This work investigates the thermal stability and retention of mechanical properties

of a custom-made AlMg3 aluminum alloy used as a substrate material in 10B4C-coated

Multi-Grid neutron detectors developed for the European Spallation Source

(ESS) in Lund, Sweden. The customized alloy was designed to minimize neutron

background contributions and neutron activation by reducing the impurity content,

while maintaining the general characteristics of commercial AA5754-H22. However,

observations have indicated substantial mechanical softening of the substrate

following the thermal cycle associated with sputter deposition of 10B4C coatings.

To evaluate the origin and extent of this degradation, tensile testing, Vickers

microhardness testing, and electron backscatter di!raction (EBSD) were performed

on untreated (as-received), coated, and coating-equivalent heat-treated specimens

of both the ESS-custom alloy and commercial AA5754-H22 reference material.

Tensile results demonstrated that thermal exposure during coating caused severe

reductions in yield strength in the ESS-custom alloy, with treated samples

exhibiting approximately a 75% decrease in yield strength relative to the untreated

condition. In contrast, the commercial AA5754-H22 samples retained substantially

greater strength after equivalent treatment. EBSD analysis revealed recovery,

grain coarsening, and partial recrystallization in the treated ESS-custom samples.

The purified alloy also exhibited greater sensitivity to grain growth and thermal

softening, due to the reduced presence of intermetallic particles that normally

contribute to grain-boundary pinning and microstructural stabilization.

The results indicate that the thermal cycle associated with 10B4C sputter

deposition is the dominant mechanism responsible for mechanical degradation

of the substrate material, rather than the coating itself. While purification

of the alloy improves neutron-related performance requirements, it also reduces

resistance to recovery and recrystallization during elevated-temperature processing.

These findings are directly relevant to the long-term dimensional stability and

mechanical reliability of Multi-Grid detectors and suggest that further optimization

of alloy chemistry, thermomechanical processing, or coating conditions may be

necessary to balance neutron transparency with structural stability.