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Title: Microstructure and mechanical properties of tin-based alloys for miniature detonating cords
Author: Liu, Guangyu
ISNI:       0000 0004 7961 7707
Awarding Body: Brunel University London
Current Institution: Brunel University
Date of Award: 2019
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Miniature detonating cord (MDC), a typical linear explosive device, is primarily used in aircraft canopy severance systems. MDCs are typically flexible cylindrical cords with an explosive core and a robust sheath/cladding material. In practical applications, upon detonation, the MDC which is bonded to the inside of canopy propels the sheath outward at a high velocity, thereby penetrating or shattering the canopy transparency to clear an escape path. Conventionally, the materials for explosive core are pentaerythritol tetranitrate (PETN), cyclotrimethylenetrinitramine (RDX), cyclotetr-amethylenetetranitramine (HMX), or hexanitrostilbene (HNS). For decades, the sheath materials of MDCs were made by antimonial or non-antimonial lead, due to high density, good ductility, and relative ease of manufacturing. However, owing to the environmental and human health concerns regarding lead poisoning and the increasingly strict requirement in environmental legislations, the replacement of lead has been compulsory for recent development of new generation metallic sheath materials for MDCs in aerospace industry. The present study aims to study the feasibility and reliability of developing a costeffective and easy-manufacturing lead-free tin-based alloy, which is suitable as the sheath materials of MDCs for the clearance of aircraft canopies. The materials requirement was analysed and Sn-Cu based alloys were firstly chosen as potential candidates. The Sn-Cu alloys (0.3-1.0wt.%Cu) were prepared by casting and rolling. Microstructures of as-cast Sn-Cu alloys comprised Sn solutions with Cu6Sn5 intermetallic phase in the matrix. The as-rolled hypoeutectic Sn-Cu alloys (0.3-0.5wt.%Cu) offered the yield strength from 26.1 to 31.9 MPa, UTS from 30.1 to 34 MPa and elongation from 86.4 to 87.5%, which were found to be appropriate for the sheath. Particularly, the Sn-Cu alloys exhibited nonwork-hardening phenomenon under tensile stress, which could benefit sheath manufacturing and subsequent processing after assembly with high-energy explosive materials. Another achievement is understanding the deformation mechanisms and microstructure characteristics of the Sn-Cu alloy under rolling, which involves boundary formation, dynamic restoration, twinning, and recrystallization texture. A bimodal grain structure was well established after rolling, ascribed to the dislocation activities and dynamic restorations including dynamic recovery (DRV) and dynamic recrystallization (DRX). The Cu6Sn5 particle stimulated nucleation (PSN) was found as the major mechanism of DRX, which was also the dominant cause of forming of 〈001〉//RD oriented nuclei. Additionally, DRX nuclei are formed along the existing boundaries, resulting in a necklace structure via continuous dynamic recrystallization (CDRX). {301} and {101} twins were identified as additional significant microstructure features. Due to the microstructural inhomogeneity of Sn-Cu alloys subjected to rolling, alternative tin alloys were also developed for applications. The assessment of the mechanical properties of Sn-3Zn-xBi (Bi: 0-5wt.%) alloys processed by rolling were undertaken to explore their feasibility as sheath materials. Effects of Bi on the microstructure and mechanical properties of Sn-Zn were studied. Bi significantly refined the as-cast microstructure, altering the configuration of Sn-Zn eutectic from well-aligned Zn-rich needles to misaligned Zn-rich flakes. After deformation, secondary phases such as Zn-rich precipitates and Bi particles, and Bi solutes were critical to grain refinement because these secondary phase particles provided more sites for nucleation and more obstacles to growth of new recrystallized grains. Tensile results confirmed that Bi addition enhanced both strength and ductility of the rolled Sn-Zn-Bi alloys. The Sn-3Zn-5Bi alloy possessed superior strength and ductility (UTS: 84.4 MPa; Yield strength: 68.3 MPa; Elongation: 75.2%) due to its finest and most homogeneous equiaxed grains. The corrosion properties of Sn-3Zn-xBi (x=0, 1, 3, 5, 7 wt.%) alloys were investigated to explore the effect of Bi on the corrosion performance of the Sn-Zn alloy. Results indicated that the addition of 1 wt.% Bi increased the corrosion susceptibility of the Sn-3Zn alloy, mainly attributed to the coarsened and more uniformly distributed corrosion-vulnerable Zn-rich precipitates, while further increasing the Bi contents decreased the corrosion susceptibility of Sn-3Zn-xBi alloys due to the higher fraction of nobler Bi particles serving as anodic barriers. The Sn-3Zn-7Bi possessed the best corrosion resistance among all tested alloys. The role of Bi on the corrosion properties was considerably discussed. Finally, numerical performance simulations and proof tests were conducted using Ansys AUTODYN-2D to verify the reliability of the tin alloys for the cord sheaths. The cut depths of the cords sheathed by varied metals/alloys incorporating Pb, Pb-Sb, Sn, Sn-Cu, Sn-Zn-Bi, Al, Cu, and Ta were attained. Simulation results indicated that newly developed tin sheathed cords showed similar cut depths compared with lead. Proof firings demonstrated that the Sn-Zn-Bi sheathed cords fulfilled the requirement of cut depths against acrylic and aluminium targets, confirming the successful development of Sn-Zn-Bi for the MDCs in the specific application.
Supervisor: Ji, S. ; Fan, Z. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Aluminium ; Steel ; Modeling ; Lightweight ; Fatigue