Do you know what is alloys?
If you study in science, you should know what is alloys?

Alloys is the use of pure metals is limited because they may be too weak or too soft for some purposes and may be corroded easily.
Many of the metals used today are not pure metals, but alloys.
An alloy is a mixture of a metal and other element(s) (which may be metal or non-metal). It is made by mixing the elements together when they are in liquid form, and then allowing them to cool and solidify.
The hardness, strength and corrosion resistance of a metal can be improved by making alloy.
An alloy is a mixture of metals
which may have different (improved) properties
from the metal elements which make it.
The figures given for percentages of elements in an alloy
are for guidance only,
as any particular alloy may be tailored for a particular use.
Alloys have more species.
Mild steel have iron 99.7% and carbon 0.3%, the properties is strong but maileable, it's uses to buildings, bridges, railway, car bodies.
Stainless steel have iron 70%, rickel 10% and chromium 20%, it's properties is hard and strong. very corrosion resistant, it's uses to cutlery, bearings.
Brass have copper 70% and zinc 20%, it's propetirs is golden appearance, maileable, corrosion resistant, harder than pure metals. it's used to ornaments, buttons, screws, door knobs, taps.
The bronze have copper 88% and rickel 12%. the properties is corrosion resistant and hard wearing, it's used to ornaments, statues, chyrch bell and machine.
Solder have lead 50-70% and tin 30-50%, the properties is low melting point, strong, it's used to for welding.
Duralumin have aluminium 95%, copper 3%, magnesium 1% and manganese 1%, the properties is light, strong, more corrosion resistant, it's used to aircrafts and spacecraft.
Gold of different carats is used to gold, sliver, copper( in various proportions), the properties is used to stronger and harder than pure gold, it's used to making ornaments, gold coins and jewellery.

Questions: Why alloys are stronger and harder than pure metals.
Because the layers of atoms in a pure metal can silp over one another easily when a force is applied. This is because all the atoms are of the same size. In an alloy, atoms of different sizes are present. This prevents the layers from slipping over one another.