制了Al2Ca的连续析出,降低了Ca对镁合金脆性的影响。更多还原

【Abstract】 Today, With Increasing scarcity of the metal resources, accelerating the scientific and technological research related to the magnesium alloy material is one of the important measures to achieve sustainable development. However, the main phase Mg17Al12 which has a relatively low melting point and is easy to soften under high temperature conditions can not effectively pin the grain boundaries and inhibit the high-temperature grain boundaries rotation in magnesium alloy.Therefore, improving the high temperature properties of magnesium alloy has become a critical issue at home and abroad.In this paper, the efforts of Ca and Ca、Nd addition on the as-cast microstructure and mechanical properties of Mg-Al alloys were investigated by the method of Ca and Ca、Nd compound modification.Experimental alloy were prepared by using as-cast forming technique.The efforts of Ca and Ca, Nd addition on the as-cast microstructure and mechanical properties of Mg-Al alloys were investigated and analyzed by scanning electron microscope(SEM),energy dispersive spectroscopy (EDS),X-ray diffraction (XRD),Brinell testing machines, electronic universal testing machine, et cetera.Results show that: The great effects of Ca addition on the microstructure and mechanical properties of Mg-Al alloy are produced. The experiments in this group, when the Al/Ca ratio is only less than 6, The addition of Ca result in refined matrix microstructure greatly. It was significantly smaller of the dendrite size of thickβ-Mg17Al12 phase and secondary dendrite spacing.The amount of them significantly reduced. Furthermore, a small amount ofβ-Mg17Al12 was refined to short strip or granules. The new phase Al2Ca appears when the Ca has been added. The morphology of Al2Ca phase is changed from bone-shaped into grid-like. Meanwhile,β-Mg17Al12 phase continued to decline. Al2Ca phase have continuous net-like along the grain boundaries when the Al/Ca ratio is more than 3.β-Mg17Al12 phase have disappeared completely at the time.(2) Effects of Ca addition on the hardness of the test alloys have been investigated. Results show that:with the increase of calcium content, the hardness of the alloy increased accordingly. The highest hardness values of the alloys are 54.7HB,56.4HB,61.8HB. When Al2Ca phase is changed to the main second phase, brinell hardness of the alloy obvious increases. Due to structure of alloy refined and the second phase which is high hardness separated. When the Al has been added, the effect of Ca is less to hardness of the alloy(3) Amount of Ca can improve the tensile properties of alloys in high temperature.With the increase of Ca content the tensile properties of alloys in high temperature of the test alloy first increases and then decreases. Results show that:the addition of Ca is useful to the tensile properties of alloys in high temperature. Due to heat separation of brittle phases Al2Ca, the properties of alloys in high temperature of both A and B group alloys reaches the maximum value when Ca addition up to 1 wt%.With the further increase of Ca content, high tensile strength began to decline, but still higher than the matrix alloy.With the increase of Ca content the yield strength of alloys in high temperature of the test alloy increases, which indicated that Ca can improve the deformation resistance of alloys in high temperature. The plastic of the alloy increased and the brittleness of the alloy increases with the addition of Ca increased both in room temperature and high temperature.(4) The addition of Nd results in refined matrix micro structure greatly and the second phase morphology changing greatly. When Nd addition up to 0.3 wt%, The compounds with Nd do not appears. However, compared to the matrix alloy, semi-continuous net-like Al2Ca phase has disappeared and the microstructure of alloy is refined. When Nd addition up to 0.6 wt%, a fine needle Al11Nd3 phase appears, and Al2Ca exists in form of bone-like.β-Mg17Al12 phase morphology is refined more than before, Which exists mainly in the form of particles and a small amount of Ca have been dissolve in the alloy. At this point the second phases are mainly Al11Nd3, Al2Ca andβ-Mg17Al12 phase. When Nd addition is up to 0.9 wt%, Al11Nd3 phase of matrix alloy is changed from a fine needle into a short stick. However, the morphology ofβ-Mg17Al12 phase and Al2Ca phase is coarsed. (5) The tensile strength of matrix alloy increases at room temperature and high temperature when Ca addition up is appropriate. The tensile strength and elongation of the test alloy first increases and then decreases. When Nd addition up to 0.6 wt%, tensile strength of alloy were both at the optimal at room temperature and high temperature and the valves are 191Mpa 173Mpa,respectively.They are increased by 17.1% and 19.3% more than the matrix alloy, respectively. Following that the tensile strength started to decline, but still higher than the matrix Alloy. The ductility of alloys have been improved at room temperature and high temperature.The elongation rates were 6.6% and 9.6%, respectively.They increased by 34.6% and 23.0% compared to the matrix alloy, respectively. With the addition of Ca, Al2Ca phase continuous precipitation has been inhibited and the brittle effect of Ca to magnesium alloy is reduced.更多还原