Lee Hsun Lecture Series   （236-L167）

Topic: Deformation mechanisms of Ni-based superalloys containing γ''' Ni2 (Cr,Mo,W) precipitates

Speaker: Prof. Javier LLorca

IMDEA Materials Institute, Department of Materials Science, Polytechnic University of Madrid，Spain

Time: 10:00-11:30, (Wed.) Jun. 24^th, 2026

Venue: Room 403, Shi Changxu Building, IMR CAS

Abstract:

Haynes 244® is a novel Ni-based superalloy that contains 40 vol. % of coherent Ni2(Cr,Mo,W) precipitates with lenticular shape. As opposed to othe Ni-based superalloys strengthened by and  precipitates, in which plastic deformation is due to dislocation slip in the g matrix, Haynes 244 deformation is associated with stacking faults created by partial dislocations that subsequently thicken into microtwins via the transmission of partials on adjacent planes. Detailed observations show that twinning initiates at the – interface within the precipitates and then extends outward into the matrix. Plastic deformation is accommodated by increasing the number of twins while twin thickening is limited. These mechanisms are dominant in a very wide range of strain rates (10-5 s-1 to 500 s-1) and temperatures (23ºC to 650ºC), leading to a rate independent mechanical behavior. Moreover, fatigue tests under fully-reversed deformation tests showed evidence of twinning and detwinning of different twin variants in the same grain to accommodate the plastic deformation. Finally, the competition between slip and twinning in this alloy was investigated at 23ºC and 650ºC by means of micropillar compression tests in grains with different orientations. The mechanical response of the grains oriented for slip at room temperature showed strain bursts, strain hardening and a fully elastic recovery upon unloading, that are indicative of dislocation plasticity. On the contrary, micropillars oriented for twinning did not show strain bursts or strain hardening and presented a large strain recovery upon unloading. In situ mechanical tests provided evidence of twin nucleation and thickening during compression, followed by detwinning during unloading. Micropillar compression tests in both orientations at 650ºC showed similar features: strain bursts during deformation, limited strain hardening, and elastic recovery during unloading. The deformation mechanisms of the micropillars deformed in both orientations and at both temperatures were carefully analyzed by transmission electron microscopy from lamellae extracted from the micropillars. In particular, the origin of the detwinning found upon unloading at ambient temperature was discussed in terms of the microstructure of the alloy.

V. A. Tucker, T. R. Mann, A. Roginski, M. G. Fahrmann, M. A. Monclús, J. LLorca, M. S. Titus. The origin of microtwinning in HAYNES® 244® superalloy. Communications Materials 7, 16, 2026.

I. Escobar-Moreno, Z. Ye, B. Yang, V. Tucker, M. S. Titus, J. LLorca. Mechanical response and deformation mechanisms of Haynes 244 Ni-based superalloy under fully-reversed cyclic deformation. Materialia, 48, 02773, 2026.