Your question is invalid because the safe voltage for any semiconductor is dependent on a number of factors, mainly core current and temperature which is also determined by duty cycle or operating frequency. And in the case of a CPU processing load as that determines how many internal circuits (transistors) are being used simultaneously.
The CPU is designed for and perfectly safe up to 1.5V. We know this because AMD (Robert Halleck, AMD's technical marketing rep) has told us so several times. But's only safe under certain processing conditions. If you set it up right it will boost to it's rated clock (4.6Ghz) on a single core at a time in light bursty type workloads and may need up to 1.5V to do so. But as the workload sustains, or becomes less bursty, it will lower the boost clock and lower the max voltage so it might also stabilize around 1.4V. So both are perfectly safe, under the appropriate conditions.
A more valid question is this: what's the safe voltage when at maximum processing load, something AVX like CB23, workload. That would be what's called it's FIT voltage. That is variable as it's determined for each CPU during binning. It will be something around 1.25-1.325V and it won't be stable anywhere close to 4.6Ghz with it so you lose a lot of performance for the light burst processing load that's typical of gaming.
And lastly: what is 'safe' for you? The process (electron migration) is cumulative in nature and doesn't 'kill' your processor immediately. It will take a while with a higher voltage (driving higher core current and temperature) making it go sooner. So essentially, any voltage can be considered perfectly safe at 4.6Ghz if you don't mind it degrading in 6 months useage to the point it's no longer stable and has to be down-clocked to 3.8Ghz (it's base clock speed) to do anything.
BTW: The above is only relevant to electron migration processes. The other process that's totally voltage dependent is dielectric breakdown: that's when a high voltage 'punches through' dielectrics inside the CPU. Again, AMD has told us many times gen 3 CPU's are designed to be perfectly safe boosting as high as 1.5V so it seems a safe assumption that the dielectrics can withstand at least that much, and higher for safety margins.