DEVELOPMENT OF LOW DENSITY TITANIUM ALLOYS

Under a research grant sponsored by the National Science Foundation, entitled “Development of Low Density Titanium Alloys” (NSF Award # DMR-9901642), a series of binary and ternary titanium alloy systems have been studied in which reduction in density of titanium alloys has been achieved. Low density alloys are attractive as they can give a direct weight reduction compared to conventional alloys such as Ti-6Al-4V.Of the elements which will reduce the density of titanium only aluminum and to a much lesser extent silicon can be alloyed with titanium using a conventional ingot metallurgy route. Thus alloying of other elements which will reduce the density of titanium requires innovative “far from equilibrium” techniques. In the present work two such techniques mechanical alloying (MA) and physical vapor deposition (PVD) were selected.

The elements considered as alloy addition  to titanium (in addition to Al) to reduce the density were B, Be and Si (which have atomic sizes and electronegativity widely different from that of titanium) and Li, Mg and Ca which exhibit boiling points below the melting point of titanium and are immiscible with titanium in the solid state, showing negligible solid solubility even at high temperatures. The light metal Sc has received no attention as an addition to Ti, even though Sc has a density of only 2.9 gm/cc and melting point as high as that of Fe, and this addition was also studied.  In preliminary MA work it was found that titanium hydride exhibited greater solid solubility extension (SSE) than straight titanium and in addition TiH_1.942 also eliminates the need for use a process control agent (PCA) during MA and reduces contamination caused by oxygen, carbon and nitrogen. Thus this approach was added to the program. Based on preliminary work the specific titanium systems studied were Ti-Mg, Ti-Sc, TiH_1.942-Mg-Si, TiH_1.942-Al-Sc, TiH_1.942-Mg-Sc and TiH_1.942-Mg-B. While MA was used to synthesize all these alloys, only Ti – Mg was produced via PVD.

In general, solid solubility extension (SSE) was achieved in many of the alloys systems. The solid solubility of Mg in a Ti – Mg alloy synthesized via MA was extended from an equilibrium (room temperature) value of 0.2 wt. % to 9 wt. %, resulting in a density reduction of 14.2%. The solid solubility of Mg in Ti – Mg alloy synthesized via PVD was extended from an equilibrium (room temperature) value of 0.2 wt. % to 10 wt. %, resulting in density reduction of 15%. In binary Ti – Mg alloys produced via PVD, hardness equivalent to a strength of 1800 MPa have been observed, giving specific strength up to twice as high as Ti-6Al-4V.

In Ti H_1.942-  Mg – Si, MA resulted in a SSE of Mg to 9 wt. %. In the Ti –Sc system MA resulted in a maximum SSE of 8 wt. % from the equilibrium solid solubility value of 2 wt. % giving a 2.5 % reduction in density. In the TiH_1.942 -Al – Sc, MA resulted in the solid solubility of Sc in Ti again increasing to 8 wt. %. In TiH_1.942  -Mg – Sc, MA resulted in a simultaneous extension of solid solubility of Mg to 7 wt% and Sc to 8 wt%. In TiH_1.942  -  Mg – B, MA resulted in SSE of Mg to 7 wt. % with no apparent SSE for B. Based on this extension of solid solubilities, the Ti-9wt.% Mg, Ti-8 wt.%Sc and Ti H_1.942-9wt.%Mg-8wt. %Sc systems appear most attractive for further evaluation via MA. Only the Ti – Mg system was studied in the present work, however based upon the Ti-Sc work it is suggested that this system be further evaluated via PVD.