It is available from National Institute for Materials Science in Japan as Metallic Material Microstructure Database:

http://creptimg.nims.go.jp/index_en.html

You can also find a number of other databases like Creep Data Sheets, Fatigue Data Sheets, Corrosion Data Sheets, Space Use Materials Strength Data Sheets etc.

http://smds.nims.go.jp/creep/en/

http://smds.nims.go.jp/fatigue/en/

http://smds.nims.go.jp/corrosion/en/

http://smds.nims.go.jp/space/en/

Registration and access are free

http://creptimg.nims.go.jp/index_en.html

http://www.asminternational.org/materials-resources/online-databases

http://www.nist.gov/srd/materials.cfm

Please see the Open Phase-field Microstructure Dataset (OPMD) curated on https://microstructures.net if you are still interested in computational microstructure datasets.

Better late than never!

I found another Database: https://www.schmitz-metallographie.de/en/structures-database/

There are not only micrographs but also SEM images and failure cases.