Effects of tensile stress on microstructural change of eutectoid Zn-AI alloy
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I.
INTRODUCTION
MECHANICAL properties of a material determine its response to application stress. The stresses may be tension or shears and may be applied to a material individually or in combination. Understanding the mechanical properties, especially those structure-sensitive properties including the strength properties and the creep, fatigue characteristics is one of the most difficult tasks, because these properties depend in a sensitive way on the extent and distribution of atomic-scale imperfections in the material and, therefore, very much depend on the past thermal and mechanical history of the particular specimen tested. Several recent publications have shown the increasing interest in the influence of extensive mechanical working on the refinement of the microstructure in hypereutectoid steels and titanium alloys,v,2] Unfortunately, rare systematic studies of phase relationships of the materials under both thermal and mechanical treatments are available, and little work has been done on the effects of the applied extra stress induced by tensile, impact, creep, and fatigue processes on the phase transformation and microstructural change of the materials. The present article will discuss the effects of the stress induced during tensile testing on the microstructural change and especially the phase transformation, based on the systematic investigations of the cast, solution-treated, and extruded eutectoid Zn-Al-based alloy53 17] II.
EXPERIMENTAL
Alloy material was cut from extruded eutectoid Zn-A1based alloy rods; the chemical composition of the alloy was Zn78.0, A120.2, and Cul.8 (wt pet). The material was solution treated at 350 ~ for 4 days and then furnace cooled to room temperature in order to obtain lamellar structure. This heat-treated material was machined into a standard
YAO HUA ZHU and ELIGIO OROZCO, Investigators, are with the Institute of Investigations on Materials and Institute of Physics, respectively, Department of Materials, Metals and Ceramics, Ciudad University, UNAM D.F. 04510, Mexico. Manuscript submitted October 12, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
specimen of 10-mm diameter with 50-mm gage length for tensile testing. The tensile test was carried out at 100 ~ on an Instron machine at a crosshead speed of 7.00 • 10 -3 mm/s. The temperature of the tensile test was controlled to ___2 ~ during the test. Load and extension were recorded automatically on chart recorders, and then engineering stress-strain curves were plotted. X-ray diffraction examination was performed on a diffractometer with nickel-filtered Cu K, radiation, scanning at a speed of 1 deg/min. Phase identifications were carried out on various parts of both specimens before and after tensile testing within the diffraction 20 angle range from 35 to 47 deg in order to obtain the strong X-ray diffraction. Scanning electron microscopy (SEM) was applied for identifying phases and showing morphologies of various parts of both etched and nonetched specimens tested by using secondary electron and backscattered ele
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