Component Wt. %
Al 95.8 - 98.6
Cr 0.04 - 0.35
Cu 0.15 - 0.4
Fe Max 0.7
Component Wt. %
Mg 0.8 - 1.2
Mn Max 0.15
Other, each Max 0.05
Other, total Max 0.15
Component Wt. %
Si 0.4 - 0.8
Ti Max 0.15
Zn Max 0.25
105-106 vickers
This should be the ball park for your material.
As for Gasing, have you checked your gas for purity?
Is the diamond plate the same thickness?
What does your mtr say about the material?
Have you ran a sample of your base material?
If so, does the chemical breakdown match?
Does the hardness test match?
Given what you've stated, I think I'd be looking at the gas or the material.
Pin holes are not atypical for flaws when it comes to thicker parts. Though it is possible. LOF and others would seem to be on the more Atypical side.
Something else amiss that you haven't looked at I'd think, hope some of this helps.
In the past, many eons ago, I ran across a sim. problem. It was contaminated gas that turned out to be the problem in my case.