a) is an electron micrograph of coarse primary TiB particles in a titanium alloy composition (Ti-6Al-4V-1.7B) above the eutectic limit;
b) is a fractograph of a tensile specimen showing preferential crack initiation at coarse primary TiB particles;
a) is a graph of ductility versus temperature in as-compacted Ti-6Al-4V-1B alloy with different carbon concentrations;
b) is a graph of ductility versus temperature in an extruded Ti-6Al-4V-1B alloy with different carbon concentrations;
a) is a backscattered electron micrograph of a Ti-6Al-4V-1B alloy compacted at 1750° F. (below the beta transus);
b) is a backscattered electron micrograph of a Ti-6Al-4V-1B alloy compacted at 1980° F. (above the beta transus);
a)) is a backscattered electron micrograph of a Ti-6Al-4V-1B-0.1C alloy extruded at a ram speed of 100 inch/min., taken along the extrusion direction;
b) is a backscattered electron micrograph of a Ti-6Al-4V-1B-0.1C alloy extruded at a ram speed of 100 inch/min., taken along the transverse direction;
c) is a backscattered electron micrograph of a Ti-6Al-4V-1B-0.1C alloy extruded at a ram speed of 15 inch/min., taken along the extrusion direction;
d) is a backscattered electron micrograph of a Ti-6Al-4V-1B-0.1C alloy extruded at a ram speed of 15 inch/min., taken along the transverse direction; and
The present invention provides a novel method of increasing the strength and stiffness while maintaining the ductility of titanium alloys by the addition of boron and controlled processing. This new and improved method causes the natural evolution of fine and uniform microstructural features. Although the description hereinafter is specific to a powder metallurgy processing technique, the invention is equally applicable to other metallurgical processing techniques.
In the pre-alloyed powder metallurgy approach, the boron is added to the molten titanium alloy and the melt is atomized to obtain boron-containing titanium alloy powder. The powder may be consolidated and/or formed via conventional techniques such as hot isostatic pressing, forging, extrusion and rolling.
The method of the present invention includes four important elements which are described hereinafter.
While boron is fully soluble in liquid titanium, its solubility in the solid phase is negligible. The binary Titanium-Boron phase diagram shown in
It has been discovered that the carbon concentration also significantly influences the ductility of boron-modified titanium alloys and it is important to keep the carbon level below below a critical limit to avoid an unacceptable loss of ductility. Unlike boron, the solid solubility of carbon in titanium is high (up to 0.5 weight %) and carbon in titanium could cause embrittlement. The carbon concentration, therefore, should be controlled depending on the alloy composition and processing parameters to achieve acceptable ductility values. For example,
Owing to negligible solid solubility of boron in titanium, excess boron is trapped (supersaturated) inside the lattice of titanium under non-equilibrium solidification conditions (e.g. powder manufacture via rapid solidification techniques such as gas atomization). Titanium alloy with supersaturated boron is inherently brittle and possesses low ductility values. It has been discovered that the supersaturated boron can be forced out via thermal exposure at a high temperature. Experiments to determine the optimum temperature for eliminating the supersaturation are illustrated in
Thermal exposure at lower temperatures results in close inter-particle spacing which restricts the ductility. Exposure above the beta transus increases the inter-particle spacing which improves the ductility. The rate at which the material is cooled after thermal exposure alters the grain size and morphology, both of which also significantly influence the ductility. Controlled slow cooling from above the beta transus produces fine-grained equiaxed alpha-beta microstructure due to the influence of TiB particles on the phase transformation reaction of high temperature beta to room temperature alpha. The beta transus varies with the composition of principal alloying elements in conventional titanium alloys, and, e.g., is 1850±50° F. for Ti-6Al-4V. Thermal exposure may be applied via hot isostatic pressing, extrusion, or another suitable consolidation method, or by thermal treatment before or after consolidation, or thermo-mechanical processing. The effects of thermal treatments in HIP compacts and extrusions are shown in
The rate at which boron-modified titanium alloy is subjected to deformation also has significant influence on the final microstructure and mechanical properties. Microstructures of Ti-6Al-4V-1B-0.1C material extruded at a fast ram speed (100 inch/mm) and slow speed (15 inch/mm) are shown in
The properties of slow-speed extruded Ti-64-1B are compared with a typical Ti-6Al-4V alloy [2] in
It will be readily seen, therefore, that the new and improved method of the present invention increases the strength and stiffness of conventional titanium alloys without significant loss in ductility, thus significantly enhancing the structural performance of titanium alloys.
Boron-modified titanium alloys could be produced using traditional processing methods and conventional metalworking (e.g. forging, extrusion, rolling) equipment can be used to perform controlled processing. Therefore, the improved performance with the use of the present method is obtained without any increase in material or processing cost.
Titanium alloys with 25-35% increases in strength and stiffness could replace existing expensive components for high performance and could enable new structural design concepts for weight and cost reduction.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.