while making submodel i have an error message says(disk image is malformed)
Try storing the .wtg.sqlite that is created when exporting to a submodel to a different location. Then try to re-create the model from this sub-model. See the article below which has details on resolving this error;
Crash or Error related to "11: database disk image is malformed"
In the above article there is an option of repairing the database yourself if you have an application that can open .sqlite files (like SQLite Express Personal) and try re-indexing all tables or run an integrity check for the .sqlite file.
Let me know if this helps.
Bentley Technical Support