Determination of Activation Energies and Modeling of Low Temperature Creep of Alpha, Alpha-Beta and Beta Titanium Alloys

University Of Maryland, College Park, College Park MD

Investigators

Abstract

0102320 Ankem This project is aimed at new understanding into low temperature creep of two-phase titanium alloys that find applications in various technologies including energy, aerospace, marine, chemical industries, consumer goods and bio-medical implants. Recent observations on the creep behavior of alpha titanium at ambient temperature by twinning led to the need for a more complete and fundamental modeling for predicting the integrity of these structures. The main goals of this study are: (1) to determine the activation energies for creep mechanisms in alpha, alpha-beta and beta titanium alloys in the low temperature range of 298- 458K, (2) to model the creep behavior of these alloys including Finite Element Modeling (FEM), (3) to compare FEM predicted creep curves and strain distributions with experimental values, and (4) to recommend optimal chemistry and microstructures of Ti alloys for improved low temperature creep resistance. To reach these goals, two different alloy classes (Ti-Mn and Ti-V) are selected for deformation tests at low temperatures combined with SEM and TEM for electron lithography, surface deformation studies and microstructure characterization such as dislocation densities and crystal structure analyses. Activation energies are determined and the creep processes are modeled from the creep tests data. The creep constants of the alpha and beta phases are used to model the creep behavior of two-phase alpha-beta Ti alloys by FEM using ANSYS computer program. The FEM modeling of creep of alpha-beta alloys gives a predictive capability in terms of the alpha and beta phases present and their morphologies. The results of the study are directly applicable for other similar systems such as zirconium and magnesium alloys that find application in energy and transportation sectors. In addition, the results related to the FEM modeling are applicable to any two-phase or composite materials. %%% The research develops new understanding of the low temperature creep mechanisms and the results are applicable for designing new titanium alloys and in optimizing the microstructures of existing alloys for improved creep performance. The research will also help in predicting component performance. *** Titanium alloys have attractive engineering properties including high strength to weight ratio, high fracture toughness, good high temperature strength, excellent corrosion resistance and bio- compatibility. Due to these properties, they find applications in various areas including energy, aerospace, marine, chemical industries, consumer goods and bio-medical implants. In some of these applications at low temperatures such as ambient temperature, loads are applied on the components for extended periods of time where creep becomes an important property. In this regard, it was recently shown that titanium alloys can creep at 95% yield stress at ambient temperature, but the activation energies for creep which can explain why and how creep occurs are not known. In addition, there are no simple models available to predict the creep behavior of two-phase materials from the knowledge of the creep behavior of individual phases, their morphology and volume fractions. The main objectives of this investigation are: (1) Determine the activation energies of creep mechanisms in alpha, alpha-beta and beta titanium alloys in the low temperature range of 298- 458K, (2) Model the creep behavior of a, a-b and b Ti alloys including Finite Element Modeling of a-b Ti alloys, (3) Compare FEM predicted creep curves and strain distributions with experimental values, and (4) Recommend optimal chemistry and microstructures of Ti alloys for improved low temperature creep resistance. For these studies, three Ti-Mn alloys and three Ti-V alloys will be used as the model systems. Tensile tests will be conducted in the temperature range 298 -458 K and creep tests will be conducted in the temperature range 298 - 458 K and in the stress level ranging from 85 -100% YS. SEM and TEM will be employed for electron lithography, surface deformation studies and microstructure characterization such as dislocation densities and crystal structure analyses. From the creep tests data, activation energies will be determined and the creep processes will be modelled. The creep constants of the a and b phases will be used to model the creep behaviour of two-phase a-b Ti alloys by Finite Element Modelling using ANSYS computer program. It is expected that the proposal work can be completed in three years. A successful completion of this work will be of great technological importance, as it will have a significant effect in designing new titanium alloys and in optimizing the microstructures of existing alloys for improved creep performance. For example, determination of activation energies gives a clue as to which species, i.e. elements, present in the material are responsible for low temperature creep and, accordingly, new alloys can be designed for improved performance. The FEM modeling of creep of alpha-beta alloys gives a predictive capability in terms of the alpha and beta phases present and their morphologies. This will also help in designing and in predicting component performance. Even though this investigation uses titanium alloys as the model system, the outcomes of the studies related to activation energies are directly applicable for any other similar systems such as Zirconium alloys. Furthermore, in general, the results related to the FEM modeling would be applicable to any two-phase or composite materials. This work will be carried out by the PI, Prof. S. Ankem and two graduate students. The PI has extensive experience in physical and mechanical behavior of Ti alloys and FEM Modeling. The graduate students will be trained in the state of the art experimental techniques such as electron lithography for drawing fiducial lines, SEM and TEM techniques and application of computers in predicting material behavior.