Thermal Aging of PM/HIP and Cast Alloys 625 and 690


Nuclear structural components are conventionally fabricated by casting or forging, but these processing methods are subject to fabrication defects and cavities, and require considerable post-fabrication machining and welding.  Powder metallurgy with hot isostatic pressing (PM/HIP) is an alternative processing method that creates dense, homogeneous microstructures containing fewer defects.  PM/HIP heavy components can also be produced near-net shape, reducing the need for machining and welding.  Hence, there is interest in qualifying PM/HIP alloys for nuclear structural applications.  Amongst the extreme conditions these alloys must withstand is long-term operation at elevated temperatures >200°C.

We focus here on evaluating the Ni-base Alloys 625 and 690 and their microstructural and mechanical evolution under thermal aging.  We compare the behaviors of the PM/HIP versions of both of these alloys to their cast counterparts.  Thermal annealing is conducted at 400°C, 600°C, and 800°C, to 100 hr, 1000 hr, and 10,000 hr.  Subsequently, we utilize light optical microscopy to characterize grain size evolution.  We also use dynamic microindentation to evaluate hardness, yield strength, and elastic modulus.

In general, we find that 100 hr aging has little effect on the grain size or the mechanical behavior of all alloys.  A notable exception is the cast 625, which appears to exhibit softening after 800°C aging for 100 hr; this softening may partly be attributed to slight grain growth.  Additionally, the cast 690 appears to exhibit grain size reduction after 600°C and 800°C aging for 100 hr, but this reduction does not appear to affect the yield strength.


  • D.W. Gandy, Electric Power Research Institute
  • D.P. Guillen, Idaho National Laboratory
  • H. Wang and V. Tomar, Purdue University


  • DOE Nuclear Science User Facilities project 15-8242
  • Network for Computational Nanotechnology SURF Program
  • Purdue SURF Program


  1. E. Bautista, A. Bullens, and J.P. Wharry. Understanding the role of fabrication method on metal alloys used in nuclear power plants. Purdue University Network for Computational Nanotechnology (NCN) Summer Undergraduate Research Fellowship (SURF) Symposium, West Lafayette IN, August 2017.
  2. A. Bullens, E. Bautista, K. Mao, and J.P. Wharry. Comparison of PM-HIP to cast Alloy 625 for nuclear applications. Purdue University College of Engineering Summer Undergraduate Research Fellowship (SURF) Symposium, West Lafayette IN, August 2017.

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