Directed melt oxidation and nitridation in aluminium alloys: B S S. Daniel and V.S. R Murthy Vapor phase Oxidising Nitriding Atmosphere Metal-Ceramic Alloy AlO Crucible Vapor phase B Metal-c。rami Composit Molten Alloy A20 Figure 1 Schematic of outward growth of ceramic/ metal reaction product(A)into free space and( B)through filler material. The corresponding microstructures in oxidizing and nitriding atmospheres are show these in-situ reactions can be utilized to obtain 'multi Additionally, these elements also improve the wettability phase composites by placing filler materials in the path of liquid alloys during wicking. In addition to the volatile of the growth direction(Figure 1), lo, The objective of species, elements such as Si, Ge and Sn are also added to using filler materials is mainly to increase growth rates, in control the reaction kinetics". In oxidation experiments addition to modifying the properties. a wide range of there is a choice of adding these elements either in materials are produced using different filler materials. elemental form (in the liquid alloy) or as surface Some examples are SiC/AlO Al, Al,O AlO,Al, dopants, Addition of surface oxides is believed to TiB,AIN/AL, SiNAIN/AL, etc,. All composit reduce the incubation period obtained by gas-metal reactions exhibit not only good In contrast, in nitridation experiments both II and IIB mechanical properties(KIC, stiffness, wear and corrosion elements are added to the alloy. In this process, group II resistance)but also improved electrical and thermal elements not only improve the wettability but also act as (shock resistance) properties compared to their ceramic a gettering agent in maintaining the oxygen partial pres- counterparts. The potential advantages of this method sure below a critical level. In the dimOx process are low processing cost, simplicity, near-net shape and AlOy Al composite growth was reported in binary alloy lexibility in filler material selection like Al-Mg, Al-Zn and Al-Li alloys where there are few limitations in nitridation For instance, AIN/Al composite growth was not seen in Al-Mg or Al-Si Process variables binary alloys up to 1450oC when a 4oC/min heating rate When pure aluminium is oxidized a thin oxide film is Al-Si-Mg exhibited accelerated weight gain beyond formed on the surface preventing further oxidation of the 1200oC19. In contrast, Scholz and Greil2D,21 reported metal. The oxidation behaviour of pure aluminium is conversion of Al to AIN in Al-2.3Li and Al-2.5Mg drastically changed with small amounts of volatile alloys. Additionally, in nitridation, small quantities of elements such as Mg, Zn, Li and Na-3. Initially, these iron were found to aid the infiltration whereas in oxida elements diffuse at a rapid rate to form a porous oxide tion, iron additions give rise to undesirable intermetallic on the surface. However, at a later stage, the surface which in turn affect the mechanical properties". In both oxide dissociates under a concentration gradient and processes, nickel addition was found to be beneficial helps to maintain the Mg level in the liquid reservoir because it refines the composite structure 156 Materials design Volume 16 Number 3 1995Vapor Phase A Metal-C~amic Composite A1203 Crucible Vapor Phase B Filler Material Metal-Ceramic Composite v ‘AI203 Crucible Directed melt oxidation and nitridation in ~lu~iniu~ alloys: B. S. S. t)anje/ and V. S, R. Mu&y Nitriding Atmosphere Figure 1 Schematic of outward growth of ceramicl metal reaction product (A) into free space and (B) through filler material. The co~esponding microstructures in oxidizing and nitriding atmospheres are shown these in-situ reactions can be utilized to obtain ‘multi￾phase’ composites by placing filler materials in the path of the growth direction (F&z 1)‘,‘*. The objective of using filler materials is mainly to increase growth rates, in addition to modifying the properties. A wide range of materials are produced using different filler materials. Some examples are SiC/AI,O,/Al, Al,O,/Al,O,/Al, TiB~AlN/~, Si~N~AlN/Al, etc7,‘*. All composites obtained by gas-metal reactions exhibit not only good mechanical properties (&, stiffness, wear and corrosion resistance) but also improved electrical and thermal (shock resistance) properties compared to their ceramic counte~a~, The potential advantages of this method are low processing cost, simplicity, near-net shape and ~exibility in filler material selection. Process variables A flay selection When pure al~inium is oxidized a thin oxide film is formed on the surface preventing further oxidation of the metal. The oxidation behaviour of pure aluminium is drastically changed with small amounts of volatile elements such as Mg, Zn, Li and Na”-‘3. initially, these elements diffuse at a rapid rate to form a porous oxide on the surface. However, at a later stage, the surface oxide dissociates under a concentration gradient and helps to maintain the Mg level in the liquid reservoir’4. Additionally, these elements also improve the wettability of liquid alloys during wicking. In addition to the volatile species, elements such as Si, Ge and Sn are also added to control the reaction kineticP. In oxidation experiments there is a choice of adding these elements either in elemental form (in the liquid alloy) or as surface dopants”,“. Addition of surface oxides is believed to reduce the in~bation period. In contrast, in nitridation experiments both II and IIB elements are added to the alloy. In this process, group II elements not only improve the wettability but also act as a gettering agent in maintaining the oxygen partial pres￾sure below a critical level. In the DIMOX process, Al,03/Al composite growth was reported in binary alloys like Al-M& Al-Zn and Al-Li alloys@ where there are few limitations in nitridation. For instance, AlN/Al composite growth was not seen in Al-Mg or Al-Si binary alloys up to 1450°C when a 4Wmin heating rate was employed. However, ternary alloys such as Al-Si-Mg exhibited acc&erated weight gain beyond 1200°C’9. In contrast, Scholz and Grei120,2’ reported conversion of Al to AlN in Al-2.3Li and Al-2.5Mg alloys. Additionally, in nitridation, small quantities of iron were found to aid the in~ltration” whereas in oxida￾tion, iron additions give rise to undesirable intermetallics, which in turn affect the mechanical properties2’. In both processes, nickel addition was found to be beneficial because it refines the composite structurez3. 156 Materials & Design Volume 16 Number 3 1995