You have to find the material and assign it to the component definition.
var app =(Application) System.Runtime.InteropServices.Marshal.GetActiveObject("Inventor.Application");
var doc = app.ActiveDocument;
var partDoc = (PartDocument)doc;
var compDef = partDoc.ComponentDefinition;
var mass1 = compDef.MassProperties.Mass;
var mat =
(from Material material in partDoc.Materials
where material.Name == "Titan"
select material).First();
partDoc.ComponentDefinition.Material = mat;
if (mat.StyleLocation == StyleLocationEnum.kLibraryStyleLocation)
mat.ConvertToLocal();
var mass2 = compDef.MassProperties.Mass;