40HNU-VI

Lore & Background

40KhNYu-VI is produced under the GOST system of Russian alloy nomenclature, with the suffix -ВИ indicating vacuum-induction-melted stock. It is supplied under technical specification TU 14-1-2740-79 and product standards GOST 22411-77 and GOST 2590-2006. The alloy is precipitation-hardened: quenched from above 1100 °C it forms a supersaturated face-centered cubic solid solution, and upon aging above ~500 °C precipitates a chromium-rich phase and ordered intermetallic Ni₃Al (γ′). Cheng et al. (2019) identified three crystalline phases at room temperature: an orthorhombic Ni₂Cr-type phase (about 97 vol%), a body-centered cubic Cr-type phase (about 0.1 vol%), and a cubic Ni₃Al-type phase (about 2.9 vol%).

The TU 14-1-2740-79 specification guarantees, for the standard solution-treated and aged condition, yield strength ≥ 1.08 GPa, ultimate tensile strength ≥ 1.275 GPa, elongation ≥ 10%, reduction of area ≥ 15%, and impact toughness ≥ 98 kJ/m² on a 25 mm round bar, with hardness ≤ 45 HRC. An alternative low-temperature aging regime yields hardness ≥ 57 HRC. For practical high-pressure cell construction, hardnesses in the range 50–55 HRC are preferred; cold work of the quenched alloy prior to aging can raise hardness to 64–67 HRC, yielding material with essentially no tensile ductility but good performance in compression, used in indenters and pistons.

Despite its high nickel content, the alloy is paramagnetic with no detectable magnetic ordering down to at least 1.9 K, as established by vibrating sample magnetometer measurements and confirmed by polarized-neutron diffraction. The mass magnetic susceptibility is 4 × 10⁻⁸ m³/kg at room temperature and 1 × 10⁻⁷ m³/kg at 4 K, with an almost flat temperature dependence. The alloy does not contain cobalt, which would turn to highly radioactive Cobalt-60 under neutron irradiation, making it attractive for high-field, low-temperature applications.

Reader's Guide

40KhNYu-VI is significant primarily as the material of choice for non-magnetic high-pressure cells used in neutron scattering, magnetometry, and diamond anvil cell experiments at cryogenic temperatures. Its combination of high mechanical strength, weak paramagnetism, no neutron activation, and low neutron attenuation makes it considered the best material for this purpose, superior to cobalt-containing alloys such as MP35N and maraging steels which become radioactive under prolonged neutron irradiation. For 4.74 Å cold neutrons, Cheng et al. (2019) measured a transmission of 47.5% through a 6 mm thickness, compared to 61.7% for beryllium copper; 40HNU-VI is less transparent to neutrons than CuBe but has superior strength and magnetic properties. A modification adding boron (56.5 wt% Ni, 40.0 wt% Cr, 3.5 wt% Al, 50 ppm B) was developed by Uwatoko et al. (2002) to improve forgeability, but the boron addition is undesirable for neutron scattering unless isotopically enriched in ¹¹B, because natural boron contains ~20% of the strongly neutron-absorbing isotope ¹⁰B. Klotz (2013) suggests that ¹¹B-modified 40HNU-VI could be used for neutron scattering applications on a wider scale, including to moderately high temperatures. The alloy has not been officially designated a superalloy, as its high-temperature creep and oxidation behavior have yet to be characterized in detail.

Did You Know?

• 40KhNYu-VI is only industrially produced in the Soviet Union and then Russia, and is often called 'the Russian alloy' in the high-pressure literature.
• The alloy does not contain cobalt, avoiding the formation of highly radioactive Cobalt-60 under neutron irradiation.
• Cheng et al. (2019) identified three crystalline phases at room temperature: an orthorhombic Ni₂Cr-type phase (97 vol%), a body-centered cubic Cr-type phase (0.1 vol%), and a cubic Ni₃Al-type phase (2.9 vol%).
• A boron-modified version (50 ppm B) was developed by Uwatoko et al. (2002) to improve forgeability, but natural boron's ¹⁰B content strongly absorbs neutrons.

Composition, Nomenclature & Industrial Provenance

40HNU-VI is a precipitation-hardened alloy whose nominal makeup is roughly 57 weight percent nickel, 40 percent chromium, and 3 percent aluminum. Its name follows the GOST system of Russian alloy nomenclature, where the Cyrillic letters Х, Н, and Ю encode chromium, nickel, and aluminum respectively, while the suffix -ВИ signals vacuum-induction melting. In the English-language high-pressure community the designation surfaces in several transliterations—40HNU-VI, 40HNYu-VI, 40KhNYu-VI, and even the shorter EP793-VI—making cross-paper identification a recurring headache. The alloy is also casually dubbed "NiCrAl" or "the Russian alloy," a tag that underscores its singular industrial origin: it has been manufactured exclusively in the Soviet Union and later Russia, governed by the technical specification TU 14-1-2740-79 and the product standards GOST 22411-77 and GOST 2590-2006. Despite its outstanding mechanical performance, 40KhNYu-VI has never been formally classified as a superalloy, because its high-temperature creep and oxidation behavior remain insufficiently characterized.

Microstructure & the Three-Phase Assemblage

The metallurgical core of 40HNU-VI lies in its precipitation-hardening cycle. Quenching the alloy from above 1100 °C dissolves chromium into a supersaturated face-centered cubic nickel matrix, producing a metastable solid solution. Aging at temperatures above roughly 500 °C then decomposes that solution, nucleating a chromium-rich phase alongside the ordered intermetallic Ni₃Al, commonly called gamma-prime. A 2019 synchrotron X-ray powder diffraction study by Cheng and co-workers resolved the room-temperature microstructure into three distinct crystalline phases: an orthorhombic Ni₂Cr-type ordered structure in the Immm space group, constituting approximately 97 volume percent of the material, whose lattice parameters are simple rational multiples of the parent fcc cell; a body-centered cubic Cr-type phase (Im3m) present at only about 0.1 volume percent; and a cubic Ni₃Al-type phase (Pm3m) accounting for roughly 2.9 volume percent. This tightly interlocked three-phase assemblage is what endows the alloy with its exceptional combination of strength and dimensional stability under extreme confining pressure.

Mechanical Performance & High-Pressure Cell Construction

In its standard solution-treated and aged condition—quenched from 1140 to 1160 °C in water, then aged at 800 to 840 °C for 12 to 24 hours with air cooling—40HNU-VI guarantees a 0.2 percent offset yield strength of at least 1.08 GPa, an ultimate tensile strength of at least 1.275 GPa, and elongation of at least 10 percent on a 25 mm round bar, with hardness capped at 45 HRC. A lower-temperature aging regime at 540 to 560 °C for five hours pushes hardness above 57 HRC. Walker (1999) reported room-temperature yield strength of 2.00 GPa and ultimate tensile strength of 2.10 GPa with about 7 percent elongation. For constructing high-pressure cells, practitioners favor the 50 to 55 HRC window, where the material retains enough plasticity to serve as a pressure liner. Cold-working the quenched alloy before aging drives hardness to 64 or 67 HRC, sacrificing tensile ductility entirely but producing a material that excels in compression, making it ideal for pistons and indenters. The alloy's Young's modulus is approximately 224 GPa and its density sits near 7.6 g/cm³.

Magnetic Quietness & Neutron-Transparency

Perhaps the most consequential property of 40HNU-VI for experimental physics is its magnetic quietness. Despite containing 57 percent nickel, the alloy is purely paramagnetic: vibrating-sample magnetometer measurements and polarized-neutron diffraction both confirm the absence of any ferromagnetic ordering down to at least 1.9 K, with magnetization varying linearly in applied field and no hysteresis loop. A slight deviation of the inverse susceptibility from simple Curie-Weiss behavior hints at an additional contribution, possibly of spin-glass character. Equally important is the alloy's complete absence of cobalt. In neutron-scattering and diamond-anvil-cell work, cobalt-bearing steels would activate to highly radioactive cobalt-60 under irradiation, rendering them unusable. By relying on a Ni-Cr-Al chemistry, 40HNU-VI remains neutron-transparent and safe for repeated use in cryogenic non-magnetic high-pressure cells, where it forms the cylinders, pistons, and gaskets that confine megabar pressures in magnetometry and neutron-scattering experiments at liquid-helium temperatures.