Mtb which is lighten alloy or alluminium?

As Yon points out, about alloy's, an alloy just means a mix of materials to change the properties of the base material. So there are alum. alloy's, steel alloy's, etc. To find if one is lighter than an other is tough to do as the properties of some alloy's allow for thinner tube diameters than other and this effects the weight of a particular frame. Therefore a steel frame could be lighter or heavier than an alum. one. So to compare one frame to another just by what material it is made of is not cut and dry. However if you are looking at box store bikes know that they are all heavier than the ones at a bike store.