Crystal plasticity analysis of deformation anisotropy of lamellar TiAl alloy: 3D microstructure-based modelling and in-situ micro-compression

Abstract

Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micromechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micromechanisms responsible for the highly anisotropic deformation response of TiAl on the intralamella and inter-lamella scales. This study focuses on two specific configurations of / 2 lamellar microstructure with the / 2 interfaces being aligned 25 and 55 to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25 micropillar while the anisotropy of the 55 micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25 micropillar less localised than that in the 55 micropillar. Moreover, the CP-FEM 2 / 42 model successfully predicted substantial build-up of internal stresses at / 2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins. IntroductionTitanium aluminide (TiAl) alloys have a density approximately half that of nickel-based superalloys [1] and specific strength that is comparable with nickel-based alloys up to temperatures as high as 700 o C [2], resulting in a potentially wide application of the alloys in aero-engines [3,4]. However, the application of TiAl alloys is limited by their low ductility, which is mainly due to the anisotropy of constituent microstructure in TiAl, and insufficient understanding of the microstructure-property relationships. In-depth understanding of the relationships would lead to wider applications of these lightweight alloys in aerospace, leading to the weight reduction, thereby, increase in the fuel efficiency and decrease in the CO2 emissions. Although there have been significant efforts in understanding the relationships between chemical composition, microstructure and mechanical properties of TiAl alloys, in particular regarding the TiAl lamellar microstructure [1, 5-7], there have been no detailed studies in which both in-situ mechanical testing and microstructure-based modelling are carried out to provide more insights into the direct relationships between constituent microstructures and mechanical properties. This present study aims to integrate modelling and in-situ tests to seek possible solutions to the low ductility of TiAl. The poor ductility of lamellar TiAl alloys is partly due to its strong plastic anisotropy because of the respective anisotropy of constituent phases. Lamellar TiAl alloys consist of two phases: -face centred tetragonal (f.c.t.) and ...

Review on globularization of titanium alloy with lamellar colony

“…Based on the study of the globularization mechanism, many researchers have established the prediction models of globularization. These models are summarized as follows: empirical model [17,18], neural network model [19], internal variable model [3,22], processing map [20,21], phase field model [23,24] and microstructure-based finite element model [13,16,25,26]. Moreover, the comparison and analysis of these prediction models are accomplished through tabular form shown in Table 1.…”

“…The shortcomings of the well-known globularization mechanisms make it difficult to control globularization during hot working. The mains are follows: (a) the applicable conditions of every globularization mechanism are inconsistent (dynamic recrystallization at high temperature and low strain rate [7], shearing mechanism [8] at low temperature and high strain rate); (b) the microstructure parameters considered in every globularization mechanism are different (dihedral angle of the boundary in boundary splitting [9,10], solute concentration gradient in termination migration [11,12]); (c) multiple mechanism coexistence due to the diversity of lamellar colony and meso-scale heterogeneous deformation [13][14][15][16]. Therefore, a unified globularization mechanism of coupling multiple factors needs to be further developed.…”

“…Meanwhile, the phase field model [23,24] can predict the morphology evolution of single alpha lamella during static globularization without consideration of hot deformation. The meso-scale heterogeneous deformation can be well captured by microstructure-based finite element model [13,16,25,26].…”

Current Situation of Titanium Research, Development and Applications in China

“…The details of the 3D modeling construction is shown in Figure 7. A crystal plasticity-based finite element modelling (CP-FEM) coupled with experiments have been used to study the micro-mechanisms of plastic anisotropy in lamellar TiAl alloys, for the in-depth understanding of micro-mechanisms responsible for the highly anisotropic deformation response of TiAl on the intra-lamella and inter-lamella scales [33] . For permance properties prediction, numerical simulations are conducted to investigate the fatigue behavior of anovel TA2-TA15 titanium alloy fabricated by the laser melting deposition (LMD) manufacturing process [34] .…”

Micromechanical fatigue experiments for validation of microstructure-sensitive fatigue simulation models

“…In prior work, Zhang et al (2016a) quantitatively compared CPFE strains in a nickel alloy grain cluster with HR-EBSD after fatigue testing. A coupling between in-situ micropillar compression of a lamellar TiAl alloy and CPFE modeling is described by Chen et al (2019).…”