An Atom Probe Study of Kappa Carbide Precipitation and the Effect of Silicon Addition

Abstract

The influence of silicon on j-carbide precipitation in lightweight austenitic Fe-30Mn-9Al-(0.59-1.56)Si-0.9C-0.5Mo cast steels was investigated utilizing transmission electron microscopy, 3D atom-probe tomography, X-ray diffraction, ab initio calculations, and thermodynamic modeling. Increasing the amount of silicon from 0.59 to 1.56 pct Si accelerated formation of the j-carbide precipitates but did not increase the volume fraction. Silicon was shown to increase the activity of carbon in austenite and stabilize the j-carbide at higher temperatures. Increasing the silicon from 0.59 to 1.56 pct increased the partitioning coefficient of carbon from 2.1 to 2.9 for steels aged 60 hours at 803 K (530°C). The increase in strength during aging of Fe-Mn-Al-C steels was found to be a direct function of the increase in the concentration amplitude of carbon during spinodal decomposition. The predicted increase in the yield strength, as determined using a spinodal hardening mechanism, was calculated to be 120 MPa/wt pct Si for specimens aged at 803 K (530°C) for 60 hours and this is in agreement with experimental results. Silicon was shown to partition to the austenite during aging and to slightly reduce the austenite lattice parameter. First-principles calculations show that the Si-C interaction is repulsive and this is the reason for enhanced carbon activity in austenite. The lattice parameter and thermodynamic stability of j-carbide depend on the carbon stoichiometry and on which sublattice the silicon substitutes. Silicon was shown to favor vacancy ordering in j-carbide due to a strong attractive Si-vacancy interaction. It was predicted that Si occupies the Fe sites in nonstoichiometric j-carbide and the formation of Si-vacancy complexes increases the stability as well as the lattice parameter of j-carbide. A comparison of how Si affects the enthalpy of formation for austenite and j-carbide shows that the most energetically favorable position for silicon is in austenite, in agreement with the experimentally measured partitioning ratios.

Structure and properties of κ-carbides in duplex lightweight steels

“…A model is applicable to predict the lattice misfit of these phases [19]. According to the orientation relationships between phases revealed by TEM (Figure 3 The low lattice misfit between κ-carbide and  grain is able to produce excellent mechanical properties [1,20,21]. On the other side, the low lattice misfit means small elastic strain energy.…”

Ab Initio-Based Modelling of the Yield Strength in High-Manganese Steels

“…The quantum-mechanical structure optimizations are performed based on calculations of lowest-energy states of the electron structure and thus, are conducted at 0 K. Results from ab initio approaches employing DFT have offered good results that are in line with experimental evidence. For example, Mössbauer spectroscopy [17], atom-probe tomography (APT) [42], in situ synchrotron X-ray diffraction measurements [23], and correlative TEM/APT approaches [43] have shown that results from ab initio simulations employing DFT are applicable to systems containing nano-sized κ-carbides that are usually formed during isothermal annealing at temperatures of 600 • C and above. Generally speaking, the effect of temperature changes on the lattice parameter and on intrinsic material properties that derive from electron effects like electrical conductivity is often approximated to be linear [44].…”

High Manganese and Aluminum Steels for the Military and Transportation Industry

“…Silicon was shown by ab initio calculations and experimental results to increase the activity of carbon in austenite and stabilize the j-carbide at higher temperatures. 18 The repulsive Si-C interaction was responsible for increasing the partitioning coefficient of carbon in j-carbide from 2.1 to 2.9 in 0.59% and 1.56% Si steels aged 60 h at 530°C. 18 Silicon was shown to partition to austenite during aging and decrease the austenite lattice parameter.…”

“…18 This study used a combination of transmission electron microscopy (TEM), 3-D atom probe tomography (APT), x-ray diffraction (XRD), and first principles modeling to study the precipitation. In solution-treated specimens, XRD and TEM results confirmed that, regardless of Si content, no spinodal decomposition or short-range ordering within the austenite had taken place.…”

“…17 Adding silicon has also been shown to increase the strength and hardness during aging but decrease work-hardening rates. 10,11,18 High-strain-rate fracture in these lightweight steels is also a function of phosphorus content and melt cleanliness. Phosphorus in levels greater than 0.06% P greatly increases the kinetics of age hardening but sharply decreases the Charpy V-notch (CVN) impact toughness by as much as 80%.…”