Microstructure and fracture mode of ballistic perforated plates made of unalloyed ADI material

Abstract

Austempered Ductile Iron (ADI) is an advance engineering material produced from ductile cast iron by austempering heat treatment, through which a unique microstructure ‐ ausferrite is obtained. Consequently, an excellent combination of mechanical properties could be achieved [1]. In this paper, ADI material has been evaluated as an alternative to steel for perforated plates applied in the ballistic protection of military vehicles [2]. The ADI materials were produced from an unalloyed ductile iron by austenitization at 900°C/2h, followed by 1 hour austempering at 275°C (ADI‐275) or 400°C (ADI 400). The microstructure was observed on Leitz Orthoplan light microscope while fracture mode was studied by SEM JEOL JSM‐6460LV, 20 kV equipped with EDS Oxford Instruments INCA system. To evaluate ballistic properties, perforated plates of the ADI materials, having thicknesses of 7 and 9 mm, were mounted in front of basic armour and 12.7x99 mm M8 API ammunition was fired from 100 m. The microstructure of the ADI‐275 and ADI‐400 was fully ausferritic consisting of a mixture of ausferritic ferrite and retained austenite (9.8 and 26%, respectively). The ADI‐275 has an acicular morphology of ausferrite, while the ADI‐400 had a more plate like morphology. Due to difference in ausferrite morphology and retained austenite amount the ADI‐275 posses higher strength while ADI‐400 is more ductile. After impact, intensive cracking occurs near the impact point, Fig. 1a and 2a. The size of the fractured fragments is considerably larger in the less ductile ADI‐275, while ADI‐400 is more plastically deformed. The microstructure after impact is shown in Fig. 1b and 2b. In ADI‐400 martensite occurs in the area of the intensive plastic deformation, Fig. 2b. The martensite is formed through SITRAM effect (Strain Induced Transformation) [3]. In contrast, the SITRAM does not occur in ADI‐275, where austenite carbon enrichment is higher, Fig 1b. The ADI‐275 have a typical ductile fracture, with the dimpled surface covered with a layer in the form of drops, Fig. 1c. However, in ADI‐400 a mixed fracture mode can be observed, a ductile fracture near the nodules and in other areas brittle, quasi – cleavage fracture, Fig. 2c. This brittle behaviour of ADI‐400 is the result of the presence of martensite. The results of debris EDX analysis reveal a mixture of Cu, Ba, Mg and Al, i.e. material of the projectile jacket and products of the IM‐11 incendiary mixture, Fig. 3. Perforated plate made of the ADI‐275, with a higher hardness and a lower ductility, were proved to be superior to the softer and more ductile ADI‐400. The ballistic testing causes a SITRAM effect to occur in the ADI‐400, causing a partial brittle fracture and thus lowering ballistic performance [4]. The perforated plates made of the ADI material have a larger damaged area, a lower cost of fabrication and a similar mass effectiveness than steel perforated plates [2, 4].