Aluminum is the one of the most common materials for bicycle frames. In fact, they are a little bit more expensive than its cousin, the steel. What is so good about the aluminum material?
Firstly, it is the lightest material of all, given a specific strength ratio. This would also mean it is a soft type of metal, compared to the harder steel. Therefore, manufacturers have been making thicker aluminum bike frames to compete with other materials. Since its (aluminum) strength is not so high, its diameter or thickness has to be increased to compete with other materials. If you take a look at the market, you will find that most aluminum frames are bigger than the other material types.
Secondly, it can resist corrosion. This mean it would not rust at all. This is very useful for people who are constantly riding even in the rainy season.
But there is a disadvantage. Most riders would describe riding a bicycle with aluminum frame to be uncomfortable. This is because of its hard property. On the bright side, having a harder material will also mean greater efficiency in terms of transferring energy to the pedals. For serious riders, this is an advantage by itself.
The most common material of aluminum is the 6061 grade aluminum. The other is known as 7000 series which is a little bit harder than the former, 6061. But for the most part, you will be better off choosing the 6061 grade type. If you like something that is light and yet reasonably hard, go for aluminum for your bicycle frame.