Influence of texture on fatigue properties of Ti-6Al-4V
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I.
INTRODUCTION
FOR (a + fl)-Ti-alloys the microstructure is known to have a dominating influence on the fatigue properties. It is generally believed and supported by sufficient experimental data that refining the microstructure results in an improvement of the fatigue strength for smooth specimens; 1-6 however, resistance against fatigue crack propagation is decreased. 5-9 Furthermore, texture effects have to be considered. 7'1~This influence is much more pronounced for commercial (c~ + /3)-Ti-alloys than for steels or Al-alloys due to the high volume fraction of the inherently anisotropic hexagonal a-phase. The experimental data available clearly reveal a strong influence of texture on the fatigue behavior of Ti-alloys, though they are not quite consistent because the textured materials tested often exhibited inhomogeneous microstructures. 11-14Therefore, it was impossible to separate clearly the various effects of texture and/or microstructure. Recently, however, it has been shown for the Ti-6A1-4V alloy that texture and microstructure can be controlled nearly independently. ~5This offers the opportunity of individually investigating the influence of texture on fatigue properties without having to consider microstructural effects, which is the intention of the present investigation. II.
EXPERIMENTAL PROCEDURE
The material investigated had the following chemical composition: 6.4 pct AI, 4.0 pct V, 0.19 pct oxygen (wt pct). The alloy was made by TIMET and forged in the ]3-phase field to a plate thickness of 30 mm by Ladish Company. The starting heat treatment for all specimens was 15 minutes at 1050 ~ followed by water quenching. Rolling was used for deformation processing. The different textures resulted from changes in rolling mode and temperature while the deformation degree was kept constant M. PETERS is Research Scientist, Deutsche Forschungs- und Versuchsanstalt for Luft- und Raumfahrt (DFVLR), Institut fiir Werkstoff-Forschung, D-5000 K61n 90, West Germany. A. GYSLER and G. LUTJERING are Professors, Technische Universit~it HamburgHarburg, D-2100 Hamburg 90, West Germany. Manuscript submitted November 28, 1983. METALLURGICALTRANSACTIONS A
(e = - 1.4). After rolling the material was water quenched and heat treated for 1 hour at 800 ~ followed by water quenching. The specimens were then aged at 500 ~ for 24 hours and air cooled. All heat treatments were done in an argon atmosphere. For light microscopy studies polished specimens were etched in a solution consisting of 2 ml HF, 3 ml HNO3, and 95 ml HzO. The texture investigations were performed on a Philips goniometer using the reflection method. To determine the full pole figure a composite sampling technique was employed as described in detail by M~iller and Knott. ~6 The results are given in (0002) pole figures. Tensile tests were performed on cylindrical specimens with a gage length of 20 mm and a gage diameter of 4 mm using an initial strain rate of 8.3 x 10-4 s -1. Two tests were performed for each condition. Fatigue life curves were measured on hour
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