Dynamic Compaction of Titanium Aluminides by Explosively Generated Shock Waves: Experimental and Materials Systems A. FERREIRA, M.A. MEYERS, N.N. THADHANI, S.N. CHANG, and J.R. KOUGH Different approaches to compact cylinders of titanium aluminide powders by explosively generated shock waves were explored. Two basic compositions of the titanium aluminide pow- ders produced by the rapid solidification rate (RSR) technique were used: Ti-21 wt pct Nb-14 wt pct A1 and Ti-30.9 wt pct A1-14.2 wt pct Nb. A double-tube design utilizing a flyer tube was used in all experiments. Experimental parameters that were varied were initial temperature, explosive quantity, and explosive detonation velocity. The major problem encountered with shock consolidation of titanium aluminides was cracking. Titanium aluminide powders were also mechanically blended with niobium powders in one case and elemental mixtures of alu- minum and titanium powders in the other case. Enhanced bonding and decreased cracking were observed in both cases. In the former case, the addition of niobium powder provided a ductile binder medium which assisted in consolidation. In the latter case, due to the additional heat generated and melting produced by the shock-induced reactions between Ti and A1, significant improvements in bonding of the titanium aluminide powders were observed. I. INTRODUCTION TITANIUM aluminides are potential materials for many of the rotating and static components in the compressor sections of gas turbine engines t~'2] in aerospace appli- cations because of their low density and high-temperature properties, t3'al The objectives of this work were twofold: (1) to present the macrostructural features resulting from dynamic compaction of rapidly solidified titanium alu- minides by explosively generated shock waves and (2) to investigate the effects of alloy composition, niobium powder additions, shock-induced chemical reaction be- tween mechanically blended powder mixtures of tita- nium and aluminum, initial temperature, amount of explosive, and detonation velocity of the explosive on the cracking density of compacts. II. EXPERIMENTAL PROCEDURE A. Material Characterization Table I lists the characteristics (alloy type, run num- ber, composition, pct carbon, and powder size) of the rapidly solidified powders as obtained from Pratt & Whitney, West Palm Springs, FL, and used for the com- paction experiments. These powders were produced by the rapid solidification rate (RSR) process. The particle size distribution obtained by sieve analysis was per- formed at Pratt & Whitney. There is a considerable de- gree of variation in the compositions reported in Table I. The main differences are in the carbon content and in the presence of erbium for part of the powders. The high- A. FERREIRA and J.R. KOUGH, Department of Metallurgical and Materials Engineering, and N.N. THADHANI are with the Center for Explosives Technology Research, New Mexico Institute of Mining and Technology, Socorro, NM 87801. M.A. MEYERS and S.N. CHANG are with the Center of Excellence for Advanced Materials, University of California, San Diego, La Jolla, CA 92093. Manuscript submitted September 5, 1989. carbon powders (Ti3A1) are the top four ones in Table I and were solely used for filler material in the containers. For the central portions of the containers, which were subsequently analyzed and tested, the low-carbon pow- ders were used. Erbium additions for a number of runs are intended to improve the high-temperature properties through the formation of erbium oxide. B. Experimental Systems The experimental setup consisted of two coaxial tubes, the external one being accelerated inward to impact the internal tube that contains the powder. A detailed de- scription of the system is presented elsewhere.iS1 The basic experimental system is shown in Figure 1, and this tech- nique is called the flyer-tube or double-tube technique. The explosive charge (an ammonium nitrate-fuel oil (ANFO) mixture with 6 pct oil) is detonated at the top, and a DuPont DETA SHEET* (a 2-mm-thick PETN-based *DETA SHEET is a trademark of E.I. DuPont de Nemours & Co., Inc., Wilmington, DE. plastic explosive) is used to create a more uniform det- onation front. The explosive charge is contained in a polyvinyl chloride (PVC) plastic tube resting on a wood base and surrounds a mild steel flyer tube, in the center of which is the assembly containing the powder (stain- less steel pipe with top and bottom steel plugs). The con- tainers are filled with the powder under argon atmosphere. The central axis of the powder container sometimes has a solid rod (mandrel) to eliminate Mach stem forma- tion. tSj Shock consolidation conditions were varied by changing the detonation velocity and amount of explosive. In order to impart a greater ductility to the starting powders and to bring them closer to the melting point, it was felt that explosive consolidation of preheated pow- ders would be effective. Preheating the powder may in- duce sufficient ductility into the powders to make them easy to deform and conform along their neighbors during METALLURGICAL TRANSACTIONS A VOLUME 22A, MARCH 1991--685