热型连铸Cu-Al-Ni形状记忆合金的性能研究

【作者】 尹占华；

【导师】 黎沃光；

【作者基本信息】 广东工业大学 ， 材料加工， 2004， 硕士

【摘要】 本文对具有柱晶组织的Cu-Al-Ni形状记忆合金丝的性能作了系统的研究，以确定Cu-Al-Ni形状记忆合金在工业化生产中实际应用的可能性。尽管Cu-Al-Ni单晶显示出优良的机械性能和记忆性能，但在实际应用的合金制备工艺中，无一例外的都是获得随机取向的多晶组织。研究表明，Cu-Al-Ni合金大的弹性各向异性因子，导致变形时在晶界处形成应力集中，引起晶间断裂。目前，几乎所有的研究工作都集中在细化晶粒，使应力集中分散，来提Cu-Al-Ni多晶体的机械性能和记忆性能，如添加合金元素、快速凝固、粉末冶金、孕育处理等。本课题用热型连铸法定向凝固来得到粗大的柱状晶组织，通过强化织构来提高Cu-Al-Ni合金的力学性能和疲劳性能，并进一步分析合金丝组织与拉伸疲劳性能的关系。 采用水平式热型连铸装置用不同的拉铸速度制得(?)1和(?)1.5的形状记忆合金丝。用微控电液伺服试验机在室温下作拉伸试验，测定其抗拉强度和延伸率；用自制的拉伸疲劳试验机，测定Cu-Al-Ni合金丝在4％，6％和8％固定应变下的疲劳寿命和记忆性能；用DSC法测试了合金丝的相变温度；用X-ray衍射仪分析了合金丝内晶粒的晶体取向；用金相显微镜观察合金丝在铸态、固溶处理后以及疲劳拉断后的组织形态；用扫描电镜观测了Cu-Al-Ni合金丝的静拉伸断口和疲劳断口形貌。 研究结果表明：用50mm／min拉铸的((?)1.5合金丝，抗拉强度为633MPa，延伸率14.95％，在4％应变下的反复拉伸疲劳寿命高达38560次；在6％的应变下反复拉伸疲劳寿命为30378次，其最大可恢复应变为10％；以75mm／min拉铸的(?)1合金丝，抗拉强度可达910MPa，延伸率为18.76％，在4％应变下的反复拉伸疲劳断裂次数达17108次；在6％的应变下反复拉伸疲劳寿命为16215次，其最大可恢复应变为10.5％。几种合金丝在疲劳断裂前，回复率一直保持为100％。 随着拉铸速度的提高，单位横截面积上的晶粒数增加，晶粒直径变小。X-ray衍射结果显示，各种拉速的合金丝内的晶粒取向基本一致，柱晶取向偏离度不超过13°。从疲劳断口形貌可以看出，影响合金丝疲劳寿命的因素有两个：(1)横向晶界面的大小和数量；(2)铸造缺陷和表面质量。纵向晶界的存在不会影响合金丝的疲劳寿命。广东工业大学工学硕士学位论文 用热型连铸法拉制具有柱晶组织的Cu一Al一Ni形状记忆合金丝，具有优良的机械性能、疲劳性能和记忆性能。通过优化工艺参数，提高合金丝表面质量，消除铸造缺陷;提高拉速，增加单位横截面积上的晶粒个数，减小横向晶界面的大小和数量，可以进一步提高合金丝的机械性能和疲劳性能。更多还原

【Abstract】 Properties have been studied to establish the possibility of industrial application of Cu-Al-Ni SMA with columnar grain structure. Although the Cu-Al-Ni single crystal shows excellent mechanical and memory properties, almost all Cu-Al-Ni SMA are polycrystalline in practice. Researches have shown that stress concentration formed at grain boundary and rupture along the boundary when deforming, which resulted in the large elastic anisotropy factor of Cu-Al-Ni SMA. At present, most researches focus on grain refinement, which can disperse the stress concentration, in order to improve the performance of mechanism and memory. For example, adding alloying elements, rapid solidification, powder metallurgy, inoculation treatment, etc. The purpose of this study is to improve the mechanical and memory performance of Cu-Al-Ni SMA by strengthening texture, i.e. obtaining a longitudinal columnar structure by OCC (Ohno continuous casting) process, and to analyse the relationship between repeat tensile fatigue and structure.Cu-Al-Ni alloy wires 1.5mm and lmm in diameter are cast by OCC process at varies casting speeds. The tensile strength and elongation are measured with tensile experiment equipment at room temperature. The repeat tensile fatigue life and memory performance is determined by a self-designed repeat tensile fatigue testing equipment under 4%, 6% and 8% strain. The characteristic phase transformation temperatures were measured by Digital Scan Calorimeter (DSC). The crystal orientation of Cu-Al-Ni SMA wires is analyzed with X-ray Diffraction (XRD). The structure of as-cast, solid solution treated and tensile fatigue fracture were observed by metallography microscope. The fracture morphologies of tensile and fatigue were observed by Scan Electron Microscope (SEM).The result shows that: The 01.5mm wires cast in 50mm/min show a ultimate tensile strength of 633MPa, elongation of 14.95%. Under 4% strain, the number of repeat tensile fatigue rupture is 38,560 circles; Under 6% strain, the number of repeat tensile fatigue rupture is 30,378 circles, and the ultimate resumable strain of the wires is 10%. The other wires lmm in diameter cast in 75mm/min show a ultimate tensile strength of 910MPa, elongation of 18.76%. Under 4% strain, the repeat tensile fatigue life is 17,108 circles; Under 6% strain, the repeat tensile fatigue life is up to 16,215 circles, and the ultimate resumable strain of the wires is 10.5%. The recovery rates of all wires keep up 100% before fatigue rupture.As the casting speed increase, the grain diameter decreases and grain number in unittransversal area increases. The XRD results show that the crystal orientation varies between 13 in all studied casting speeds. The fatigue fracture morphologies showed that two factors affect the fatigue life: (1) the area and quantity of transverse grain boundary. (2) casting defects and surface quality. Longitudinal grain boundary has little effect on fatigue life.The Cu-Al-Ni SMA cast by OCC process have excellent mechanism, fatigue and memory performance. Through optimizing the process parameters, improving surface quality of wires, eliminating casting defects, enhancing casting speeds, increasing grain number in unit transversal area, reducing the size and quantity of transverse grain boundary, can be further improved the mechanical and fatigue performance.更多还原