The Effect of Order on the Martensitic Transformation in Fe 3 Pt
- PDF / 1,480,429 Bytes
- 6 Pages / 415.8 x 637.2 pts Page_size
- 36 Downloads / 202 Views
THE EFFECT OF ORDER ON THE MARTENSITIC TRANSFORMATION IN Fe3Pt. D.P. DUNNE,* M. STOBBS** AND N.F. KENNON* *Dept. of Metallurgy, University of Wollongong, NSW, Australia. ** Dept. of Metallurgy and Materials Science, Cambridge University, U.K. ABSTRACT The lattice parameter and Curie temperature of austenite in Fe 3 Pt increase sharply on ordering for times up to 2 hr 0 at 650 C. These effects, together with a rapid decrease in the martensitic transformation temperatures, are interpreted in terms of increased short-range order. The onset of thermoelastic reversibility of the martensitic transformation is associated with the development of a significant volume fraction of long-range ordered domains. INTRODUCTION Order in the parent phase can have a profound effect on the characteristics of a martensitic transformation and in many alloys, such as the Cubased shape memory alloys, ordering occurs so rapidly during cooling from above the critical temperature, Tc, that it is difficult to suppress by quenching. In Fe 3 Pt however, the ordering reaction is sufficiently sluggish that the degree of order can be controlled by isothermal treatments below Tc, allowing systematic investigation of the effect of order on the characteristics of the martensitic transformation [1-3]. One of the most dramatic effects of ordering in Fe3Pt is the change from a nonthermoelastic martensitic transformation in disordered or weakly ordered austenite, to a thermoelastic one in well-ordered austenite. The factors controlling this thermoelastic behaviour have been investigated recently by Umemoto and Wayman [3] who concluded, suprisingly, that the degree of order in the austenite is not the dominant factor determining thermoelastic behaviour. They suggested that the Ms must be below about -20°C to obtain thermoelastic transformation and that thermoelasticity is due to a combination of factors, including martensite tetragonality and small transformation volume change. Leaving aside the specific factors responsible for thermoelasticity, the single necessary and sufficient condition for thermoelastic reversibility is the relative absence of plastic accommodation of the transformational shape strain [4]. Ordering in Fe3Pt results in an increase in martensite tetragonality and a substantial decrease in volume change [5], favouring elastic rather than plastic accommodation. The mechanical properties of austenite (y) and martensite (a), as well as the chemical (thermodynamic) properties of the transformation, are also relevant to the above criterion and the relative importance of these various factors has been discussed [4,6,7]. The change from nonthermoelastic to thermoelastic transformation is associated with a distinctive morphological change from thick lenticular plates to thin parallel-sided plates which form in self-accommodating four-plate groups [1,3]. Umemoto and Wayman [3] reported some indirect evidence that these two types of martensite can coexist in alloys with limited long-range (LR) order. These workers also reported that transformation
Data Loading...
