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UDC 669.017The iron corner of the Fe-B-P phase diagram is plotted for room temperature based on metallographic and x-ray analyses. The structure and properties of phosphorus-doped iron hemiboride in Fe-3.8 wt.% B alloys containing 0 to 5 wt.% P are examined.Iron hemiboride Fe 2 B crystals grow as tetragonal prisms in slow-cooled alloys [1] and can contain fine inclusions of iron monoboride FeB [2,3]. Doping improves the mechanical characteristics and strength of iron hemiboride. It is shown in [4] that the hardness of iron hemiboride increases when it is doped with chromium. Doping with nickel insignificantly decreases the hardness of Fe 2 B crystals. The effect of vanadium on the structure and properties of Fe 2 B in alloys of the Fe-B-V and Fe-B-V-C systems is studied in [5]. It is established that the microhardness of iron hemiboride decreases, and its anisotropy remains constant. The anisotropy of the critical stress intensity factor K 1c decreases.No data have been found on what effect phosphorus has on the structure and mechanical properties of iron hemiboride Fe 2 B. Doping with phosphorus can help to obtain corrosion-resistant materials. Hence, our objective is to examine the effect of phosphorus on the structure, phase composition, micromechanical characteristics, and oxidation resistance of Fe-B alloys containing 3.8 wt.% B.To study borides Fe 2 B, we used alloys of the Fe-3.8% B and Fe-P-3.8% B systems. Phosphorus addition varied from 0.5 to 5 wt.%. The content of carbon was no more than 0.018%. To make alloys, we used high-purity carbonyl iron, amorphous boron (to 99.5 wt.% B), and ferrophosphorus . The melt cooling rate was 100°C/sec over the range of temperatures at which iron boride crystals grow. The chemical and phase compositions of the alloys are summarized in Table 1.The structure of the alloys was determined using chemical and thermal etching. A quantitative metallographic analysis of the Fe-B-P alloys was carried out with an Epiquant analyzer. The linear dimensions were determined with an accuracy of ±2% and the volume content of the phases with an accuracy of 3%. The phase composition of the alloys was examined in Fe-K α radiation using x-ray diffraction and a DRON-3M diffractometer. The microhardness, microbrittleness, and critical stress intensity factor (fracture toughness) K 1c were determined with a PMT-3 microhardness tester under a load of 0.5-2 N and calculated with the formulas given in [6,7].

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