Accessing Colony Boundary Strengthening of Fully Lamellar TiAl Alloys via Micromechanical Modeling

Abstract

In this article, we present a strategy to decouple the relative influences of colony, domain and lamella boundary strengthening in fully lamellar titanium aluminide alloys, using a physics-based crystal plasticity modeling strategy. While lamella and domain boundary strengthening can be isolated in experiments using polysynthetically twinned crystals or mircomechanical testing, colony boundary strengthening can only be investigated in specimens in which all three strengthening mechanisms act simultaneously. Thus, isolating the colony boundary strengthening Hall–Petch coefficient KC experimentally requires a sufficient number of specimens with different colony sizes λC but constant lamella thickness λL and domain size λD, difficult to produce even with sophisticated alloying techniques. The here presented crystal plasticity model enables identification of the colony boundary strengthening coefficient KC as a function of lamella thickness λL. The constitutive description is based on the model of a polysynthetically twinned crystal which is adopted to a representative volume element of a fully lamellar microstructure. In order to capture the micro yield and subsequent micro hardening in weakly oriented colonies prior to macroscopic yield, the hardening relations of the adopted model are revised and calibrated against experiments with polysynthetically twinned crystals for plastic strains up to 15%.

In situ fracture observations of distinct interface types within a fully lamellar intermetallic TiAl alloy

“…The amount of α 2 phase and ζ-Ti 5 Si 3 silicides may be optimized by the alloying concept in order to enlarge the number of strengthened α 2 /γ interfaces. However, macroscopically thinner lamellae within the α 2 /γ colonies would result in a strain localization along the colony boundaries and thus higher tendency for crack initiation [32,39]. Also, a change in the α 2 /γ ratio and significantly lower lamellar spacing are reported to affect coherency [19,62].…”

“…Furthermore, crack-tip blunting by ductile phases, shear ligament toughening, twinning, and crack-tip interface debonding are also considered to enhance the crack resistance at the micro-scale [12,35,37]. Therefore, the deformability of γ lamellae due to mechanical twinning or slip of dislocations within the plastic zone in front of the crack tip and the interaction with adjacent γ or α 2 lamellae has to be considered [19,21,23,32,38,39].…”

Quantifying the High-Temperature Separation Behavior of Lamellar Interfaces in γ-Titanium Aluminide Under Tensile Loading by Molecular Dynamics

“…Crystal plasticity models can help to account for specific deformation mechanisms (slip and twinning) and for describing the anisotropic material behavior in TiAl. Recently, such models (Schnabel and Bargmann, 2017;Ji et al, 2018;Schnabel and Scheider, 2020) were used to predict the colony boundary strengthening coefficient as a function of lamella thickness and to map the deformation in lamellar TiAl. Microstructureinformed multi-scale models bear promise to accelerate the alloy development in γ-TiAl.…”

Cellular Automata Modeling of Ostwald Ripening and Rayleigh Instability

“…The latter is a phenomenon of the breakup of rod-like (high-aspect-ratio) structures due to the progressive growth of axial perturbations, leading to these rod-like structures breaking up into an array of equiaxed structures [ 1 ]. Modern engineering materials often contain high-aspect-ratio features (e.g., titanium alloys [ 2 , 3 ] and TiAl alloys [ 4 , 5 ] with lamellar structures, Al–Si Alloys [ 6 ] with eutectic microstructure, fiber reinforced composites [ 7 ]). When applied at elevated temperatures, the high-aspect-ratio structures in these materials undergo transformation which could ultimately lead to their breakup.…”