What type of metal is used in dentures?

Depends. Most partial denture frameworks use an alloy or mixture of metals. These are typically gold, nickel, or chromium alloys. Full dentures rarely contain metals. There are partial dentures available today that are made without metal.
Chromium Cobalt. Metal based removeable partial dentures and even sometimes full dentures have frameworks of metal. Usually chromium/cobalt. Fixed dentures such as bridge work is generally a high nobel metal specifically gold.
None usually. Unless it is a partial denture, metal is not used however, for partial dentures the metal is cobalt-chrome.
There are a few... Historically, removable partial and complete denture frames were composed of a gold alloy. This alloy has fallen out of favor due to its high cost. These days, the most popular casting alloys used for this purpose are chromium-cobalt alloys; the prototype was a proprietary product called "vitallium". This alloy combines strength, rigidity, resistance to corrosion, and bio-compatibility.