Deformation-induced phase transformation in zinc sulphide

There are two crystalline modifications of zinc sulphide. The high-temperature hexagonal (e) phase and the low-temperature cubic (/~) phase exhibit a reconstructive transformation at a transition temperature of about 1020 ° C. The transformation of e phase back to ~ phase is a sluggish process compared to that of/3 to :~ [1], therefore once it is formed the a phase will remain as a metastable phase at room temperature. The presence of e-ZnS in a/?-ZnS matrix produces a twophase material; this is detrimental to the optical properties because of the resultant scattering loss for transmitted light due to a small mismatch in the refractive indices of the two ZnS phases [2]. It has been shown that the transformation of a metastable hexagonal phase containing ZnS single crystal to a complete cubic structure can be achieved through plastic deformation at room temperature by applying shear stress along a (1 1 1) slip plane [3]. In this letter we report our observation of deformationdependent phase transformation in polycrystalline ZnS near its phase transition temperature. ZnS pellets were prepared by hot-pressing from a high-purity ZnS powder (G.E. > 99.9%) under pressure of 200 MPa and temperature range of 900 to 1050°C for a period of 15 to 90 min. The temperature and time varied to control the ~ phase content. Some of the starting ZnS discs were obtained from Eastman Kodak Co. Specimens for the deformation experiments were cut as rectangular parallelepipeds of approximate size 4.5mm x 4.5mm x 6mm or 7mm x 7mm x 9ram. The ~ phase content of the starting material ranged from 2.9 to 43.3%. The testing was done in uniaxial compression at constant displacement rates, in argon at one atmosphere pressure. The displacement rates used were from 0.01 to 1 mm rain ~, corresponding to initial strain rates of 1.4 x 10 5 to 2 x 10 -3sec -~. X-ray diffraction was employed to determine the e-ZnS contents of the samples before and after deformation testing, using the procedure described elsewhere [4]. These results were then compared with those obtained from static annealing of samples under otherwise identical conditions. Fig. 1 shows the a-ZnS content as a function of deformation or annealing time at 1000°C for various initial ~ contents. It is clear that at this temperature deformation accelerates the/~ to ~ phase transition in the whole c~-content range studied. From the figure, it would seem that the transformation is strongly strainrate or flow-stress dependent. This is not really the case. In Fig. 2 we plot data in the form of A~ (difference of a content between deformed samples and annealed ones at a given time) against the strain for samples having initial ~ contents of 2.9 and 5.5%. There is a nearly linear relationship of Aa and the strain. The same trend was observed at other temperatures in the cubic ZnS range. For example, results obtained at 950 ° C give a similar strain dependence of the transformation, as shown in Fig. 3. Such a correlation between the strain and Ac~ during the dynamic phase transformation of polycrystalline ZnS is consistent with the findings in single-crystal ZnS deformed at room temperature [3]. This suggests that the conversion of metastable hexagonal ZnS structure to the cubic one may result from successive glide of partial dislocations creating the same shear in each of the slip planes. In polycrystalline ZnS, however, due to co-existence of the dislocation and diffusional