Microstructure and Mechanical Properties of Ti-based Lightweight Multi-Principal Element Alloys


Student thesis: Doctoral Thesis

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Awarding Institution
Award date19 Jul 2021


Multi-principal element alloys (MPEAs) or high entropy alloys (HEAs) have attracted extensive attention due to their excellent mechanical properties. However, the exploration on low-density MPEAs has seldom been studied. In this thesis, we develop a new promising Ti-based low density MPEAs, i.e., Tix(AlCrVNb)100-x and their microstructure and mechanical properties are systematically explored.

The Tix(AlCrVNb)100-x (x=20, 40, 60 and 80) lightweight MPEAs were prepared by vacuum arc-melting. The density of alloys decreases with increasing Ti content from 5.81 to 4.79 g/cm3. The microstructure of Ti20 consists of body-centered cubic (BCC) and minor amount of nano-sized particles segregating along grain boundaries. But Ti40, Ti60 and Ti80 all consist of a single BCC solid-solution phase. In addition, the experimentally observed phase constitutions are compared with the equilibrium and non-equilibrium thermodynamical modeling results. Good specific strength and ductility combination was found in both Ti40 and Ti60. The solid-solution strengthening was found to be the dominant strengthening mechanism, making up more than 90% of the yield strength of Ti40-Ti80.

In addition, the influence of crystalline orientation and loading rate on the pop-in behavior of Ti60(AlCrVNb)40 MPEA or medium entropy alloy (MEA) under nanoindentation was investigated. The incipient plasticity showed a clear orientation and loading rate dependence. The onset of yielding is associated with a mixture of two mechanisms: heterogeneous nucleation of dislocations through atomic-sized precursors and/or expansion of dislocation loops by pre-existing dislocation multiplication. The crystalline orientation effects can be rationalized by de-pinning the different dislocation configurations on each orientation due to the different numbers of activated slip systems and resolved shear stresses.

Then, we studied the influence of lattice distortion on the initiation of plasticity in Ti-based MPEAs at a loading rate of 500 μN/s. The surface dependence was observed in the Ti20 MPEA, where the lowest first burst load was determined in the (110) oriented grain. The same was also observed in Ti60. This implies that this kind of crystallographic dependence is an intrinsic feature of present studied alloys and is not completely covered by the lattice distortion effect. In spite of similar orientation dependence, a much higher load at the first burst was identified in Ti20 as compared with that in Ti60, attributed to the fact that the more severe lattice distortion in T60 would cause a higher critical resolved shear stress (CRSS) to nucleate dislocations and a greater lattice friction to overcome during dislocation multiplication and emission process. In addition, based on the activation volume value, the mechanism in Ti20 is regarded to be similar to that in Ti60, inferring that altering the lattice distortion does not change the basic pop-in mechanism in MPEAs. The lattice distortion influence on the homogeneous dislocation nucleation process was investigated and compared with the heterogeneous dislocation nucleation events in both Ti20 and Ti60.

Finally, to get a detailed real-time monitoring of atomic scale evolutions during nanoindentation testing, we carried out the nanoindentation simulations using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). The simulation results show that the critical load for inception of yielding is determined by the number of defective FCC and HCP atoms, transformed from the original massive BCC lattice atoms. This implies that there would occur stress-induced phase transformation (from BCC to FCC or HCP). Compared with the transformed FCC atoms, HCP atoms would play a more dominant role in determining the critical load. In contrast to the (110)AA model (where AA stands for average-atom), LD in the (110)LD model would decrease the critical force at the onset of yielding due to the its role in promoting the formation of HCP atoms. The CSRO effect would further facilitate the formation of HCP atoms, lowering the load for initiation of plasticity.

As for the orientation effect, based on the simulation, the onset of yielding mechamism for the (100) and (110) orientations originates from homogeneous nucleation from newly formed defective atoms, while the (111) orientation exhibits a heterogeneous nucleation from the surface step. In spite of the inconsistence of the load order with experiments, our simulations provide a valuable atomic-scale observation for the incipient behavior and reveal the atom configurations on different orientations which are inaccessable by experimental observations. The inconsistence may be due to the fact that different mechanisms in simulations and experiments for incipient behavior. The mechanisms for the pop-in behaviors of all three orientations appear to be inhomogeneous, based on experimental observations. Other reasons like the much higher loading rate and the extremely lower temperature as well as the composition difference applied in our simulation could also lead to the inconsistence between simulations and experiments. Further exploration will continuously be conducted.