Effects of Trace Additions of Magnesium on Microstructure and Properties of Fe–36Ni Invar Alloy

Abstract

extremely low coefficient of thermal expansion (CTE). [1,2] It is widely used in a variety of applications, including the standardization of the metric system, the mold of composite materials, liquid natural gas tanks, and long-distance power cables. [3][4][5] The Invar family has developed vigorously, which has expanded to Fe-Ni-Co, Fe-Ni-Cr, and Fe-Ni-Mn series. [6] Invar alloy by the pyrometallurgy method possesses the facecentered-cubic (FCC) structure and thus generally exhibits outstanding ductility and relatively poor strength for structural applications. [7] Extensive applications of the Invar alloy are severely restricted due to poor mechanical performance.Recently, numerous studies have paid attention to the behavior of magnesium (Mg) microalloying in metallurgy. It is well known that Mg exhibits fairly high chemical activity and extremely high vapor pressure. [8,9] The trace amount of Mg can be enriched in the molten steel after Mg treatment. However, the trace amount of Mg in the molten steel can play a significant role in the evolution of microstructure and properties of materials. Zhu et al. reported that generating Mg-containing nanosized particles can be pinned on the grain boundaries and inhibit the austenite grain growth, resulting in a decrease in grain size after Mg treatment. [10] The average austenite sizes were found as 385 and 86 μm for 0 and 50 ppm of Mg additions, respectively. Jiao et al. demonstrated that the low-density Mg-containing inclusions easily floated out of the molten steel, which contributed to the matrix achieving effective purification by Mg treatment. [11] Furthermore, the improvement of hot ductility of Invar alloy by utilization of Mg treatment was widely reported. He et al. reported that the hot ductility behavior of wrought Fe-36Ni Invar alloy with 49 ppm Mg had a notable increase. [12] Abbasi et al. studied the functions of microalloying additions with Al, Zr, Ti, and Mg on the hot ductility of Fe-36Ni Invar alloy. [13] The hot ductility of Fe-36Ni Invar alloy with 0.0030 wt% Mg addition was improved most obviously, which was attributed to effectively decreasing the harmful effect of S on the grain boundaries. Adding trace Mg is also recognized as an effective way of enhancing mechanical performance. Gong et al reported that the mechanical properties of Cr12Mo1V1 steel steadily were enhanced with the increase of Mg additions. [14] The tensile strength and elongation increased from 1320 to 1670 MPa and from 17.5% to 22.0%, respectively, when the Mg content increased from 0 wt% to 0.0041 wt.%. Lv et al. investigated that the yield strength, tensile strength, and elongation were