The material 60-Nitinol (60wt%Ni-40wt%Ti) has a unique combination of physical properties, including high hardness, low apparent elastic modulus, and resistance to saltwater corrosion. These properties give the material tremendous potential for use in aerospace and defense-related components such as bearings, gears, and other apparatuses. Various methods of primary processing are being explored for fabrication of high-performance components that are free of metallurgical defects that might lead to premature failure. Hot isostatic pressing (HIP) is one process under consideration. The steps in the HIP process include (a) filling a sealed canister of the appropriate dimensions with powder, (b) heating the canister under vacuum to remove volatile and gaseous contents, (c) applying heat and pressure to the evacuated and sealed canister to consolidate the contents, and (d) removing the canister.

When used to consolidate metal powders, HIP has distinct advantages over other processing techniques such as casting; namely, the mechanical properties of the consolidated material tend to be more isotropic due to the random orientation of grains, which are dictated by the random orientation of the powder particles. Likewise, the bulk material tends to be more chemically homogeneous due to reduced chemical segregation. Also, the defect size within the bulk material tends to approximate the particle size. One of the major disadvantages of HIP is the cost. The purchase cost of the powders, HIP processing (especially if long cycle times are needed), and canister removal can represent a barrier to technological development and adoption by industry. In addition, due to the fact that hydrostatic forces are applied during consolidation, there are no shear forces that might otherwise act to disrupt adsorbed oxide films on particles. These oxide films then persist in the bulk material, serving as potential fracture propagation paths. This study was undertaken to provide information to reduce the economic burden of developing HIP processing of 60-Nitinol.

The effects of varying the time, temperature, and pressure during consolidation of 60-Nitinol by HIP were examined. Six HIP cycles with a cycle time of either 2 or 20 hours, temperature of 900 or 1000 °C, and a chamber pressure of either 100 or 200 MPa were used. While the longest (thus, most expensive) cycle time at the highest temperature produced material with the least porosity, there was also modest grain growth. The cycle representing the shortest cycle time at the highest temperature and pressure (2h/1000 °C/200 MPa) produced well-consolidated material with the highest hardness (720 HV).

This work was done by Malcolm Stanford of Glenn Research Center. NASA is seeking partners to further develop this technology through joint cooperative research and development. For more information about this technology and to explore opportunities, please contact here . LEW-19341-1

NASA Tech Briefs Magazine

This article first appeared in the March, 2017 issue of NASA Tech Briefs Magazine.

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