The Effect of Microstructure on the Strength of VDM Alloy 780

Abstract

Nickel-base alloys have been developed, which offer higher temperature capability from ingot metallurgy than Alloy 718Alloy 718. However, these alloys have higher volume fractions of gamma primeGamma prime (γʹ) precipitates, require further processing steps and consequently, have higher material processing costs, show a greater propensity for freckleFreckle and are not readily electron beam (EB) welded. There is an appetite for an alloy that shows improved forgeability, that is EB weldable and can be used at temperatures of up to 700 °C. This study examines VDM Alloy 780VDM Alloy 780, first using laboratory compression tests and heat treatmentHeat treatment experiments to determine suitable thermo-mechanical processing (TMP) conditions. Subsequently, pancakes that were 21–22 mm in height and 130–133 mm in diameter were forged from 70 mm in diameter × 76 mm high bars, which were extracted from mid-radius locations of 8-inch diameter billet. From these, test piece blanks were extracted, and heat treated for tensile testsTensile tests at temperatures of 20, 650 and 750 °C. Results from these tests were compared with microstructureMicrostructures and data from 30 mm thick pancake forgingsForging that received different TMP conditions from work at VDM Metals International. The combined experiences provide an insight into the effect of forgingForging and heat treatmentHeat treatment conditions on the microstructureMicrostructures and tensile testTensile tests properties of VDM Alloy 780VDM Alloy 780. They show that strength levels for fine grain Alloy 720Li can be achieved if specific sizes of γ grains and γʹ precipitates can be produced.