Boron nitride nanotubes induced strengthening in aluminum 7075 composite via cryomilling and spark plasma sintering

Abstract

Al7075 is among the strongest commercial aluminum alloys with low density, making it a standout choice for structural metals. However, the never-ending quest for higher strength and low-density materials demands structural metals stronger than Al7075. In this study, high-strength and chemically inert one-dimensional boron nitride nanotubes (BNNTs) are used to reinforce Al7075 alloy, making ultra-high strength aluminum matrix composite. Al7075-BNNT composite is fabricated using a multi-step process involving ultrasonication, cryomilling, and spark plasma sintering (SPS). Ultra-fine grains were efficiently achieved in 2 h of milling, resulting in an impressive ultimate strength of ~ 636.8 ± 18.9 MPa and elongation up to necking of 10.1 ± 0.5% in heat-treated Al7075-BNNT composite. The obtained strength is 1.3 times higher than SPS Al7075 and 2.9 times higher than cast Al7075 alloy. The cryomilling facilitated a homogeneous dispersion of BNNTs, fostering effective interfacial bonding, albeit leading to variations in BNNT length ranging from 1–50 µm. The interplay between BNNT lengths and their impact on mechanical properties is explored, showcasing a synergistic improvement in strength and elongation. The comprehensive understanding of the resulting strengthening mechanisms encompasses Hall–Petch, Orowan, dislocation-induced strengthening, and dominant load transfer mechanisms. These findings offer valuable insights into fabricating high-performance aluminum matrix composites surpassing conventional strength. The Al7075-BNNT composite's unprecedented mechanical strength could further extend the use of aluminum alloys to more demanding aerospace applications, such as spacecraft structures and next-generation vehicles, as well as racing and automotive parts where the need for ultra-lightweight yet ultra-strong materials is paramount for fuel efficiency and performance under extreme conditions.