Ti-6Q2 was created for the aerospace industry. Ti-6Q2, which is a brand name for Ti-6Al-2Zr-2Sn-2Mo-2Cr-0.25Si, is a proprietary alloy produced by RTI metals. With cutbacks in US defence spending, lots of excess Ti-6Q2 was apparently dumped onto the market for cheap a couple of years ago.
This isn't to say Ti-6Q2 is a lousy material, just consider the economics of acquiring raw material.
Ti-6Q2 modulus is 16.2 msi (111.7 GPa) in bar form. Ti-6Al-4V (more commonly known as grade 5) has a modulus of 16.9 msi (116.5 GPa) in bar form according to the US Metallic Materials Properties Development and Standardization-01 document. So, it is actually marginally LESS stiff than Ti-6Al-4V, but for all intents and purposes its essentially the same. not something that can be felt out on the road. no one is strong enough to bend/deform pedal spindles on the road. I don't know about density, but they are probably very close.
Ti-6Q2 and Ti-6Al-4V will have differing properties in fracture and fatigue resistance, also depending on how you heat treated it, you could make Ti-6Q2 stiffer than T-6Al-4V, but not by much. They are meant for different applications- Ti6Q2 most likely has to deal with higher temperatures than Ti6Al4V. I don't know which one is better in fatigue/fracture resistance, but if it's bar stock going to the aerospace or medical industry it's going to be high grade.
You are mistaken that stiffer is always better, and new materials are not always adopted based on the economics involved. The medical industry actually wants LESS stiff titanium to better match the modulus of bone. in aerospace some flex is sometimes designed into aircraft parts. You need to understand what engineers mean by TOUGHER and STIFFER, (toughness is the resistance to fatigue, stiffness is the resistance to elongation under stress) and that depending on the desired part, these properties arent always desired.
As for the economics of adopting a new material, any material developed for the Medical and Aerospace industry (the 2 biggest industries to use titanium, also , oil and gas) has to go through very rigorous and expensive standardisation and testing. unless our new alloy has a HUGE advantage or a very specific niche application, you aren't displacing an alloy which has been tested and proven (e.g. Ti-6Al-4V ) because it takes a very long time ( 3-5 years for most medical applications) and alot of money, something most enterprises are not willing to undertake.- i.e. if it ain't broke, don't fix it. The bicycle industry is not big enough for anyone to actually go develop a Ti alloy blend custom for the needs of bikes. Ti-6Al-4V serves us quite well enough.
Thanks for the answer. I did not say that stiffer is always better though (did I?). At least that's not what I ment. I do understand that stiffness and toughness isn't the same thing (although often reflecting one another). I'm studying for a masters degree in construction engineering, so I do understand some basic principles. But as I'm not studying mechanical engineering and I'm only in my second year out of five, my knowledge is indeed limited. Especially when it comes to industries adopting new materials and such. But if the material is better for some purposes (although marginally), one would think there was at least some manufacturers using it, for products that's mostly marketed against industry purposes, but available for consumers (i.e. bolts/fasteners etc.)?
ummm no. Switching costs are high. you want to switch an entire assembly line over? then you'd have to explain to your long time Ti6Al4V supplier why you're ordering less, risking some valuable relationships. you'd need to consider if you're a big player you're probably holding on to futures contracts of Ti6Al4V already, so theres sunk costs there. then you'd need some downtime in your production line to recalibrate processes, clean ( no contamination!) re-train workers.... and time is money. Then you'd have to convince customers that your new magic titanium is superior to the normal titanium they've been using for years and worth the extra $1 per part you'll be charging them.
Then what happens when you switch back? same thing again. No one gives a rats ass if your bolt has an 10MPA UTS more than the normal bolt, because engineering products are already designed with a safety factor in mind - i.e. if your part is expected to see loads of 200MPA, you dont design for 200 MPA- you design for 300MPA (in aerospace, safety factor is 1.5-2.5 . safety factor for bolts is 8.5) . so a bolt that fails at 850MPA is actually only rated for use in 100MPA. who gives a rats ass if your 100MPA rated screw fails at 860MPA over the 850MPA screw? is it worth the extra $1?
As I said unless your material has a very clear and significant advantage, no one invests that kind of money and time just for a small advantage, especially over something that has to meet very stringent certifications, is good enough, and everyone is familiar with.
The desire for niche weight weenie parts is a very, very small component of manufacturing. for bicycles hobbyists, it's even smaller. The titanium spindles guys are a unique case you are not likely to see again.