Thermal stress and strain effects on phase transition temperatures in differential thermal analysis testing
- PDF / 418,419 Bytes
- 5 Pages / 630 x 792 pts Page_size
- 14 Downloads / 195 Views
I.
INTRODUCTION
D I F F E R E N T I A L thermal analysis (DTA) is an accurate, fast, and economical method to determine phase transition temperatures tl,2j and has been used to construct phase diagrams, t3m Equilibrium phase transition temperatures are often needed to construct equilibrium phase diagrams but cannot be determined directly in solid-phase transitions due to overheating or overcooling in the DTA testing. In a previous work by Zhu and Devletian, tS] an attempt was made to determine the equilibrium phase transition temperatures by extrapolation of nonequilibrium data obtained in DTA experiments using the following equation: T~ = C[STs exp (Qb/RT~)] 1/3 + To
[1]
where T~ is the onset nonequilibrium phase transition temperature obtained during heating (or cooling), S is the heating (or cooling) rate, R is the gas constant, To is the equilibrium phase transition temperature, and Qb is the diffusion activation energy for phase transformation reaction. The value of Qb equals the diffusion activation energy on defects such as grain boundaries, second-phase boundaries, dislocations, etc., since a phase transition starts most readily on these defects. But Eq. [1] does not consider the thermal stress caused by the nonuniform distribution of temperature in the sample and the strain caused during some solid-state phase transformations. According to basic thermodynamic principles, it is known that thermal stress and strain have an effect on the solid-state phase transition temperature. Therefore, without investigating the influence of thermal stress and strain, the accuracy of Eq. [1] is limited. The objectives of this article are to (1) investigate the effects of thermal stress on solid-state phase transition temperatures and (2) modify the previous model
(Eq. [1]) so that the effect of strain energy is included in the calculation of equilibrium transition temperatures from DTA data. II.
The differential thermal analyzer used in this study was a DU PONT* 9900 DTA analyzer with the high*DU PONT is a trademark of E.I. Du Pont de Nemours, Inc., Wilmington, DE.
temperature 1600 cell. The cell was purged for 2 hours with high-purity argon before the start of each test. A high-capacity hot gas purifier was used to further reduce the oxygen content in the argon gas before it entered the DTA cell. A contamination test described in Reference 5 showed that the DTA cell was free of contamination. The method to determine the onset nonequilibrium phase transition temperature, Ts, from a DTA curve is illustrated in Figure 1. [5,6] As shown in Figure 1, two tangential lines, which are drawn from the baseline and the peak slope of the DTA curve, meet at one point. The temperature represented by this point is defined as the onset temperature, which corresponds to the starting phase transition temperature, t61 The materials tested were polycrystalline high-purity Zr, commercially pure Ti (02 < 0.15 wt pct) and highpurity Fe. Each sample was machined into a 3.3-mmdiameter • 3.0-mm-long cylinder with a 1.65-mm radius on one end
Data Loading...
