Solid State Phenomena Vols. 172-174

Abstract: Austenite/ferrite phase transformations in Fe-xCu-10Ni alloys, 0<x<15 (mass%), are studied under two different cooling conditions, ice-brined quenching or slow cooling in the dilatometer. The influence of copper addition and cooling rate on the microstructure of the alloys is studied. Metallographic examinations of quenched samples show that metastable transformations occur during cooling. As for Fe-Ni alloys, it is impossible to stabilize the high temperature phase (γ_FeNi) in the Fe-Ni-Cu alloys. Dilatometry measurements of the γ → α transformation temperature with a cooling rate of 2°C/min also indicate a metastable phase formation despite the low cooling rate. For all alloys, a mixture of massive and lath ferrite is observed, one being predominant depending on the cooling conditions and composition. It is shown that the cooling rate has nearly no influence on the microstructure of alloys with a small amount of Cu unlike the alloys containing more Cu. In all alloys containing Cu, nanometric γ_Cu precipitates, much finer in the quenched samples, are detected in the ferrite grains.

Abstract: The effects of low temperature isothermal treatments on a quenched AISI D2 tool steel was studied using Barkhausen Noise, X-ray diffraction, dilatometry and optical and electronic microscopy. The specimens were austenitized at 1040°C and quenched in oil. The isothermal treatments involved immersion in hot oil baths at 80° or 130°C for 0.1, 0.6, 1, 3, 10 and 30 hours, except for the dilatometry, in which the specimens were submitted to a single thermal cycle for 30 hours. These thermal treatments are industrially known as “stress relief treatments”, and are used to prevent cracks and catastrophic failures during cryogenic treatments. The comparison of global and local (microscopy) measurements allows the discussion of the phenomena involved in the aging process.

Abstract: The precipitation behaviour of the Fe20Co18W (wt%) alloy was studied by transmission electron microscopy during aging treatments at 800°C. The decomposition of the matrix produces the C14 phase. At the beginning of the heat treatment, the observation at the atom scale indicates that the structure of the precipitates does not coincide exactly with the Laves phase. Using the orientation relationship between the Fe based matrix and the precipitates it is shown that simple atomic shifts can lead to the transformation from the bcc matrix to the C14 Laves phase.

Abstract: The growth of ferrite from alloyed austenite can take many forms: Widmanstätten ferrite and “plessites” in meteoric Fe-Ni-Co; ferrite layer growth under decarburization conditions; grain boundary precipitation and Widmanstätten ferrite and bainite in alloy steels. This contribution considers ways in which these different aspects of austenite decomposition can inform one another.

Abstract: Ferrite growth behavior in Fe-C-Mn alloys has been studied using controlled decarburization experiments. Two types of kinetic transition are considered. A first transition is proposed which involves a change from ParaEquibrium (PE) contact conditions at short times to Local-Equilibrium with Negligible Partitioning at longer times (LENP). This transition is attributed to the gradual build up of an alloying element spike due to the diffusion of Mn across the interface. The cross-interface mobility of Mn is estimated based on the experimental results. In some alloys, we observe a transition to extended PE states at high temperatures. A simple model which quantitatively describes the experimental observations over a range of composition and temperature is proposed. A key feature of this model is the introduction of an alloying element capacity of the moving ferrite/austenite interface, X*. The introduction of this quantity is purely guided by the experimental data and, at present, there is no physically based method for calculating it.

Abstract: Discontinuous precipitation is a solid-state transformation involving the decomposition of a supersaturated matrix into two phases arranged periodically as alternate lamellae or rods, which is accompanied by a grain boundary migration. The rate-limiting step of this process is supposed to be boundary diffusion of solute along grain boundaries. However, volume diffusion is generally present as well, and its influence on the occurrence of the discontinuous precipitation reaction is at present not well understood. We investigate this problem using a phase-field model in which the bulk diffusivity, surface diffusivity and grain boundary mobility can all be varied independently. The main results are that (i) when volume diffusion is the dominant mechanism, a close analogy is observed between the precipitate growth and the growth of a crystalline finger in a channel, and (ii) both the geometry of the precipitate’s tip and the growth velocity are strongly influenced by the relative magnitudes of the bulk and surface diffusivities as well as by the grain boundary mobility. Steady-state growth is possible only for a finite range of precipitate spacings, which is limited for low spacings by a fold singularity and for large spacings by an oscillatory or a tip-splitting instability. The values of these limits are found to depend on the supersaturation as well as on the ratio of bulk and surface diffusivities.

Abstract: Discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions have been observed in numerous alloy systems [1]. DP has been observed in the U-Nb system [2, 3, 4, 5]. The U-Nb phase diagram (Fig. 1) exhibits a continuous γ-BCC solid solution at high temperatures and a two-phase mixture of a-orthorhombic and γ-BCC below the 647°C monotectoid isotherm. The DP reaction occurs during continuous cooling and isothermal aging over 300-647°C. No metallographic evidence of a DC reaction in U-Nb has been published, although this is suggested from x-ray observations of distinct changes in the Nb content of the γ phase upon prolonged holding after the DP reaction [2, 3, 6]. This study will provide direct evidence for a DC reaction. Discontinuous and other aging reactions [7] are undesirable in U-Nb alloys, since they degrade corrosion resistance [5], ductility [8], and the shape-memory effect [9]. Hence, an improved understanding of the kinetics of these discontinuous phase transformations in U-Nb alloys is of practical interest.

Abstract: The original mixed-mode model is reformulated by considering the soft impingement effect and applying a general polynomial method of dealing with the concentration gradient in front of the interface. Comparison with the numerical solution shows that the reformulated mixed-mode model is more precise than the original model. The effect of soft impingement on the kinetics of partitioning phase transformation depends on both the growth mode and the degree of super-saturation.

Abstract: The microstructure formed in a steel after a partial martensitic transformation contains martensite-austenite assemblies with similar chemical composition in the two phases. In the absence of carbide precipitation, further annealing or tempering of this microstructure is believed to promote carbon partitioning from the martensite to the austenite. Experimental observations suggest that this carbon diffusion might take place in combination with migration of martensite-austenite interfaces. In this work, the effect of the martensite and austenite dimensions on the interaction between the carbon partitioning from martensite to austenite and the interface migration during annealing of martensite-austenite microstructures is analyzed. With that aim, simulations have been done by using a model in which the chemical potentials of carbon in martensite and austenite are assumed to be the same at the interface and motion of the phase interface is occurring when an appropriate driving force is present. Carbide precipitation is precluded in the model, and three different assumptions about interface mobility are considered, ranging from a completely immobile interface to the relatively high mobility of an incoherent ferrite–austenite interface.

Abstract: The excellent mechanical properties of austempered ductile iron are due to the unique matrix microstructure called “ausferrite”. Such microstructure is obtained from the first stage reaction during isothermal transformation of austenite into ferrite and high carbon austenite. The second stage reaction is the decomposition of high carbon austenite into ferrite and carbide. In this study, the microstructure of Cu-alloyed ductile iron treated by two-step austempering was investigated using SEM and TEM. The two-step austempering consists of 1) quenching from 900°C to 300°C and hold for 6 minutes in a salt bath, and 2) increasing the temperature of the salt bath by 30°C at the rate of 3°C per minute and holding until the total time was 120 minutes. The SEM samples were prepared by grinding to 1200 grit, polishing with 0.3 micron alumina powder and etching using 2% nital. Thin foil discs of 3 mm diameter for TEM observation were prepared by mechanical thinning to 80-100 microns and were then dimpled to 20 microns. Final thinning was carried out using Gatan precision ion polishing system (PIPS^TM). The thin foils were then examined using Jeol-JEM2010 operating at 200 kV. TEM results show that the matrix microstructure is ferrite subunits interspersed with retained austenite films. The subunits and austenite films exhibit either Kudjamov-Sachs (K-S) or Nishiyama-Wasserman (N-W) orientation relationships. Occasionally, fine carbide precipitate was observed.