Do note that hardness isnt the only property that makes a material suitable to cut things/people up with. Hardness is merely the amouth of force needed to deform a material, not when that material loses edge or breaks.
Irirdium would be very unsuitable for a weapon. It would not only be unhandlable due to being allmost twice as dense as lead, but also very prone to breaking. Thats what hard things do, and also why we didnt make swords out of stone or glass. That and its impossible to forge iridium due to its hardness and high melting point.
First, a heavier weapon is mostly a problem with long weapons like longswords and two-handed weapons, at a weight of 1.1 kg ordinarily a short sword will weigh 3.15kg or around 7 pounds which is less than things in the same size i've handled like one, it gives me ache and makes me tired, but i have weak arms too...
About how brittle it is... well i have not had any breaking of iridium not that i've handled that much, but it didn't seem that brittle to me.
Also, alloys arnt as simple of taking an avarage of each ingredients 'stats'.
I was quite close to taking metallurgy as the main subject in chemistry at the university, and as I checked it out first, i so now things like that, but thanks for the thought...
Of course im not telling you to not mod in iridium, it could be fun ingame to make some weird alchemical metals. Im just had to say it would be useless for weapon making in reality.
Maybe pure Iridium would be, i will check just how brittle it is... but the weight is not a limiting factor for short swords and such weaponry (maybe axes and maces as i don't know even an example weight of such weaponry...
Also if the brittleness will be a huge problem for weapon smithing i will probably focus more on iridium alloys...
A big problem however is the temperatures needed... but i have thoughts about that too...
A weapon coated in iridium would never rust though.. and would look sweet when seen from a distance as the colors refract and dazzle off of it..
It would most likely not be destroyed by any corrosive substance either... but the dwarf holding it most likely will be...
/Zorbeltuss