Directed melt oxidation and nitridation in aluminium alloys: B S. S. Daniel and V. S R. Murthy Table 3 Mechanical properties composites,37. Materi Flexural Tensile Young's Shear Poissons persian (MPa) (MPavm)(MPa) (GPa) (×10)(wmK)(kgmm2) AIN/Al 66-40095-11.1 AIN/AIN/Al TiB2/AIN/Al 13.5 9.5 42-457165-18065-720.260.54 2865064 ALo/AIN/AI275+50°8+1 523+5410.74+2.1 92-94 "Naturally aged with 0. 4 um reaction layer with 1.5 um reaction layer As the liquid wicks through the preform, occasionally Applications and limitations the reaction front is blocked and results in macroporos- Keeping the advantages(simplicity, flexibility and cost ity, whereas local solidification shrinkage of metal in the of the process in mind, a large number of patents have composite gives rise to microporosity. These porosities already been filed on these materials. Landini et are claimed to increase with temperature, magnesium made valve inserts using SiC and Al O, particulate rein- content in the alloy and reinforcing particle size forced Al2O Al composites. These components were Porosities can be controlled to a limited extent either by successfully tested on a diesel engine. Similarly, compo- sothermal holding between the oxidation temperature nents made from SiC particulate -reinforced Al20-Al and the liquidus of the alloy or by altering the alloy composites(AInax products, USA) have been used for composition hydrocyclone cone liners and apexes and the perform- Finally, there were attempts to model the complex ance was superior compared to conventionally used infiltration process 5-37 However, so far there are no parts". SiC(Nicalon) fibre reinforced AL,O,/Al com- realistic models to estimate the growth rates in any of posites are seen as future materials for gas turbine hese system applications". The other probable applications of oxide composites include wear parts for pumps, jet mill noZZ les,automotive parts, etc On the other hand Mechanical properties AIN/Al composites(with their superior thermal conduc- Like many composite materials, the properties of tivity) are likely to find a place in the electronics pack- The major limitations of these processes are the type and volume fraction of the reinforcing phase. The properties of reinforcing phase are listed in 1. It takes a long time to develop oxide-nitride rein Tables 2 and 3. Although the mechanical properties of forced composites and hence the process cannot be AIN and Al,O, are almost similar, ain has a greater used for producing very large components thermal conductivity (ten times higher)and lower 2. Residual porosities(both micro and macro)are thermal expansion coefficient as compared to Al 03 limitation in achieving best mechanical perform AIN exhibits excellent thermal shock resistance and ance. due to these inherent defects these materials stable up to -1000oC in an oxidizing atmosp re unlikely to find applications in critical areas Additionally, ain has excellent corrosion resis 3. ain base composites are sensitive to moisture and good wettability with aluminium. The only limita attack and hence applications are restricted to tion of ain is that it reacts with H,0 in a hydrolysis selected areas such as the electronics industry. If the reaction to form various hydroxide phases, liberating AIN evolved in the process can be made water monia and heat. As a result, mechanical properties resistant like the recently developed AIn powders deteriorate to a large extent (chemically altered composition) AIN/Al compos- The mechanical properties of oxide and nitride base ites can be used successfully in various applications omposites are summarized in Tables 2 and 3. Average flexural strength(280-400 MPa)and fracture toughness values(3-9 MPa vm) of both composites are compa- The recently developed direced melt reaction processes rable even when filler materials are used.,. However, (oxidation and nitridation) are flexible and attractive in- properties are supcrior when fibre reinforcements are situ methods for developing metal-ceramic composites utilized.Further,these properties of fibre reinforced Composite formation in aluminium alloys is strongly composites are dependent on chemistry, strength of the dependent on composition, processing temperature, fibre and its interfacial interaction time and partial pressure of the gaseous species. For 160 Materials Design Volume 16 Number 3 1995Directed melt oxidation and nitr~dation in ~iu~ini~~ alloys: B. S. S. Daniel and V. S. R. bulky Table 3 Mechanical properties of AlNlAl compositef~2s~37~39 Material Flexural Fracture Tensile Young’s Shear Poisson’s Thermal Thermal strength toughness strength modulus modulus Knoop ratio NW expansion (MPat/m) (MPa) conductivity hardness KW (X lodK-‘) (W m-‘K-‘) (kg mm*) Sintered AIN 28CL3.50 3.3 1200 AlNiAi 366-400 9.5-11.1 186240 223-i 124 AINJAINIAI 340 5.9 240 8-11 -100 TiBdAIN/AI 540 13.5 240 Al~O~~AlN/Al 340 9.5 242-451 165-180 65-72 0.26-&54 286506 Al,O,~AlN/Al 275450’ 8Flb 523+554’ 10.74%!.1’ SiCjAIN/Af 9.2-9.4 “Naturally aged bWith 0.4 pm reaction layer With 1.5 nm reaction layer As the liquid wicks through the preform, occasionally the reaction front is blocked and results in macroporos￾ity, whereas local solidification shrinkage of metal in the composite gives rise to microporosity. These porosities are claimed to increase with temperature, magnesium content in the alloy and reinforcing particle size34. Porosities can be controlled to a limited extent either by isothermal holding between the oxidation temperature and the liquidus of the alloy or by altering the alloy composition22. Finally, there were attempts to model the complex infiltration process 35-37 . However, so far there are no realistic models to estimate the growth rates in any of these systems. Mechanical properties Like many composite materials, the properties of directed melt in-situ reacted composites are dependent on the type and volume fraction of the reinforcing phase. The properties of reinforcing phase are listed in Tables 2 and 3. Although the mechanical properties of AIN and Al,O, are almost similar, AlN has a greater thermal conductivity {ten times higher) and lower thermal expansion coefficient as compared to AI,O,. AIN exhibits excellent thermal shock resistance and is stable up to -1000°C in an oxidizing atmosphere. Additionally, AIN has excellent corrosion resistance and good wettability with aluminium. The only limita￾tion of AlN is that it reacts with Hz0 in a hydrolysis reaction to form various hydroxide phases, liberating ammonia and heat, As a result, mechanical properties deteriorate to a large extent3’. The mechanical properties of oxide and nitride base composites are summarized in Tables 2 and 3. Average flexural strength (280-400 MPa) and fracture toughness values (3-9 MPa y’m) of both composites are compa￾rable even when filler materials are used37s39@. However, properties are superior when fibre reinforcements are utilized. Further, these properties of fibre reinforced composites are dependent on chemistry, strength of the fibre and its interfacial interaction4’. 160 Materials & Design Volume 16 Number 3 1995 Applications and limitations Keeping the advantages (simplicity, flexibility and cost) of the process in mind, a large number of patents have already been filed on these materials. Landini et ~1.~’ made valve inserts using SiC and A&O3 particulate rein￾forced Al,O,/Al composites. These components were successfully tested on a diesel engine. Similarly, compo￾nents made from SIC particulate-reinforced A120j-Al composites (Alnax products, USA) have been used for hydrocyclone cone liners and apexes and the perform￾ance was superior compared to conventionally used parts4’. SIC (Nicalon) fibre reinforced Al,03/A1 com￾posites are seen as future materials for gas turbine applications43. The other probable applications of oxide composites include wear parts for pumps, jet mill nozzles, automotive parts, etc3’. On the other hand, AlN/Al composites (with their superior thermal conduc￾tivity) are likely to find a place in the electronics pack￾aging industry. The major limitations of these processes are: 1. It takes a long time to develop oxide-nitride rein￾forced composites and hence the process cannot be used for producing very large components. 2. Residual porosities (both micro and macro) are a limitation in achieving best mechanical perform￾ance. Due to these inherent defects, these materials are unlikely to find applications in critical areas. 3. AlN base composites are sensitive to moisture attack and hence applications are restricted to selected areas such as the electronics industry. If the AlN evolved in the process can be made water￾resistant like the recently developed AlN powders (chemically altered composition)38 AlNiAl compos￾ites can be used successfully in various applications. Summary The recently developed direced melt reaction processes (oxidation and nitridation) are flexible and attractive in￾S&X methods for developing metal-ceramic composites. Composite formation in aluminium alloys is strongly dependent on composition, processing temperature, time and partial pressure of the gaseous species. For