March 2004 Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction 467 elastic part of the stress-strain curves to the maximum stress. In Table 1, wt%o monoclinic phase estimated sample 2, the longitudinal strain deviated from the straight line by from Macroscopic Volume Change a maximum of-1331(He), whereas the transverse strain deviated from the straight line by-5887(He tress(MPa) Hold time (min) (2) Diffraction Patterns 1000 4.66 The diffraction patterns recorded during compression testing of sample 2 are shown in Fig. 3. On close scrutiny, in the loading 1150 1200 154 2336 process, the(I 11) reflection at% 26, grew gradually as the 1225 109 applied stress increased. The reflection appears to increase in size Total 989920.2 ven at relatively low loads. An obvious increase occurred when the applied stress was >1000 MPa. There is a corresponding decrease in the(013)and (121) peaks of tetragonal phase near 65 20 especially after o exceeded 1 100 MPa. At higher stresses, the To provide an independent measure of the real phase comp (I1I)m peak continued to increase as the load increased, until the tion In 9. 4Mg-PSZ under each applied stress, quantitative phase maximum stress, 1225 MPa. During the unloading process, no change in the size of the (11 1)m peak was observed In addition to analysis was conducted using the Rietveld refinement scale fac- peak intensity changes, some additional line broadening can be tors where the wt% of phase p is given by observed, e.g., in the double peak near 6520, where there is a (SZMV progressive loss of resolution y (SZMV 3)t→ m Transformation where Wp is the weight fraction of phase p, S is the scale factor, Z From the strain data, it is clear that the sample volume increased after the applied stress exceeded 800 MPa. It is well known that the one unit cell, and V is the unit cell volume of each phase. In total tetragonal-to-monoclinic phase transformation will give rise to a four phases were considered: the cubic, tetragonal, monoclinic 4.9%e volume increase. If it is presumed that the volume increase and 8-rhombohedral phases. The phase composition for the start in this experiment came totally from the f-m phase transforma- ing material is listed in Table II with error estimates. tion, the volume increase can be used to estimate the percentage of The calculated weight percentage of tetragonal phase and new monoclinic phase, The estimated fraction of monoclinic phase corresponding weight percentage of monoclinic phase during the produced during each hold at constant load was calculated and is whole mechanical testing process are shown in Figs. 4(a) and(b) shown in Table L. These results neglect a small amount of The arrows indicate the sequence in which the external stress was transformation that occurred essentially instantaneously during applied. When the load was between 0 and 800 MPa. the fraction application of the load increment In the loading half-cycle. a total of monoclinic phase was constant to within the estimated errors. of -20 wt% of new monoclinic phase was predicted by this As expected from the raw neutron data( Fig 3), during loading, the calculation greatest increase in the monoclinic phase was observed at stresses 20000 MPa 15000 420 058 4567 120 1200 5000}1150 1000 400 20(degree) Fig 3. A portion of the diffraction patterns recorded during compression testing of the 94Mg- PSZ. Note the growth of the monoclinic-phase peaks, e.g. at31°20