Understanding creep in TiAl alloys on the nanosecond scale by molecular dynamics simulations

Hariprasath Ganesan,Ingo Scheider,Christian J Cyron

doi:10.1016/j.matdes.2021.110282

Hariprasath Ganesan, Ingo Scheider + Show 1 more

Open Access

https://doi.org/10.1016/j.matdes.2021.110282

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Abstract

Molecular dynamics (MD) simulations of creep generally face the problem that the creep most often evolves on time scales hard to capture with MD due to their typically short time step size. Consequently, MD studies of creep often use unrealistically high temperatures and stresses and simplified atomistic models to make creep-like processes happen on computationally accessible time scales. Apparently, this compromises the physical reliability of such studies. To alleviate this problem, we designed an MD model of titanium aluminide (TiAl) with a microstructure matching at least many of the key parameters of experimentally observed microstructures. We applied this MD model with stresses much lower than the ones used in most previous creep studies (well below yield stress) and in the temperature range 0.55TM-0.7TM, with melting temperature TM. Compared to typical previous MD studies, this much more realistic setup produces creep rates more than three orders of magnitude smaller and thus much closer to reality. We identified the driving mechanisms of primary creep on the nanosecond scale that agree very well with recent experimental observations, thus contributing towards the overarching goal of bridging the gap between atomistic creep simulations and continuum-scale creep simulations for engineering applications.

Highlights

A recent study [8] pointed out the shortcomings of the existing creep models of fully lamellar TiAl alloys and concluded that for realistic creep simulations, bulk creep and interfacial creep should be taken into account, e.g., by cohesive zone models [9,10,11]
As the main difference between the mono-colony simplified atomistic models (SAMs) and the poly-colony SAM is the existence of colony boundaries, the faster creep in the poly-colony SAM can most likely be attributed to the presence of these boundaries, highlighting their key role in TiAl alloys creep
This nanomechanical insight from molecular dynamics (MD) simulation holds in perfect agreement with the experimental works [41,45,59], which claim a decrease in primary creep strain with decreasing lamellar spacing in fully lamellar microstructure

Summary

Introduction

While over the last years, a considerable number of MD studies of TiAl alloys have been published [12,13,14,15,16,17,18,19,20,21,22], only two recent studies addressed creep [23,24], with several serious limitations like unrealistic TiAl atomistic models combined with heuristic stresses for creep simulation This is surprising, noting that over the last two decades MD simulations of creep in metallic materials have attracted rapidly increasing attention [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. Note that the rapid deformation resulting from such high applied stresses, some times above ry, violates creep conditions per definition

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