First of all, a point regarding the difference in the material, material system and structure, because IMHO they are being used interchangeably in this Question, giving rise to confusion.
With the term:
material: I usually refer to specific element or alloy with specific density properties and more or less determined strength (there are ways to increase strength, chemical and mechaninal
material system: I usually refer to composite materials. Materials that have two (or more) distinctive phases at different ratios.
structure: an engineering construction with load bearing characteristics that can be made from one or more materials or material systems in different geometric configurations.
I believe this post is implicitly referring to the later (structure), however since material is used I will start with the material. One thing to point out that based on my interpretation of the term, one cannot change the density (therefore the weight) only the strength properties. That improves the strength to weight ratio.
However, even then there are so many things to do in so many different levels.
Chemical or thermal process
To adjust the strength properties you can use :
CVD or PVD in different ways and combinations to change the outer layers of the material and improve the strength (and chemical resistance of a material).
Heat treatment processes to change the phase of the material
Case hardening (in all its variations)
you can use
- work hardening
- shot peening to introduce compressive stresses that in turn increase tensile properties.
When you are getting into material system, then you can affect the density by changing the ratio of the materials in the system.
Also the strength properties can be tailored by a few things:
- the type of reinforcement material (particle, short randomly oriented fiber, long fibers)
- the combination of reinforcement material and matrix material (and more precisely the way they interact and how well do they adhere).
When it comes to structures, then the options truly increase exponentially. Because you can combine the materials/material systems with different geometries.
I will give only one example I found impressive when I stumbled upon it in a science journal a few weeks ago, about a structure which mimics the structure of Euplectella aspergillum. Apparently their structure offers significantly higher buckling resistance to weight ratio compared to a solid structure.