Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Chair

Marisol Koslowski

Committee Member 1

Thomas Siegmund

Committee Member 2

Anter El-Azab

Committee Member 3

David Bahr

Abstract

Strength of nanolayered metallic composite, stacking fault strengthening mechanism in random alloys especially high entropy alloys and solid state amorphization during mechanical milling are investigated in detail using phase field based method. Nanolayered metallic composite are comprised of alternating layers of two or more different metallic phases. It exhibits superior strength and ductility in ambient and extreme environment [1–4]. The key phenomenon characterizing the strength of this material is the slip transmission of single dislocation from one layer to the other. It is observed that both the modulus and the lattice parameters of different phases affect the slip transmission strength. However, until now there is no design criteria which includes both the modulus and the lattice parameters into considerations. Random alloys are used in a variety of engineering fields. Such as the Ni-based super alloys used in the aerospace industry in hot environment (turbine engine) [5], aluminum alloys used in automotive applications [6], and the emerging high entropy alloys (HEA) [7]. These alloys can exhibit improved functional properties, including thermal, electric and environmental resistance as well as mechanical properties. High entropy alloys are relative new concept alloys, in which five or even more elements are mixed nearly equiatomically. The stacking fault energy in such alloys varies as the local elemental concentration varies. The interaction of the dislocations with the local fluctuation in the stacking fault energy is very important in determining the yield strength of the material. On the other hand, Mechanical milling is one of the most common manufacturing processes in a variety of industries, such as pharmaceutical, semiconductor, and metal alloy productions. During this process, the material is subjected to extensive deformation to reduce its crystalline and particle size. However, this also leads to some undesired microstructural changes, such as polymorphic transformations and solid state amorphization (SSA) [8–23]. In some cases, SSA may directly affect specific properties of the material under concern, such as increasing mechanical strength of metal alloys and lowering efficacy of pharmaceutical medicines [8,10,14]. Close correlations between SSA and the plasticity are observed in experiments. Surface conditions such as roughness and loading conditions also seem to affect the SSA tremendously.

A phase field dislocation dynamics (PFDD) approach is applied to study the strength of the nanolayered metallic composites and the stacking fault strengthening mechanism in HEA. On the other hand, a phase field based finite element model is established to investigate the surface effects on SSA of pharmaceutical materials. For the nanolayered material, modulus inhomogeneity, interface induced misfit strain and residual dislocation at the interface are considered and incorporated into PFDD model. A new design criteria includes both lattice parameter mismatch and modulus mismatch is established. Comparison between the PFDD result and the results from current literatures are made. The γ−surface energy is incorporated into PFDD model to investigate the stacking fault strengthening mechanism in random alloys especially in HEA. A critical stacking fault region size is observed where the maximum strengthening can be achieved. This critical region size is comparable with the effective equilibrium stacking fault width. The waviness of the dislocation is closely related with the strengthening of the material. A phase field based model is established to study mechanically induced SSA. The model incorporates the mechanical properties of both the crystalline and amorphous phase of the material. The total energy of the system consists of three parts: the elastic strain energy, the plastic dissipation energy and the chemical energy. The model is thermodynamically self-consistent. It is general and can be applied to study mechanically induced SSA in any kinds of the materials. The model is applied in four different pharmaceutical compounds. Validation with experiment results is performed. Surface roughness effect is investigated in detail.

Download

Share

COinS