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Alpha-Beta Titanium Alloys

#3184


Evolution of Substructure and Phase Morphology in Alpha-Beta Titanium Alloys during Plastic Deformation and Their Influence on Mechanical Properties

Tech Area / Field

  • MAT-ALL/High Performance Metals and Alloys/Materials
  • MAN-MPS/Manufacturing, Planning, Processing and Control/Manufacturing Technology

Status
8 Project completed

Registration date
12.01.2005

Completion date
29.01.2009

Senior Project Manager
Tyurin I A

Leading Institute
Russian Academy of Sciences / Institute of Metals Superplasticity Problems, Russia, Bashkiria, Ufa

Supporting institutes

  • Makeyev Design Bureau of State Rocket Center, Russia, Chelyabinsk reg., Miass

Collaborators

  • Ladish Co., USA, WI, Cudahy\nAir Force Research Laboratory / Wright-Patterson Air Force Base Ohio, USA, OH, Wright-Patterson\nMcGill University, Canada, QC, Montreal

Project summary

The objectives of the present Project are to study the relationship between the substructure parameters (the type of intraphase grain boundary and misorientation spectrum) and kinetics of transformation of the lamellar-colony microstructure into globular morphology during hot/warm working of two-phase titanium alloys as well as the influence of substructure and phase morphology on their mechanical properties.

Transformation of lamellar microstructure of alpha-beta titanium alloys into globular one during hot working/post working annealing operations at temperatures below the beta transus plays a key role in development of desired microstructure for final shaping or service. Therefore, the globularization process has received considerable attention reasoning from the microstructure evolution and phase transformation kinetics and mechanisms. However, a number of principle questions remain unexplored. One of the main is an influence of substructure on lamellar-to-globular transformation since it is greatly differs both within plates, and from colony to colonies. There are also no systematic data on conversion of interphase boundaries from semi-coherent boundaries to non-coherent ones during globularization process. Kinetics of transformation of the lamellar microstructure depends on processes occurring both in - plates and -interlayers. However, there are no data on the influence of processes occurring in -phase on spheroidization in the literature. High stacking fault energy is typical for -phase. That is why, due to high rates of recovery processes, recrystallization does not occur in this phase during hot deformation and there are observed low and middle angle boundaries, which pide -interlayers into parts. Evidently, the formation of additional interfaces stimulates diffusion processes that contribute to pision and spheroidization of -plates. The literature data on the influence of -phase substructure on globularization process are almost absent.

Another situation takes place at low temperatures of deformation. In recent years titanium alloys with submicrocrystalline (SMC) structure attract much interest. Large plastic deformations at low processing temperatures are used for formation of SMC structure. In this case -phase is more enriched by alloying elements than at high temperatures. That is why one can expect the development of recrystallization in -phase because rates of recovery should be less. However, on the other hand, the amount of -phase decreases essentially, interlayers become much thinner than -plates and their strengths are equal. Unlike high temperatures, at which there occurs pision of -plates, at low temperatures one should expect pision and spheroidization of -interlayers. The process of globularization occurs under conditions of high stresses that is why the type of dislocation structure formed should be different from the one at high temperatures. However, investigations in this direction were not performed.

So, during plastic deformation, in and phases of different colonies of the lamellar microstructure there occurs formation of substructures distinguished by different origin and parameters. Formation of intraphase boundaries can be resulted from both the redistribution of dislocations under stresses and development of polygonization and recrystallization processes in phases, as well as the intersection of colonies by shear bands. This assumes a significant difference in spectrum of misorientations for intraphase boundaries not only from one grain to another grain but also from one colony to another. Since this fact surely leads to essentially different kinetics of globularization further investigations in this direction seem very important. The present work proposes to perform investigations relating to formation of grain boundary misorientation spectrum in alpha-beta titanium alloys Ti-6Al-4V (Ti-64) and Ti-5Al-4.75Mo-4.75V-1Cr-1Fe (VT22 in Russian) with lamellar structure under warm and hot working.

The substructure and phase morphology strongly affect on mechanical properties of titanium alloys. The formation of globular microstructure allows increasing their strength and fatigue sufficiently. The growth of strength and fatigue at least up to 30-50% simultaneously can be achieved in submicrocrystalline condition of two-phase titanium alloys. Such alloys show the excellent superplastic properties at temperatures of deformation, which are lower down to 200-350С than that in microcrystalline condition. The primary emphasis in this study will be placed to achieve the highest mechanical properties of strength and fatigue in two-phase titanium alloys. On the base of the results to be obtained the method for producing the thread studs out of titanium alloys with the highest mechanical properties will be developed.

The participants of the Project have a significant experience in study of fine microstructure of metals and alloys, particularly, of titanium alloys. Investigations of various aspects of the behavior of titanium alloys are performed during 30 years. In all, IMSP's scientists published no less than 50 articles in this field and have got tens of patents.

Expected results and their application

The main milestone of the Project assumes performance of basic research relating features of substructure effect on the transformation of lamellar microstructure to the equiaxed one, as well as applied research aimed to develop a method for producing the thread studs out of titanium alloys with the highest mechanical properties. The Proposal concerns the development of new approaches for obtaining of desired microstructure in alpha/beta titanium alloys for metal working technologies oriented to manufacturing products for different branches of industry.

The Project implementation will allow:


· Establish the mechanisms and kinetics of transformation of lamellar structure of two-phase titanium alloys into globular one depending on thermomechanical conditions of deformation.
· Develop a comprehensive approach to performing globularization process of metal working, which includes:
a) analysis of substructure evolution of alpha and beta phases of titanium material during hot/warm deformation and post working annealing at temperatures below the beta transus;
b) analysis of intraphase grain boundary misorientation spectrum depending on thermomechanical conditions of metal working;
c) evaluation of mechanical properties of titanium alloys with various phase morphology and parameters of substructure.
· Determine the optimum thermomechanical conditions (temperature, strain, strain rate) conditions of the globularization process in two-phase titanium alloys under various relations between the substructure parameters and phase morphology of initial microstructure of material.
· Develop the method to produce the thread studs out of titanium alloys with the highest mechanical properties.

The Project possesses large scientific and commercial potential since it combines the basic and applied researches. Results to be obtained during the Project realization may be successfully used in other branches of industry thus stimulating the transition from military purpose technologies to development of new technologies of civil products operating during long periods of time under conditions of high temperature and cyclic loadings, that are used in gas- and oil pumping stations, steam turbines, power and transport machines. For weapons scientists and engineers the Project gives opportunities to redirect their talents to peaceful activities. It is proposed the close cooperation with named scientists and Project collaborators – Prof. Lee. Semiatin (Air Force Research Laboratory, Wright-Patterson AFB) and Dr David Furrer representing the leading metallurgical Ladish Company that will promote to integration of scientists from Russia into the international scientific community.

Results to be obtained during the Project realization will contribute to the progress of science as well as to the development of techniques of production of semi- finished products with equiaxial fine-grained microstructure out of titanium-based alloys. Employment of equiaxial fine-grained materials manufactured according to these novel techniques in various units of mechanisms and machines will allow creating unique components with high service characteristics.

Main technical approach to be developed will be investigation of lamellar-to-globular microstructure transformation under conditions of hot and warm working as well as post deformation annealing. At that the two different types of microstructure micron-sized and submicron-sized structures will be obtained. The methods of ttransmission electron microscopy and scanning electron microscopy will be attracted to analyze the evolution of microstructure during thermomechanical treatments and special software will be elaborated to find the spectrum of grain boundaries misorientations. Based on these studies a new method for obtaining the highest strength and fatigue resistance will be developed.

The following activities will be performed in the course of the Project implementation:


1. Investigation of the influence of thermomechanical conditions of deformation on the mechanical behaviour and kinetics of transformation of initial lamellar microstructure into globular one.
2. Study of structure evolution of alpha and beta phases of titanium materials depending on thermomechanical conditions of hot and warm deformation.
3. Evaluation of the spectrum of intraphase boundaries misorientations as well as the transformation of semi-coherent interphase boundaries to non-coherent ones during hot/warm plastic deformation.
4. Investigation of crystallographic texture evolution of alpha and beta phases of titanium materials during plastic deformation.
5. Investigation of the effect of post deformation annealing on the globularization kinetics.
6. Ascertainment of the relationship between the substructure parameters (the type of intraphase grain boundary, their misorientation spectrum) and phase morphology (lamellar, partially globularized and fully globularized structures).
7. Ascertainment of the relationship between the substructure parameters (the type of intraphase grain boundary, their misorientation spectrum) and phase morphology (lamellar, partially globularized and fully globularized structures)..
8. Establishing of the phase morphology/substructure/mechanical properties relationships for two-phase titanium alloys.
9. Development of the method for producing the thread studs out of titanium alloy VT22 with the highest mechanical properties.
10. Preparation of the final report and development of proposals on industrial use of the developed processing.

Role of Foreign Collaborators

In the course of the Project realization it is planned to interchange information with collaborators and to discuss, cross-check and analyze the results obtained.


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