【作者】 董桂馥；

【导师】 蔡伟；

【作者基本信息】 哈尔滨工业大学 ， 材料物理与化学， 2008， 博士

【摘要】 本文采用扫描电镜观察、透射电子显微分析、X射线衍射分析、示差扫描热分析、交流磁化率测试、室温压缩试验及磁感生应变测量等方法系统研究了Ni53Mn23.5Ga23.5-xTix (x=0, 0.5, 2, 3.5, 5, 8)合金时效析出行为、相变行为、力学行为和磁学特性,阐明了时效过程中组织演化规律及其对马氏体相变温度、力学性能和磁感生应变的影响规律,揭示了时效改善合金力学性能的微观机制。研究表明,固溶处理的Ni-Mn-Ga-Ti合金室温为四方结构5M马氏体。在773K-1173K范围内时效,基体中弥散析出六方结构的Ni3(Ti,Mn,Ga)相。Ni-Mn-Ga-Ti合金在加热和冷却过程中发生一步热弹性马氏体相变,随Ti含量的增加,相变温度升高。时效处理对合金的相变温度有显著影响。当时效时间为3h时,相变温度随时效温度的升高先降后升,在973K时达到最小值;当时效温度为873K时,相变温度随时效时间的延长而降低。这是因为富Ni的Ni3(Ti,Mn,Ga)相的析出,改变了基体中Ni含量所致。试验结果表明,随Ti含量增加,合金的断裂强度及断裂应变先增后降,在Ti含量为5at.%时达到最大值。合适的时效处理显著改善Ni53Mn23.5Ga23.5-xTix合金的力学性能。在823K-1173K温度区间3h时效时,合金的断裂强度和断裂应变均随时效温度的升高先增加后降低。在873K时效时,Ni-Mn-Ga-Ti合金的断裂强度和断裂应变均随时效时间的延长而降低,经873K/0.5h时效后,断裂应变达到最大值11.9%,比Ni-Mn-Ga合金高1倍以上。经873K/3h恒应变约束时效处理Ni53Mn23.5Ga18.5Ti5合金的断裂强度高达1403MPa。时效改变了合金的断裂方式,固溶Ni-Mn-Ga-Ti合金的断口形貌呈现脆性沿晶断裂特征;随时效温度的升高或时间的延长,断裂方式逐渐转变成穿晶断裂,断口上存在较多的韧性撕裂棱;当时效温度高于973K时,合金的断口形貌为沿晶剥离,导致脆性增大,其原因在于析出相在晶界面上的数量增多、尺寸增大。固溶Ni-Mn-Ga-Ti合金的居里温度及饱和磁化强度均随Ti含量的增加而近线性降低;其居里温度随时效温度的升高先降低后升高,在973K时为最低值;而饱和磁化强度随时效温度的升高而略有下降。研究发现,合金的磁感生应变量随Ti含量的增加而下降,时效及热机械训练均可显著能提高合金的饱和磁感生应变量,其微观机制在于:合金在时效过程中形成弥散共格Ni3(Ti,Mn,Ga)析出相,在基体中产生共格应力场,在冷却发生马氏体相变时,处于有利取向的马氏体变体首先形核、长大,室温形成择优取向的马氏体,显著提高合金的饱和磁感生应变量。更多还原

【Abstract】 The aging precipitation behavior, martensitic transformation characteristics, mechanical properties, magnetic properties of Ni53Mn23.5Ga23.5-xTix (x=0, 0.5, 2, 3.5, 5, 8) alloys have been systematically investigated by means of SEM, TEM, XRD, DSC and compression test as well as ac susceptibility measurements and MFIS tests. The precipitation growth behavior of the second phase has also been illuminated. Moreover, the effect of the aging second phase on the martensitic transformation, mechanical properties and magnetic properties has been revealed. In addition, the mechanism of improving the mechanical property by the aging second phase has been studied.The results show that the crystal structure of solution-treated Ni-Mn-Ga-Ti alloys is 5M martensite at room temperature. The second phase of Ni3(Ti,Mn,Ga) with hexagonal structure is precipitated in the matrix of Ni53Mn23.5Ga23.5-xTix alloys between 773K and 1173K.Ni-Mn-Ga-Ti alloys exhibit one-step thermoelastic martensitic transformation during the heating and cooling procedure. The martensitic transformation temperatures of the studied alloys increase with increasing Ti content. Moreover, it is found that aging treatment has obvious effect on the martensitic transformation temperatures. For Ni-Mn-Ga-Ti alloys with 3 h aging time, the martensitic transformation temperatures first decrease and then increase with the increase of aging temperature. When the aging temperature is fixed at 973 K, the martensitic transformation temperatures reach its minimum value. When the aging temperature is 873 K, the martensitic transformation temperatures decrease with the increase of aging time.The experimental results show that the fracture strength and strain first increase and then decrease with increasing the Ti content, and reaching the maximum value when Ti content is 5 at.%. In addition, it is noted that proper aging treatment improves the mechanical property of Ni-Mn-Ga-Ti alloys significantly. When the studied alloys aging between 873 K and 1173K, the fracture strength and strain increases first increase then decrease with the increase of aging temperture.When the aging temperature is fixed at 873K, the fracture strength and strain decrease with the increase of aging temperature. In particular, the fracture strain of Ni53Mn23.5Ga18.5Ti5 alloy aged at 873 K for 0.5 h is 11.9%, which is two times higher than that of Ni-Mn-Ga alloy. Ni53Mn23.5Ga18.5Ti5 alloy after constraint aging at 873 K for 3h, the fracture strength of it reaches 1403 MPa.The aging-treatment changes the fracture type of Ni-Mn-Ga-Ti studied alloys. The fracture morphology of solution-treated Ni-Mn-Ga alloys exhibit brittle intergranulal fracture. With the increase of aging temperature and time, the fracture type gradually becomes ductile intragranular fracture. However, as the aging temperature is higher than 973 K, the fracture morphology changes to peeling along the grain boundary due to high ratio of precipitation in grain boudary, resulting in the increases of brittle.The Curie temperature and saturation magnetization decrease linearly with the increase of Ti content in solution-treated Ni-Mn-Ga-Ti alloys. The Curie temperature first decreases and then increases with increasing aging temperature, reaching the minimum value at 973 K. While, the saturation magnetization decreases slightly with increasing aging temperature. In addition, it is found that the MFIS decreases with increasing Ti content. Both the aging and thermo-mechanical training can raise the saturated MFIS significantly. The mechanism for the effect mentioned above can be attributed to the coherent stress-field in the matrix, which is generated by the dispersed coherent Ni3(Ti,Mn,Ga) precipitation. When the martensitic transformation occurs during cooling, the martensitic variant with the highest orientation factor would grow first and become the preferred orientation martensite at room temperature, which leads to the significant raise of the MFIS.更多还原