I'm not sure what kind of turbine blades you've inspected that aren't steel?? Back in the late 80's/early 90's, we ran our own machining and finishing facility. Our major contracts were with suppliers for the military aircraft programs. We did all of the final machining and polishing on the fan blades for various jet engine turbines, and I can tell you for a fact that every one of the them came to us straight out of the investment casting molds. And they were steel. I will say that also did finish work on some blades that were titanium experimental parts, but since we closed the company, I don't know if they ever went into production. You can bet that 99% of the blades in engines in use today are steel, since most of the aircraft in use are going on 20 years or longer.
Here's a quote from from a website on investment casting of steel:
Applications
Investment casting is used in the aerospace and power generation industries to produce single-crystal turbine blades, which have more creep resistance than equiaxed castings. It is also widely used by firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost. Other industries that use standard investment-cast parts include military, medical, commercial and automotive.