RELATIONSHIPS BETWEEN MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TI-B19 ALLOY

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The phase transformation kinetics of titanium alloys

The solid phase transformation kinetics in alloys can be established by a continuous characterization of changes of physical properties which are sensitive to the structural evolutions leading to a relationship of the relative change of property with temperature and time[86~90]. These properties can be expansion coefficient, volume, hardness, electrical resistivity as well as enthalpy changes etc.. Assuming a linear relationship between those relative properties and the relative structural change, the equation between the phase transforming degree (f) and those changes of physical parameters can be given as: 1 0 0 p p p p f –  – = , 0 £ f £ 1 ( 1-1).
where, P is the value of the measured physical parameters at time t; P1 and P0 are the values of the measured physical parameters at the beginning and at the end of the phase transformation, respectively.
The isothermal kinetics of the transformation will be expressed as f = f(t). For diffusive transformations, the transformation kinetics often presents a sigmoid shape that is expressed by JMAK law (Equation 1-2). (1 exp( ( ) )) max 1 0 0 n b f k t t p p p p f = – – – – – = ( 1-2).
Here, k means the temperature coefficient which is sensitive to the variation of temperature; n is the Avrami exponent; t is time at transformation temperature and tb is the beginning time for the transformation. The Avrami coefficient is dependant on the nucleation and growth mechanisms.

The aim of the present research work

Excellent combination of mechanical properties can be obtained in titanium alloys by controlling the heat treatment process. So it is very important to understand in detail the phase transformations occurring during the process in order to improve the mechanical properties by controlling the microstructure formation and design the deformation and heat treatment. Anyway, it is also an important guide to design and develop new alloys.
Ti-B19 is a kind of new metastable beta titanium alloy developed by China. Remarkable combination of mechanical properties as strength, ductility and toughness can be obtained by appropriate heat treatments. The microstructure of the alloy is strongly depended on the parameters of the heat treatment, that is to say the difference of temperature, time and the rate of heating and cooling will lead to differences in volume fraction, shape, size as well as distribution of the formed phase, which introduce differences in mechanical properties. So the aim of the present work is to study the phase transformation kinetics in Ti-B19 alloy during heating and cooling as well as isothermal periods, and to understand the phase transformation behaviors. The results of this work will provide some important data to understand the microstructure evolution as well as some guides to design the processing route (deformation and heat treatment) for a new composition of titanium alloys.

Study of isothermal phase transformation kinetics

In order to investigate systematically the isothermal decomposition of b metastable phase in Ti-B19 alloy a wide range of temperature has been selected. The temperature is ranging from 300oC to 700oC. The holding time at the transformation temperature is varying, depending on the end of the transformation time. The treatment applied to the specimens is schematized in Figure 2-2. For all specimens, a first solution treatment in the b temperature range is realized to control the initial state and the b grain size. The heating rate is 5°C/s, the temperature is 900oC, with 10 minutes holding time and a cooling rate of 20oC/s is used to freeze the microstructure obtained after the solution treatment at high temperature and keep a metastable b state.

Study of the influence of heating on isothermal phase transformation

Heating rate may influence the isothermal phase transformation kinetics and microstructure evolution remarkably. So the influence of heating rate on isothermal phase transformation behaviors have been studied as schematically shown in Fig.2-3.
The first solution treatment in the beta temperature range is the same as previously leading to a similar beta metastable state. A single aging temperature is selected as 500oC and the holding time for all conditions is four hours. Three heating rates have been applied 10oC/s, 1oC/s and 0.1oC/s, respectively. After transformation, a rapid cooling rate is applied to freeze the microstructure formed during aging. The cooling rate was more than 20oC/s.
Moreover, according to the variation of resistivity obtained during heating at 0.1°C/s, samples were quenched from 350oC, 450oC and 500oC, respectively, in order to analyze the structure and microstructure evolutions.

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Study of the phase transformation kinetics during continuous cooling.

For the previous conditions, we applied a cooling rate from the beta temperature range leading to a beta metastable state. However, during cooling processes phase transformation may occur depending on the local cooling rate in the material. So the phase transformation kinetics and microstructure evolutions have been studied in Ti-B19 alloy during cooling from 900oC to room temperature according to the programs showed in Fig.2-4. The solution temperature of all samples are 900oC and holding time was 10 minutes with a heating rate of 5oC/s. The cooling rates considered were the following: 1oC/s, 0.2oC/s, 0.1oC/s, 0.06oC/s and 0.03oC/s, respectively. 500 ? T Tb 900 ? ,10 min 10oC/s 1oC/s 0.1 oC/s t 5? /s >20 ? /s >20 ? /s.

Influence of plastic deformation of the beta metastable phase on isothermal phase transformation kinetics during aging.

At last, we analyzed the influence of plastic deformation of the beta metastable phase on isothermal phase transformation during aging. The aging temperature considered was 500°C. Two levels or pre deformation of 43% and 52% were considered.

Table of contents :

I. INTRODUCTION
I.1. Brief introduction of titanium alloys
I.2. Phase transformation in titanium alloys
I.2.1. Main phase transformations in titanium alloys
I.2.2. Martensitic phase transformation
I.2.3. Phase transformation
I.2.4. Decomposition of metastable b phase
I.3. The phase transformation kinetics of titanium alloys
I.4. The aim of the present research work
II. RESEARCH SCHEME AND EXPERIMENTAL PROCEDURE
II.1. Research contents
II.2. Researching routes
II.2.1. Study of isothermal phase transformation kinetics
II.2.2. Study of the influence of heating on isothermal phase transformation
II.2.3. Study of the phase transformation kinetics during continuous cooling.
II.2.4. Influence of plastic deformation of the beta metastable phase on isothermal phase transformation kinetics during aging.
II.3. Experimental materials
II.3.1. Brief introduction of Ti-B19 alloy
II.3.2. Preparation of materials and samples
II.4. Experiment methods and devices
II.4.1. In-situ resistivity measurement method and device
II.4.2. High energy synchrotron X-ray diffraction
II.4.3. Other research methods
III. ISOTHERMAL PHASE TRANSFORMATION KINETICS AND MICROSTRUCTURE EVOLUTIONS OF TI-B19 ALLOY
III.1. Variations of electrical resistivity with time and further analysis method
III.1.1. Variation of electrical resistivity with time generally
III.1.2. Variations of electrical resistivity with time during isothermal treatment Phase transformations and microstructure evolutions in metastable beta titanium alloy Ti-B19
III.1.3. Method to obtained phase transformation kinetics from in-situ resistivity variations
III.2. Isothermal phase transformation kinetics
III.3. Microstructure observations
III.3.1. Initial state
III.3.2. Structure and microstructure evolutions for aging at 300~ 350oC
III.3.3. Structure and microstructure evolutions for aging at 400~ 450oC
III.3.4. Structure and microstructure evolutions for aging at 500 ~ 550oC
III.3.5. Structure and microstructure evolutions for aging at 600°C and 700°C
III.4. The design of TTT diagram for Ti-B19 alloy
III.5. Summary
IV. INFLUENCE OF HEATING RATE ON ISOTHERMAL PHASE TRANSFORMATION IN TI-B19 ALLOY
IV.1. Evolutions during the heating.
IV.1.1. Variation of resistivity with temperature (time) during the heating process
IV.1.2. Variation of structure and microstructure during the heating for a heating rate of 0.1°C/s.
IV.2. Evolutions during the holding step.
IV.2.1. Evolutions of electrical resistivity variations.
IV.2.2. Microstructures and structure obtained after isothermal holding.
IV.3. Brief summary and discussion
V. EFFECT OF PLASTIC DEFORMATION ON PHASE TRANSFORMATION OF TI-B19 ALLOY DURING AGING
V.1. Resistivity variation during heating and aging processes
V.1.1. Resistivity variation in the heating range
V.1.2. Resistivity variations during holding at 500°C.
V.2. Isothermal phase transformation kinetics
V.3. Influence of deformation on microstructure evolutions.
V.4. Brief Summary
VI. PHASE TRANSFORMATION OF TI-B19 ALLOY DURING CONTINUOUS COOLING
VI.1. Influence of the cooling rate on the microstructure evolution in Ti-B19 alloy.
VI.2. Resistivity variations during continuous cooling.
VI.3. Establishment of phase transformation kinetics functions
VI.4. Brief summary
VII. RELATIONSHIPS BETWEEN MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TI-B19 ALLOY
VII.1. Mechanical characteristics of Ti-B19 alloy after solution aging
VII.2. Influences of volume fraction of precipitated phases on mechanical characteristics of Ti-B19 alloy.
VII.3. Summary
CONCLUSIONS
APPENDIX
REFERENCES

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