From DT Online
As material samples are stretched, initially, the Stress is directly proportional to the Strain - producing the straight line as shown on the graph. The material is behaving in much the same way as a spring would under tension and, if the force is removed, the material returns to its original length. The material is obeying Hooke's Law and this phase is known as the Elastic Stage .
Elastic deformation stretches the bonds between atoms without applying enough energy to break their Inter-atomic Bonds and so when the applying force is removed, the atoms ‘spring back’ to their original positions.