The role of grain size and selected microstructural parameters in strengthening fully lamellar TiAl alloys

Abstract

More than 5 years ago, wrought processing was first used to produce fully lamellar (FL) microstructures in TiAl alloys having grain sizes less than Ϸ400 m. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (500 to 1000 MPa at low temperatures) while maintaining attractive high-temperature properties. The improvements appeared to be related to an unusually high apparent sensitivity of strength to grain size. Studies reported an apparent value for the slope of the Hall-Petch (HP) plot approaching 5 for FL gamma alloys, while that for single-phase or ͌ MPa m duplex microstructures is near unity. The present investigations examine the slope of the HP plot for FL microstructures, paying particular attention to the lamellar microstructural variables. Results show that the ␣ 2 lamellar thickness and spacing and the ␥ lamellar thickness can vary over more than two orders of magnitude with typical process methods. These spacings influence the value of k y in the HP (grain size) relationship. Since they often change concomitantly with grain size in processing, they can give rise to a large scatter in the HP plot. The investigations also examine the flow behavior, glide barriers, and slip multiplicity for polysynthetically twinned (PST) crystals (the single-grain analogue of FL material), and then map this behavior into an explanation of the yield behavior of high-strength FL gamma alloys.

Versatile algorithms for the computation of 2-point spatial correlations in quantifying material structure

“…Some examples of such microstructure measures include volume fraction, average particle/ grain size, average particle/fiber spacing (mean free path), tortuosity and coordination number [1][2][3][4][5][6][7][8][9][10][11][12]. However, it is easily seen that this simple set of microstructure measures is unlikely to be the best possible set or even an adequate set, because it is easy to imagine multiple instantiations of microstructures that would exhibit the same values of these simple microstructure measures while displaying vastly different values of macroscale properties of interest.…”

Monitoring Microstructure Evolution in TiAl Using TEM

“…This is in contradiction with [1], stating that bright and dark lamellae have the same L1 0 lattice but with different orientation, but in strict correspondence with [2][3][4][5].…”

Thermomechanical modelling of polysynthetically twinned TiAl crystals

“…interfaces between lamellae and domain boundaries within the γ lamellae) is best described via the Hall-Petch relation (see e.g. [34][35][36][37])…”