Chu et al. from the School of Materials Science and Engineering at Shanghai University of Technology adopted an innovative three functional co nano precipitation strategy to improve the mechanical properties of low dislocation energy (SFE) 17Mn steel in low-temperature applications. By combining intense cold deformation and subsequent annealing, a layered structure was observed characterized by (Ti, Nb) C carbides (approximately 10 nm) and Cu rich intermetallic compounds (approximately 2 nm) in the austenitic matrix, with uneven grain size distribution. Compared with single precipitation (SP) steel, eutectoid (CP) samples exhibit excellent properties, with a yield strength of 1150 MPa, a tensile elongation of 44.8%, and an impact toughness of 110 J at liquid nitrogen temperature (LNT), even exceeding the matrix -17Mn steel. The CP-17Mn sample exhibits higher density and thinner nanotwins under larger strains, leading to a rapid increase in geometrically necessary dislocations (GND). In tensile and impact tests, harmful martensitic transformation was effectively suppressed. The observed trade-off between inverse strength ductility and strength toughness can be attributed to the role of three functional eutectoids: they provide dispersion strengthening, induce structural heterogeneity, and serve as effective barriers for twin thickening. Large sized (Ti, Nb) C carbides contribute to grain refinement and nail boundary migration, while smaller Cu rich intermetallic compounds inhibit the growth and thickening of nanotwins due to their strong stress field in twin precipitation interactions, preventing further dislocation movement. This new mechanism paves the way for the development of high-performance steels with fine and dense nano twins at low temperatures.
Figure 1. (a) Engineering tensile stress-strain curves of samples annealed at different temperatures under RT and LNT, (b) Real strain hardening rate curve, (c) Dynamic compressive stress-strain curve, and (d) Comparison of mechanical properties of various metal materials under LNT.
Figures 1a-b show the engineering stress-strain and corresponding strain hardening rate (SHR) curves of various 17Mn steels under RT and LNT. At room temperature, the YS (about 633 MPa) and ultimate tensile strength (about 903 MPa) of SP-17Mn sample are significantly lower than those of CP-17Mn sample (about 744 MPa and about 982 MPa), with differences of about 111 MPa and 79 MPa, respectively. On the contrary, the tensile elongation (TEL) of SP-17Mn is higher, at 55.2%, while CP-17Mn is 43.6%. The TEL of the base-17Mn material is the highest (about 62.0%), but the YS is the lowest (about 505 MPa). Under LNT conditions, the YS (about 1150 MPa) and UTS (about 1470 MPa) of the CP-17Mn sample significantly increased, while its tensile ductility slightly increased (about 44.8%). However, the tensile strength of the SP-17Mn sample increases with decreasing temperature, but its TEL significantly decreases to about 32.7%. The strength ductility trade-off phenomenon also appeared in the base-17Mn sample. Table 1 summarizes the tensile properties of various 17Mn steels at different temperatures. CP-17Mn exhibits the best performance in terms of strength and tensile elongation, at approximately 57GPa.%. At the same time, compared with RT, LNT also showed a stable high SHR value (about 3000 MPa) throughout the low-temperature deformation stage.
Figure 2. IPF diagrams (a1, a2), phase diagrams (b1, b2), and corresponding particle size distributions of CP-17Mn and SP-17Mn samples characterized by EBSD. (d) XRD patterns of CP-17Mn and SP-17Mn samples.
Figure 2 shows the initial inverse pole figure (IPF), phase diagram, particle size distribution, and XRD pattern of CP-17Mn and SP-17Mn samples. Both samples exhibit predominant face centered cubic (FCC) recrystallized gamma austenite with random orientation, accompanied by a small amount of hexagonal close packed (HCP) e-martensite, as confirmed by XRD patterns characterized by e-martensite peaks. Both types of steel exhibit heterogeneous grain structures, characterized by a wide range of grain sizes, including coarse grains (CG>1 um) and fine grains (FG<1 um). Table 2 provides quantitative data on the distribution, number fraction, and average size of these heterogeneous domains in these two steels. Surprisingly, the average grain size (AGS) values of CP-17Mn and SP-17Mn samples showed only a small difference, with SP-17Mn having a slightly higher CG proportion. This is in sharp contrast to previous studies on precipitation free 22Mn steel, where a higher annealing temperature of 740 ℃ resulted in significant grain growth and significantly larger AGS values.
Figure 3. (a1, b1, c1, d1) Phase diagrams, (a2, b2, c2, d2) KAM plots, and corresponding average KAM distributions of CP-17Mn and SP-17Mn samples under LNT with intermittent tensile strains of 15% and 30%, (e, f, g, h).
Figure 3 shows the EBSD images of deformed CP-17Mn and SP-17Mn samples under LNT. As the strain increases, both samples retain a large amount of austenite, and the volume fraction of ε - martensite only slightly increases. In addition, the austenite grains are significantly elongated and refined. The nuclear average misorientation map (KAM) provides insight into the distribution of geometrically necessary dislocations (GND), which also increases with the progression of deformation. Both deformed samples exhibit non-uniform KAM distribution. To analyze heterogeneity, two representative FG regions (represented by white boxes as sub1) and CG regions (represented by yellow boxes as sub2) were selected.
The relevant research results were published in the International Journal of Plasticity (Volume 178, July 2024, Article number 104014) under the title "Tri functional co nanoprecipitates enhanced cryogenic conductivity by inducing structural heterogeneity and refining nano winds in a low stacking fault energy 17Mn steel". The first author of the paper is Xiaoli Chu, and the corresponding authors are Xiaoshuai Jia and Yu Li.
