The P-51 made extensive use of aluminum (the entire basic airframe was constructed of it) and magnesium (used prominently in the engine mount) in its airframe, with a lesser amount of steel, copper, titanium, and other metals. Other materials were used where called for, such as fabric, rubber, wood, plexiglas, and plastic. Here are the "materials of construction" for the P-51 Mustang (essentially a list of heat-treated aluminum, magnesium, and steel castings, forgings, and members used in the construction of the airframe), taken from AN-01-60JE-2 Erection and Maintenance Instructions For Army Models P-51D-5, -10, -15, -20, -25, P-51K-1, -5, -10, -15, and British Model Mustang IV Airplanes; it gives a good overview of what types of materials were contained in the "guts" of the airframe.

Part	Material
Fitting - Wing Rear Spar Attachment	Aluminum alloy drop forging
Bracket - Aileron Inboard Hinge	Aluminum alloy casting
Bracket - Wing Flap Inboard Hinge	Magnesium alloy casting
Bracket - Wing Flap Center Hinge	Aluminum alloy casting
Bracket - Wing Flap Outboard Hinge	Magnesium alloy casting
Angle - Wing Rear Spar to Fuselage Attachment Fitting Stabilizer	Aluminum alloy extrusion
Fitting - Wing Station 75 Rear Spar Center Flap Support	Aluminum alloy extrusion
Fitting - Nose to Fuselage Attachment	Magnesium alloy casting
Fitting - Wing Center Bulkhead to Fuselage Upper Center	Magnesium alloy casting
Angle - Wing Station 0 Jack Fitting Support	Aluminum alloy extrusion
Fitting - Aileron Inboard Hinge	Magnesium alloy casting
Fitting - Aileron Center Hinge	Aluminum alloy casting
Fitting - Aileron Outboard Hinge	Aluminum alloy casting
Bracket - Elevator Center Hinge	Magnesium alloy casting
Fitting - Horizontal Stabilizer Rear Beam to Fuselage Attachment	Chrome-molybdenum steel forging
Fitting - Elevator Trim Tab End Hinge	Aluminum alloy casting
Fitting - Elevator Trim Tab Intermediate Hinge	Aluminum alloy casting
Fitting - Elevator and Rudder Intermediate Hinge	Aluminum alloy extrusion
Fitting - Elevator Outboard and Rudder Upper Hinge	Aluminum alloy extrusion
Horn - Elevator Trim Tab	Mild carbon steel sheet
Tube - Elevator Torque	Aluminum alloy tube
Fitting - Rudder Lower Hinge	Magnesium alloy casting
Fitting - Rudder Trim Tab Intermediate Hinge	Aluminum alloy extrusion
Fitting - Rudder Trim Tab Lower Hinge	Aluminum alloy extrusion
Fitting - Rudder Trim Tab Upper Hinge	Aluminum alloy extrusion
Fitting - Rudder Horn and Lower Hinge	Magnesium alloy casting
Bracket - Rudder Static Balance Lower Weight	Alclad sheet
Support - Tail Wheel Door Operating Link	Aluminum alloy extrusion
Support - Engine Removable Cowling Firewall Upper Former Rear	Mild carbon steel sheet
Support - Engine Removable Cowling Side Former Rear Upper	Mild carbon steel sheet
Support - Engine Removable Cowling Side Former Rear Lower	Mild carbon steel sheet
Angle - Fuselage Firewall-to-Wing Center Bulkhead Attachment	Aluminum alloy extrusion
Bolt - Engine Mount Firewall Lower Attaching	Chrome-molybdenum steel bar
Bolt - Engine Mount Firewall Upper Attaching	Chrome-molybdenum steel bar
Nut - Engine Mount Fuselage Attaching	Chrome-molybdenum steel bar
Bracket - Engine Mount Lord Bushing Housing Front	Aluminum alloy extrusion
Brace - Engine Mount Jury	24ST-E [aluminum alloy extrusion]
Link - Landing Gear Fairing Attachment Rear	Aluminum alloy casting
Bracket - Landing Gear Fairing Door Rear Hinge Support	Aluminum alloy casting
Arm - Landing Gear Fairing Door Lock Support Actuating	Chrome-molybdenum steel sheet
Yoke - Landing Gear Fairing Door Lock Support	Chrome-molybdenum steel sheet
Support - Landing Gear Fairing Door Lock Rear	Aluminum alloy casting
Fork - Tail Wheel Steering Turnbuckle	Chrome-molybdenum steel bar
Pin - Tail Wheel Down Position Strut Stop	Chrome-molybdenum steel bar
Retainer - Tail Wheel Lock Spring	Magnesium alloy casting
Bellcrank - Tail Wheel Declutching	Alclad sheet
Bellcrank - Tail Wheel Steering Pulley	Aluminum alloy casting
Retainer - Tail Wheel Lock Spring [different part number]	Magnesium alloy casting
Fitting - Tail Wheel Bellcrank Support	Magnesium alloy casting
Retainer - Tail Wheel Door Operating Bellcrank Support	[not given]
Bellcrank - Propeller Central Front	Chrome-molybdenum steel sheet
Stop - Central Stick Aileron Torque Tube	Magnesium alloy casting
Arm - Aileron and Elevator Control Lock	Aluminum alloy casting
Tube - Elevator Control Stick Connecting	Chrome-molybdenum steel tube
Clevis - Elevator Control Stick Tube Adjustment	Chrome-molybdenum steel bar
Arm - Surface Control Locking	Aluminum alloy casting
Bellcrank - Elevator Control	Magnesium alloy casting
Horn - Elevator Left	Aluminum alloy plate
Horn - Elevator Right	Aluminum alloy plate
Wheel - Aileron Fork Actuating Pulley	Magnesium alloy casting
Fork - Aileron Control	Aluminum alloy rough forging
Sector - Aileron Control Cable	Magnesium alloy casting
Fork - Rudder Pedal	Aluminum alloy casting
Bellcrank - Rudder and Steerable Tailwheel Control	Magnesium alloy casting
Bracket - Rudder Control Lock	Magnesium alloy casting
Lever - Rudder Control Locking	Chrome-molybdenum steel
Housing - Aileron Trim Tab	Magnesium alloy casting
Support - Aileron Trim Tab Drum	Magnesium alloy casting
Handle - Flap Cockpit Control	Magnesium alloy casting
Support - Flap Torque Tube	Aluminum alloy drop forging
Support - Flap Torque Tube Left	Aluminum alloy drop forging
Link - Flap Actuating Cylinder to Arm Connecting	Chrome-molybdenum steel tube
Support - Flap Torque Tube Right	Aluminum alloy drop forging
End - Flap Control Rod	Chrome-molybdenum steel bar
Rod - Hydraulic Wing Flap Control Valve	Mild-carbon bar
Lever - Hydraulic Auxiliary Landing Gear Up-lock Timing Valve	Aluminum alloy bar

Sources:

• Design Breakdown of the P-51 Mustang, by George Gehrkens, P-51 project engineer

• AN-01-60JE-2 Erection and Maintenance Instructions For Army Models P-51D-5, -10, -15, -20, -25, P-51K-1, -5, -10, -15, and British Model Mustang IV Airplanes