Gerhard Inden

Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation

Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility).

Experimental determination of phase equilibria in the FeAlC system

Abstract The phase equilibria in the FeAlC system have been determined between 800 °C and the liquidus surface. From cast alloys the liquidus surface was established. From electron microprobe analyses (EPMA) of quenched samples three isothermal sections at 800, 1000 and 1200 °C have been obtained. Additional high temperature X-ray diffraction experiments (HT-XRD) yielded three vertical sections and the temperatures of four invariant reactions. The influence of carbon on the transition temperature between the disordered (A2) and the ordered (B2) α-solid solution was determined from HT-XRD experiments with single crystals. This transition is shifted remarkably to higher temperatures by the addition of carbon. Special emphasis was placed on establishing the homogeneity range of the K-phase and its dependence on temperature. This dependence is discussed in terms of order/disorder. In addition, the properties of the K-phase, lattice constant as function of chemical composition, microhardness and thermal expansion coefficient α, have been determined.

Deformation behaviour of iron-rich iron-aluminum alloys at low temperatures

Abstract The deformation behaviour of binary monocrystalline and polycrystalline Fe-Al alloys with Al contents up to 18 at.% and only low unavoidable impurity contents—in particular less than 100 wt.ppm C—has been studied at room temperature and −100 °C. The effects of quenching and annealing treatments on the behaviour of as-cast materials were investigated in order to clarify the dependence of strength and ductility on Al content and short-range ordering. It was found that the stress-strain behaviour at low temperatures is controlled primarily by Al solid-solution hardening and quenched-in excess vacancies with only minor effects of short-range ordering.

Ordering and phase separation in the b.c.c. phase of the Fe-Al-Ti system

Ordering and phase separation in {alpha}-Fe alloys of the Fe-Al-Ti system have been investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and electron-probe microanalysis (EPMA). The results indicate that the partial replacement of Fe with Ti in the Fe-rich Fe-Al binary alloys results in drastically stabilizing the B2 (FeAl) and D0{sub 3} (Fe{sub 3}Al) phases and increasing the A2-B2-D0{sub 3} successive order-disorder transition temperatures. It is confirmed that there are two kinds of phase separations of the BCC phase, (A2 + D0{sub 3}) and (B2 + D0{sub 3}) in the composition range below 25 at.% Al. The width of the two-phase region drastically increases with decreasing Al composition. The two-phase regions close at ternary tricritical points. Calculations of BCC phase equilibria have been performed with the cluster variation method (CVM) using the irregular tetrahedron approximation. First and second nearest pair interactions as well as tetrahedron interactions are taken into account. Starting from binary and ternary phase equilibria the numerical values of these interactions have been determined and an excellent agreement between calculation and experiments was obtained.

The Fe-A1-Ti system

An extensive experimental investigation of the Fe-Al-Ti system by metallography, microprobe analysis, and XRD on quenched specimens and on diffusion couples is presented. Two isothermal sections at 800 and 1000 °C were established; they differ substantially from the existing (800 °C) or partly determined (1000 °C) diagrams. From these results, existence of the τ1 phase (Fe2AlTi) can be ruled out. Existence of the ternary compounds, τ2 (Al2FeTi) and τ3 (Al22Fe3Ti8), is confirmed. The composition limits of both phases were determined; they differ considerably from those given in earlier reports. The τ2 phase apparently exists in a cubic and a tetragonal polymorph, depending on composition. The cubic form exists at high titanium contents. At 1000 °C, the two polymorphs are separated by a miscibility gap. At compositions where the “X phase” (Al69Fe25Ti6) was previously reported, single-phase samples were obtained at both temperatures. From the present results, there is no evidence to assume that this is a new ternary phase rather than the ternary homogeneity range of the Al3Fe phase. In addition, extensions of the binary intermetallic phases into the ternary system were determined.

Ordering and segregation reactions in b.c.c. binary alloys

Abstract Within the Bragg-Williams-Gorsky approximation the most stable atomic configurations of b.c.c. solid solutions will be determined in dependence on two energy parameters W (1) and W (2) , the interchange energies between nearest and next-nearest neighbors. The most stable atomic configurations of homogeneous solid solutions (configuration diagrams) are presented for a representative set of values out of the whole range of W (1) and W (2) . From these results the most stable states of solid solutions, if heterogeneous concentration distributions are permitted (in the phase diagrams), are determined.

Phase equilibria in the AlNbTi system at high temperatures

Abstract Phase equilibria in the AlNbTi system were experimentally determined. From the investigation of diffusion couples and alloys of fixed composition by optical microscopy, electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and X-ray diffraction (XRD) two isothermal sections at 1000 ° and 1200 °C were established. At 1200 °C earlier investigations postulated the existence of several intermetallic phases. From the present study the existence of the phases γ1 (NbTiAl3, tetragonal), T1 (“Ti45Al35Nb20”, cubic B2) and T2 (“Ti45Al45Nb10”, cubic B2) at this temperature can be ruled out and the stability of the phase α ∗ (“Ti4Al3Nb”, hexagonal) is at least questionable. At 1000 °C a ternary single-phase field of isolated B2-ordered β-Nb,Ti is observed which stems from a quasi eutectoid reaction below 1200 °C. While there has been only scarce information of the phase equilibria at 1000 °C the results presented at 1200 °C are discussed in respect to existing isothermal sections which have been determined recently at this temperature.

Solid-phase equilibria in the Ti-rich part of the TiAl system

Abstract The solid-state-phase equilibria in the Ti-rich portion of the TiAl system (0–50%Al) have been investigated at temperatures between 800°C and 1415°C by electron probe microanalysis (EPMA) and by transmission electron microscopy (TEM) on diffusion couples as well as on annealed alloys. The TEM observations on the diffusion couples show that the phase field α 2 , (ordered hexagonal DO 19 ) extends up to the β (bcc A2) phase field, leading to two peritectoid transformations, β + α → α 2 at T = 1210 ± 10° C and β + α 2 → α at T = 1160 ± 10° C (α: hexagonal A3). The diffusion-couple experiments yield the tie lines of the observed phase equilibria. The present results confirm the existence of a eutectoid transition α → α 2 + λ (λ: ordered fcc Ll 0 ) at about T = 1120 ± 10° C . An updated phase diagram based on these data is presented.

Phase transformation in Fe-Mo-C and Fe-W-C steels : I. The structural evolution during tempering at 700°c

Mechanism and kinetics of carbide transformation during tempering at 700°C have been studied in Fe-Mo-C and Fe-W-C steels (with up to 2.5% W or Mo) by transmission electron microscopy (TEM) and X-ray diffraction. The sequence of carbide formation is Fe3C→Mo2C→(Fe2MoC, M23C6) in molybdenum steels and Fe3C→M6C→M23C6 in tungsten steels. Increasing the alloying element level increases the rate of carbide replacement reaction. In Fe-Mo-C steels the Fe2MoC carbides nucleate preferentially at the Fe3C-α interface and grow into cementite, whereas in Fe-W-C steels the M6C carbides usually precipitate inside cementite giving rise to the in situ transformation Fe3C→M6C. The software DICTRA and THERMO-CALC were used to simulate cementite growth and to show the possibility of the in situ transformation. The M23C6 carbide is first confined to prior austenite grain boundaries and penetrates into the grains with increasing tempering time. During growth the M23C6 carbide absorbs surrounding pre-existing carbides. As a result, after tempering for 500 h, patches of two-phase areas with (M23C6 + α) or (M6C + α) are observed in tungsten steels, and patches of (M23C6 + α) or (Fe2MoC + α) in molybdenum steels. The alloying element partitioning between α and precipitated carbides was determined using TEM-EDS. It was established that the M23C6 carbide is stable at 700°C in both investigated steels. The Fe2MoC carbide is stable in the Fe-Mo-C system at this temperature. The MC carbide was not observed even after tempering for 3000 h.

Tetrahedron approximation of the cluster variation method for b.c.c. alloys

Abstract The tetrahedron approximation of the cluster variation method is formulated for b.c.c. alloys for any number of components. This includes the possibility of treating ferromagnetic alloys. First and second nearest neighbour interactions are taken into account. For a representative selection of values of these interactions the phase diagrams are presented. Tricritical points have been determined accurately by an analysis of the second Hessian determinant. With the same energy parameters Monte Carlo simulations have also been performed to test the quality of tetrahedron approximation for the calculation of phase diagrams. Good agreement was obtained between the phase diagrams calculated by both methods. The tetrahedron treatment of the cluster variation method is thus a good approximation for the calculation of phase equilibria in b.c.c. alloys.