Synthesis, Crystal Structure, Magnetic Properties, and Stability of the Manganese-Rich “Mn3AlC” κ Phase

κ-carbides
of varying composition, seemingly responsible for age hardening in
high-Al steel alloys, have been detected to precipitate both at grain
boundaries and in the bulk grain of steels. Herein we report the bulk-phase
synthesis of “Mn3AlC” by arc plasma sintering
and rapid solidification. Single crystals have been found suitable
for X-ray diffraction using Mo radiation and yield a lattice parameter
of a = 3.875(2) Å. We find a mixed occupation
of the 1a position by Al and Mn, which, together
with the C position being fully occupied, leads to the actual composition
Mn3.1Al0.9C. Additional energy-dispersive X-ray–scanning
electron microscopy measurements support the composition and corroborate
the homogeneity. SQUID data collected on the polycrystalline ferromagnetic
sample exhibit a Curie temperature of about 295 ± 13 K and a
soft magnetic behavior. The small but significant nonstoichiometry
on 1a leads to a slightly larger lattice parameter,
a higher electron count, and, thus, a lowered density of states at
the Fermi level, indicative of increased phase stability.

本研究发现，不同组成的κ-碳化物，似乎是导致高铝钢合金时效硬化的原因，这些碳化物在钢的晶界和晶粒体中均被发现有析出。本研究通过电弧等离子体烧结和快速凝固技术，实现了“Mn3AlC”的体相合成。研究发现，单晶适合使用Mo辐射进行X射线衍射，并得到晶格常数a = 3.875(2) Å。我们观察到Al和Mn在1a位置上存在混合占据，而C位置则完全占据，导致实际组成为Mn3.1Al0.9C。额外的能量色散X射线-扫描电子显微镜测量结果支持了该组成，并证实了其均匀性。在多晶铁磁样品上收集的SQUID数据表现出约295 ± 13 K的居里温度和软磁性行为。在1a位置的微小但显著的非化学计量比导致了晶格参数的略微增大、电子数增加，进而导致费米能级处的态密度降低，这表明了相稳定性的增加。