Thermal Expansion:
What is the nature of invar armor? Does it have special properties? I can't find anything on it anywhere.
I'm not sure, but it's probably higher durability than iron, with similar protection (and at minimum it saves you a third in iron). Try and experiment, see how much armor it gives you, and how high a damage value it can accrue before breaking.
Thermal Expansion:
Does the current version of TE in FTB Unleashed still allow for rich slag to be produced? I've smelted around 200 ores (not dusts) with sand in my induction smelter so far, with not a single piece of rich slag produced. Am I just incredibly unlucky?
If it is possible to get it, the recipe will show up in NEI. If so, then you're very unlucky.