1 Prof. C. H. XU School of Materials Science and Engineering Henan University of Science and Technology Chapter 6: Metallic Matrix Composites (MMCs) Subject: Composite Materials Science and Engineering Subject code:

2 Introduction In comparison with bulk (monolithic) metals, MMCs have higher specific strength/modulus, better properties at high temperature, lower coefficients of thermal expansion better wear resistance In comparison with PMCs, MMCs have higher transverse strength ( 横向强度 ) and stiffness, better high temperature capability ( 性能）

3 Introduction Most of metallic matrix composites (MMCs) in the development stage Manufacture MMCs at high temperature

4 Introduction Processing Solid state Liquid state Deposition (vapor) Interface reaction Properties of MMCc Some commercial MMCc

5 Metal Matrix Composites - processing Solid state processing Diffusion bonding (a) sandwich a fibre mat. (b) to form ply （板层） (c) stacking plies (d) hot press (diffusion bonding) (e) clean and trim Foil matrix : titanium, copper, nickel, aluminium; Fibre mat: polymer bonding fibers. E.g. aluminum reinforced with boron fibres Expensive and parts with simple shape Low temperature →less interface reaction (compare with liquid processing)

6 Solid state processing Powder metallurgy （粉末冶金） Matrix: metal particles; reinforcement: discontinous fiber, whisker; maximum of reinforcement to 50% Processing Mixture of matrix and reinforcement Heat and pressure under inert gas Large surface area and high energy solid-gas interface Low temperature →less interface reaction (compare with liquid processing) E.g. SiC whisker reinforced aluminum Metal Matrix Composites - processing

7 Liquid processing (Casting technique) Barrier 1: fibre non-wetting and interface reaction between matrix and reinforcement at melting temperature. Interface reaction products may reduce the properties of composites Precoating the reinforcement with an appropriate materials to protect against any reaction and to enhance wetting: e.g. pyrolitic graphite coating on SiC fibers Modify matrix: e.g. add lithium (Li) to Al liquid to form Li 2 O · 5Al 2 O 3 at the interface between alumina fibre and aluminum (Al) to enhance wetting; Liquid processing can't be used for Ti alloys because of Ti high reactivity. Barrier 2: non-uniform mixture of metal and reinforcement

8 Metal Matrix Composites - processing Liquid processing (Casting technique) Melt stirring （搅动） : particle or short fiber reinforcement + liquid metal matrix Stirring the mixture Improve non-uniform mixture of metal and reinforcement Rheocasting （流变铸造，固 - 液态搅动 ) (in order to modify melt stirring): particle or short fiber reinforcement + liquid metal matrix Cool the melt mixture to a more viscous two phase solid-liquid state Stirring the mixture Limit of reinforcement <20% Preform ( 预成形 ) reinforcement casting: liquid metal (matrix) infiltrates （ 渗透） a preform (reinforcement) under a pressure （ P ） Limit of reinforcement <30%

9 Metal Matrix Composites - processing Preform 1: squeeze （模压） casting: Processing (a) insert preform into die cavity; (b) meter in a precise quantity of alloy; (c) close die and apply pressure; (d) remove ram; (e) extract component High cost of die E.g. Aluminum piston crowns ( 活塞顶） locally reinforced with a discontinuous alumina fibres

10 Metal Matrix Composites - processing Preform 2: liquid melt infiltration under a gas pressure; Processing (a) insert preform and close die; (b) evacuate （排出） air; (c) apply gas pressure during solidification Small parts Low cost Common fibers include SiC, B, C, alumina

11 Metal Matrix Composites - processing Deposition: Spray （喷溅） co-deposition Processing Atomizing a melt (e.g. Al) exists as discrete ( 不连续的 ) droplets for short time Introducing the reinforcement particle (e.g. SiC) into the spray of fine metal droplets Metal and reinforcement are co- deposited on to a substrate High density, less interface reaction Diagram of spray co-deposition production SiC particulate reinforced metal

12 Metal Matrix Composites - Interface Form Interface layer between matrix and reinforcement during service or fabrication at high temperature Coupling agent for wettability and interface bonding: Li or Mg for Al-Al 2 O 3 Graphite or TiB 2 for SiC – Ti alloys Interfacial layers affect the mechanical properties of the composite: Effect of interfacial layer thickness on the mechanical properties of a Ti-6%Al-4%V alloy with 35% SiC fibers coated with C Axial transverse Thickness of brittle layer (  m) Impact energy (Kj/m 2)

13 Metal Matrix Composites - Properties of MMCs materialscoefficient of thermal expansion Mg /K Al /K Mg-Al 2 O /K Al-SiC /K Physical properties The coefficient of thermal expansion Parts with close tolerance materialsConductivity (Wm -1 K -1 ) Al 201 Al-15%SiC 140 Epoxy 0.3 Epoxy- 60%Glass fiber 1.6 Conductivity

14 Metal Matrix Composites - Properties of MMCs Mechanical Properties Elastic modulus Reinforcements increase elastic modulus of composite with matrix, Al, Mg…. Modulus at longitudinal direction is higher than that transverse direction for composite with continuous fibers Effect of reinforcement on the Young’s modulus of Al Difference in the longitudinal and transverse modulus for Al-Li alloy matrix with Al 2 O 3 fibers

15 Metal Matrix Composites - Properties of MMCs Strength Effect of volume of reinforcement on the tensile strength of Al matrix Effects of angle between tensile axis and fiber axis on the strength of continuous fiber reinforced Ti alloy

16 Metal Matrix Composites - Properties of MMCs Ductility and Toughness Reasons Fiber-matrix interface reaction In-homogeneity of reinforcement distribution Surface properties of reinforcement internal stresses matrixreinforce ment Toughness K 1C Al MPa m 1/2 AlSiC5-25 MPa m 1/2 matrixreinforce ment Ductility (%) Al-40 AlAlumina4.0 Al-10%MgAlumina1.3 Al alloySiC7.0

17 Metal Matrix Composites - Properties of MMCs Specific strength and specific modulus of MMCs is superior to that of alloys

18 Properties at elevated temperature (short time test) Metal Matrix Composites - Properties of MMCs Tensile strength Young’s modulus at elevated temperature

19 Properties at elevated temperature (long times test) Creep Permanent strain under a stress with time Metal: 3 stages creep curve Continuous fiber MMC: reinforcement hinders creep Discontinuous reinforcement MMC: 3- stage creep curve and creep resistance Metal Matrix Composites - Properties of MMCs Metal MMC (long fiber) MMC (short fiber) time strain

20 Fatigue （疲劳） resistance Fatigue: the failure of a component under cyclic stress MMCs fatigue resistance may increase or decrease More crack initiation sites for MMC: Large ceramic particles, Unbonded clusters of particles Reduce of propagation rate of crack Metal Matrix Composites - Properties of MMCs Cycles to failure

21 Al reinforced with SiC particles SiC particles are cheaper than SiC long fibers Control vol%SiC in this MMC can match different materials (see top-figure) Techniques for this MMC: heat treatment superplastic forming diffusion welding Metal Matrix Composites - some commercial MMCs Coefficient of thermal expansion of Al- SiC versus vol% SiC, showing matching with a range of metals Aircraft panel produced by superplastic forming of an Al-SiC composite

22 Cermets: metal matrix (cobalt: Co or nickel: Ni) + ceramic particles (tungsten carbide: WC or titanium carbide, TiC) Mechanical Properties: hard and enhanced toughness Ceramic particles provide the cutting surface Metal matrix withstands the cutting stress. Application: cutting tools for hardened steels, glass, Metal Matrix Composites - some commercial MMCs

23 Multifilamentary （多纤维） superconductors Nb 3 Sn Nb 3 Sn brittle, difficult to form Nb 3 Sn Bronze route: （ niobium) Nb+Sn (Cu-Sn) → Nb 3 Sn layer The superconducting properties are a function of Nb 3 Sn layer thickness and the grain size Use as windings （线圈） for superconducting magnets Metal Matrix Composites - some commercial MMCs Multifilamentary superconducting composite with filaments of approximately 5mm diameter (a) cross-section and (b) matrix etched away to show the filaments (a) (b)

24 Production of Multifilamentary superconductors composite by bronze route (a) holes drilled in bronze block and niobium rods inserted (b) swaging （模锻） to reduce the cross-section of niobium (c) sectioning, rebuilding, canning in Cu can, and final reduction (d) heat treatment to form 2  m Nb 3 Sn between Nb and bronze Metal Matrix Composites - some commercial MMCs

25 Metal Matrix Composites - MMCs for airspace application MatrixFiberapplication Cu baseC SiC W Combustion chamber Nozzle (rocket, space shuttle) Heat exchanger Fe baseWtubing Ni baseAl 2 O 3 blades Ti baseSiC TiB 2 TiC Housings, tubing Blades, Shafts, honeycomb Al baseSiC Al 2 O 3 C Housings, mechanical connects, satellite, structures, wings, blades Fuselage Structure members Mg baseAl 2 O 3 Structure members

26 Further reading Text Book: Composite Materials: Engineering and Science (pages ). Other reference: Note 6