The PO should provide a description of requirements for the delivered product, including the associated engineering drawing release. Before taking any action, you need to carefully review all of the related documentation listed on the engineering drawing and PO. Also make sure the customer provided all related change documentation such as advance ECNs that may not be listed on the drawing. If you dig down deep enough in the customer's process documentation, you might find something that allows this type of material substitution.
On the other hand, as IRstuff noted, if this part is used on a product whose design was certified based on specific material properties for that part, you should not use an alternate material without authorization. The correct approach would be to submit a written request to the customer for a formal design change or an allowance to use a substitute material for the existing purchase contract. If you deliver the finished part with non-conforming material certs, it is quite likely the customer's receiving QA will reject the parts, and your company will have to eat the cost.