Fe-B合金是一种新型的金属耐磨材料，主要由金属基体和硼化物组成，前者起到支撑和固定硼化物的作用，后者主要抵抗磨料颗粒的磨损作用以保护基体。与传统用于磨料磨损的铁碳合金相比，Fe-B合金中硬质相Fe₂B的硬度更高，热稳定性更好；硬质相Fe₂B的体积分数主要受B含量的影响，而B在金属基体中固溶度极地，可以实现对硬质相体积分数和金属基体中碳含量的独立控制，兼顾合金的硬度和金属基体的韧性。然而，Fe₂B属于金属间化合物，其脆性较大，在磨损过程中，容易发生断裂和破碎现象，导致其保护金属基体的能力急剧下降，严重影响Fe-B合金的耐磨性。因此，如何提高硬质相Fe₂B韧性是当前Fe-B合金应用推广的关键问题。 课题组前期研究发现，掺杂Cr、Mo、W等合金元素可实现Fe₂B断裂韧性的提高，显著提高了合金的抗磨料磨损性能。然而，Mo、W等元素固溶于Fe₂B的量较少，且容易在合金中形成新的硼碳化合物，故本文选取Cr为掺杂韧化元素，以Fe₂B体积分数约为22 vol.％的Fe-3.0 wt％B合金为基础，通过改善Fe₂B的断裂韧性，以提高合金的抗磨料磨损性能。研究了Cr元素的在合金中的分布规律，及其对合金综合性能的影响，阐明了Cr掺杂韧化Fe₂B的机理，分析了Fe-B合金在不同工况下的磨料磨损机制。主要得出以下结论： Cr掺杂后，Fe-3.0 wt％B合金的相组成没有发生改变，经淬火后得到马氏体基体和硬质相Fe₂B两相组织。Cr元素倾向于富集在M₂B晶粒内(M代指金属原子Fe或Cr)，其浓度约为马氏体基体中的3.83倍。随着Cr含量增加，淬火后Fe-3.0wt％B合金的宏观硬度基本不变，保持在HRC54左右；马氏体基体(约HV530)和M₂B(约HV1520)的显微硬度均基本不变，而M₂B的断裂韧性先升高后降低。当合金中Cr掺杂量为2.0 wt％时，M₂B取得最高的断裂韧性值(约4.7 MPa·m〓)，比未掺杂时提高约9.2％。同时，Fe-3.0 wt％B合金的断裂韧性随Cr含量增加先升高后降低，Cr掺杂量为2.0 wt％时取得最大值为30.4 MPa·m〓，提高约38.5％。 韧化机理研究结果表明，Cr掺杂后，置换了Fe₂B晶粒中的部分Fe原子，但不改变其体心四方的晶体结构，引起B原子周围电子密度升高，增强了沿[002]方向的B-B共价键，提高了M₂B的断裂韧性。当Cr含量为2.5 wt％时，过量Cr原子掺杂导致晶粒内的晶格畸变累积，形成明显的晶格畸变区，导致断裂韧性反而降低。 Fe-3.0 wt％B合金主要的两体磨损机制为显微切削。随着Cr含量的增加，合金的两体磨损失重先降低后升高，Cr含量为2.0wt％时两体磨损失重最低，耐磨性能最好。在两种磨料条件下，两相交互作用失重均为负值，有利于降低两体磨损失重，提高合金的两体磨损耐磨性。M₂B对两相交互作用的贡献高于马氏体基体，且在硬度较低的SiO₂磨料条件下两相交互作用对合金耐磨性的影响高于SiC磨料。Cr掺杂韧化M₂B后，两相交互作用对耐磨性提高的贡献增大。M₂B断裂韧性的改善能显著提高Fe-3.0 wt％B合金的两体磨损耐磨性。对比在两种不同硬度磨料磨损条件下两相各自对另一相交互作用的贡献率可知：硬度较低的SiO₂磨料磨损条件下可进一步通过提高M₂B的含量来提高合金的耐磨性，而在硬度较高的SiC磨料条件下应适度降低脆性的M₂B含量来提高合金的耐磨性。 在SiC和SiO₂两种磨料条件下，Fe-3.0 wt％B合金主要的三体磨损机制为短程切削和接触疲劳。随着Cr含量的增加，合金的三体磨损失重均先降低后略有升高， 2Cr试样的磨损失重最小。Fe-3.0 wt％B合金中M₂B的断裂韧性对三体磨损失重变化产生主要影响。在低硬度的SiO₂磨料条件下，合金试样的三体磨损失重明显低于SiC磨料下，表现出更加优异的耐磨性能。在两种磨料条件下，M₂B对两相的三体磨损交互作用贡献均为正值，而马氏体基体对两相交互作用贡献均为负值，说明M₂B的存在不利于合金的三体磨损耐磨性能，而马氏体基体的存在有利于合金耐磨性的提高。因此，适度降低M₂B的含量会有利于合金耐磨性的提高。在Cr掺杂韧化M₂B后， M₂B对两相交互作用的贡献值降低，说明提高M₂B韧性有利于减弱其对三体磨损耐磨性的不利影响，因此可进一步改善M₂B的韧性以获得更高的耐磨性。此外，当磨料的硬度降低时，M₂B对两相交互作用的不利影响明显减小，说明在低硬度磨料条件下合金可表现出较好的耐磨性。对比两种不同硬度磨料下M₂B对两相交互作用的贡献率可知，虽然M₂B的存在不利于Fe-B合金的三体磨料磨损性能，但如能进一步改善M₂B的韧性或在硬度比SiO₂更低的磨料条件下可使M₂B发挥抵抗磨料磨损的作用，从而使合金的耐磨性提高。 关键词：Fe-B合金；M₂B；断裂韧性；磨料磨损；交互作用 论文类型：应用基础 本研究得到国家自然科学基金(编号：51371138和51571159)资助

 Fe-B alloy is a new kind of metallic wear-resistant material, composed of the metallic matrix and boride (Fe₂B). The former supports and fixes the hard phase (Fe₂B), and the latter resists the abrasive to protect the matrix. Compared with Fe₃C in the traditional Fe-C alloy, Fe₂B, as the hard phase in Fe-B alloy, has higher hardness and better thermostability. In addition, the volume fraction of Fe₂B is mainly affected by B content. So it is possible to separately control the volume fraction of the hard phase and C content in the matrix to simultaneously adjust the alloy's hardness and matrix's toughness. However, Fe₂B belongs to intermetallic, showing high brittleness. And Fe₂B is easy to be fractured and broken due to the impacting during the wear process. Therefore, the brittle Fe₂B cannot protect the metallic matrix effectively, extremely reducing the wear resistance of Fe-B alloy. Hence, how to improve the toughness of Fe₂B is the most urgent problem for the application of Fe-B alloy. The previous study results showed, with the addition of the transition metal elements such Cr, Mo, W etc., the fracture toughness and wear resistance of Fe-B alloy can be dramatically improved. In this work, Fe-3.0 wt% B alloys with 22 vol% Fe₂B were prepared firstly. Through the addition of a series of Cr, we tried to improve the toughness and wear resistance. The distribution of Cr and the influence on Fe-B alloy's properties were analyzed quantitatively. The toughening mechanism of Cr addition and the improvement of wear resistance were clarified systematically. The main results are as follows: After Cr addition, the phase composition of Fe-3.0 wt% B alloy changes little, composed of martensite and Fe₂B. Cr tends to enter into M₂B grains (M representing for Fe or Cr). Cr content in M₂B is 3.83 times as that in martensite. With the increase of Cr content, the macrohardness of Fe-3.0 wt% B alloy has little change, floating around HRC54; the micro-hardness of martensite and M₂B has little change; the toughness of M₂B increases firstly and then decreases. With Cr content of 2 wt%, M₂B shows the highest fracture toughness of 4.704 MPa·m〓, improved by 9.2% compared with Fe₂B without Cr addition. At the same time, the fracture toughness of Fe-3.0 wt% B alloy increases firstly and then decreases. With Cr content of 2 wt%, the alloy obtains the highest toughness of 30.4 MPa·m〓, improved by 38.5%. The study of toughening mechanism shows that Cr can replace for part of Fe in Fe₂B after added. Without changing the tetragonal lattice, Cr addition can cause the increase of electron density surrounding B atom. The B-B covalent bond along [002] is strengthened, improving the toughness of M₂B. When Cr addition reaches 2.5 wt%, excessive Cr will enter into M₂B, resulting in the accumulation of the mismatch of the crystal lattice, causing the reduction of the fracture toughness. The two-body wear mechanism of Fe-3.0 wt% B alloy is micro-cutting. With the increase of Cr content, the two-body wear weight loss decreases firstly and then increases. 2Cr sample shows the lowest wear weight loss, showing the best wear resistance. In case of both abrasives, the wear interactions between two phases are negative, benefiting to decreasing the wear weight loss. M₂B makes a higher contribution to the interaction than martensite. The interaction in case of SiO₂ is higher than that in case of SiC. After toughening M₂B by Cr addition, the contribution of the interaction increases to the help improve the wear resistance. The improvement of M₂B toughness can play a positive role in improving the two-body wear resistance. Comparing the contributions to the wear interactions from each phase, it can be concluded that, further improving the toughness of M₂B can help improve the wear resistance in case of SiO₂, while decreasing the volume fraction of M₂B in some degree can improve the wear resistance of Fe-B alloy in case of SiC. In case of both SiC and SiO₂ abrasives, the three-body wear mechanisms are short-distance microcutting and fatigue. With the increase of Cr content, the wear weight loss decreases firstly and increases, and 2Cr sample obtains the lowest wear loss. The fracture toughness of M₂B plays a vital role in the improvement of the wear resistance. In case of SiO₂, the alloy shows better wear resistance. In case of both abrasives, M₂B makes a positive contribution to the wear interaction while martensite makes a negative one, implying that M₂B is bad for the wear resistance of the alloy. Therefore, appropriately decreasing the fraction of M₂B is good for the higher wear resistance of Fe-B alloy. After toughening M₂B by adding Cr, the contribution of M₂B to the interaction gradually decreases, implying that improving the toughness of M₂B can help weaken the bad influence on the three-body wear resistance. Additionaly, with the decrease of the abrasive's hardness, the bad affection of M₂B to the interaction decreases, implying the wear resistance can be improved by decreasing the abrasive's hardness. In this case, in spite of the bad affection of M₂B on the three-body wear performance, it's possible that, improving the toughness and decreasing the abrasive's hardness can make the role of M₂B transform from negative to positive, in order to improving the wear resistance of Fe-B alloy. KEY WORDS: Fe-B alloy; M₂B; Fracture toughness; Abrasive wear; Interaction TYPE OF DISSERTATION: Application Fundamentals