Michael R. Splinter has been elected president, CEO and a member of the board at Applied Materials Inc., a supplier of products and services for the semiconductor industry. Splinter succeeds James C. Morgan, who will continue as chairman of the board, and as president he succeeds Dan Maydan, who will remain a board member. The company also announced that Sasson Somekh, executive vice president and chairman of its global executive committee, was appointed a director, effective immediately.   . . .   Boeing vice president and general manager Allen Ashby has announced his retirement after 36 years with the company, effective May 30. Ashby has been responsible for the Battle Management Command, Control & Communications and Strategic Systems organization based in Anaheim, Calif. His successor has not been named.   . . .   Nikon Instruments Inc. and EXFO Electro-Optical Engineering Inc. announced they have signed a reseller agreement under which Nikon Instruments will offer EXFO's X-Cite 120 Fluorescence Illumination System with its microscopes in the US for fluorescence microscopy applications.