| Popis: |
The influence of forging schedule and subsequent heat treatment on the microstructure and mechanical properties of three a + β titanium alloys has been investigated. The alloys studied were Ti-6Al-4V, Ti-6Al-6V-2Sn, and Ti-6Al-2Sn-4Zr-6Mo. The microstructural comparisons included (a +β) finished and β-processed material, variations in primary a volume fraction, primary a particle aspect ratio, and the nature of the transformation products. Careful selection of forging history and heat treatment permitted comparisons of fracture properties at essentially constant strength levels. This paper describes the influence of forging and heat treatment on the microstructure of these alloys as shown by light and thin foil electron microscopy. The microstructural information was then used to analyze the variations in topography of both fatigue and fast fracture regions as observed by scanning electron microscopy. In some cases, good correlation between fracture topography and microstructure has been established but such correlations depend on microstructure and on loading conditions. For example, fast fracture in the β-processed materials occurs largely along prior β-phase grain boundaries, whereas fatigue cracks propagate by a predominantly transgranular mode. Further, the occurrence of fatigue striations strongly depends on microstructure and on the cyclic stress intensity, with cyclic cleavage predominating at low growth rates in those alloy conditions containing primary a particles which have at least one dimension greater than ∼25 μm. Other correlations between fracture topography, microstructure, and properties were obtained and comments for microstructure selection to control properties are included. |
