Retained free energy as a driving force for phase transformation during rapid solidification of stainless steel alloys in microgravity

Abstract

Ternary Fe-Cr-Ni stainless steel alloys often exhibit a multi-step transformation known as double recalescence where primary ferrite converts to austenite during rapid solidification processes such as casting and welding. In addition to the volume free energy associated with undercooling between the phases, the free energy driving the transformation comes from two additional sources that are retained within the metastable solid—one from the primary phase undercooling and one from melt shear. A new physical model is proposed based on accumulation of defects, such as dislocations or tilt boundaries, and lattice strain. A dimensionless analysis technique shows that the free energy associated with metastable solidification is conserved and the contribution from melt shear can be predicted based on a modification of the Read-Shockley dislocation energy equation. With these additional terms the incubation time between nucleation events becomes inversely proportional to the total free energy squared for bulk diffusion and cubed for grain boundary diffusion mechanisms. In the case of the ferrous alloys studied, the grain boundary mechanism provides a better fit and when the model is applied the delay time behavior collapses to a single master-curve for the entire alloy family.

MHD surrogate model for convection in electromagnetically levitated molten metal droplets processed using the ISS-EML facility

“…Future studies may be able to correct for the influence of convection on their measurements, improving accuracy. For solidification studies, the model can also quantify the convection at the time of solidification, which is recognized as a key variable 6,17 .…”

Influence of Induced Convection on Transformation Kinetics During Rapid Solidification of Steel Alloys: The Retained Damage Model

“…5. 19 The constants in this equation are empirically determined from experimental data from a plot of the dependent variable, DG c =_ c, as a function of the independent variable, ln (_ c), which yields a straight line with a negative slope of D m and an intercept, D b .…”

“…This is functionally equivalent to assigning an appropriate b to characterize the system attachment flux, based on an empirically evaluated constant cluster attachment success rate. 19 The two dimensionless groups that are used are independently identified by the two measurable quantities, the undercooling and the delay time. The dimensionless free energy parameter, N M , is defined as the ratio of the total free energy, DG T , (which in turn is a function of observed undercooling) to the baseline reference minimum undercooling defined by the phase diagram, DG R .…”

“…2 produces a relationship where the delay, s, becomes inversely proportional to the free energy change, DG, raised to a characteristic exponent which is geometry-and mechanism-dependent. [17][18][19] Evaluation of experimental data indicates that the best fit for FeCrNi is obtained when the attachment rate, b, is constant with time such that this exponent becomes a value of four, 20 as shown in Eq. 4; these results are functionally consistent with the work proposed by Kantrowicz 21 and Kashchiev: 22…”

The Effect of Cu-Substitution on the Microstructure and Magnetic Properties of Fe-15%Ni alloy Prepared by Mechanical Alloying

“…Recently, nanostructured materials have attracted great attention due to their excellent and unique electrical, magnetic, optical, catalytic, mechanical, and biological properties for various applications [1][2][3][4][5][6][7][8][9][10][11]. Several techniques including electrodeposition, rapid solidification [12][13][14], gas condensation, solid-state processing like friction stir welding (FSW) and mechanical alloying (MA) have been utilized to make nanostructured materials [15].…”