Elevated-temperature high-strength h-BN-doped Al2014 and Al7075 composites: Experimental and theoretical insights

Abstract

High-strength Al2014 (Al2) and Al7075 (Al7) series composites with and without addition of hexagonal BN ( h -BN) flakes (1, 3, and 5 wt%) were fabricated from the powder mixtures of individual elements using a combination of high-energy ball milling (HEBM) and spark plasma sintering (SPS). Phase compositions of Al2 and Al7 composites were different from standard alloys obtained via casting and subsequent heat treatment. Thorough structural study revealed the presence of the following phases: Al(Mg,Si,Mn,Fe,Cu), AlCu x , MgO x , and Al 5 Cu 6 Mg 2 [Al2], Al(Mg,Si,Mn,Fe,Cu), AlCu x , Al 5 Cu 6 Mg 2 , Al 6 CuMg 4 , MgO 2 , AlB 2 , and SiN x [Al2-BN], Al(Cu,Zn,Mg), Fe(Al,Cu), AlCu 3 , Al 2 Cu/Fe 3 Al, Al 5 Cu 6 Mg 2 , Al 4 Cu 9 , MgO 2 [Al7], and Al(Cu,Zn,Mg), AlCu x , MgO x , MgN x O y , MgB 2 , Mg 3 (BO 3 ) 2 , BN, and BNO [Al7-BN]. The important role of h -BN additives in the microstructure formation during HEBM and SPS was demonstrated. Classical molecular dynamics simulations were carried out to estimate critical shear stress between Al nanoparticles with and without intermediate h -BN layers. The obtained results indicated that the h -BN nanosheets had provided solid lubrication, prevented nanoparticle agglomeration during HEBM, led to a reduced porosity and more homogeneous reinforcing phase distributions in the powder mixtures and resultant composites. Structural analysis showed, that during SPS, one part of BN additives had reacted with Al, Si, and Mg to form AlB 2 , SiN x , and MgB 2 /Mg 3 (BO 3 ) 2 inclusions, while the other part remained unreacted and contributed to the material strength. Doping with 3 wt% of BN led to an increase in hardness from 76 HV 10 to 123 HV 10 (Al2 series), and from 97 HV 10 to 130 HV 10 (Al7 series). The maximum room-temperature tensile strength of 310 MPa (Al7-BN) and 235 MPa (Al2-BN) was observed for the samples with 3 wt% of BN, which corresponds to an increase in strength by approximately 74% and 16%, respectively. At elevated temperatures, the tensile strength values were 227 MPa (350 °C) and 221 MPa (500 °C) for Al2–3%BN, and 276 MPa (350 °C) and 187 MPa (500 °C) for Al7–3%BN. The superior mechanical properties were attributed to the combination of high thermal stability of the reinforcing phases, solid solution hardening, and Orowan (precipitation) strengthening.