Size effect on flow stress in uniaxial compression of pure nickel cylinders with a few grains across thickness

Abstract Uniaxial compression tests were carried out on pure nickel polycrystals with constant thickness ( t ) of 1.0 mm and various grain sizes ( d ). Experimental results show that flow stress does not decrease with grain size continuously according to Hall–Petch law. For specimens with fewer grains across the thickness ( t / d less than about 9), the Hall–Petch law is not valid anymore and even an inverse Hall–Petch effect occurs as the t / d ratio decreases from 6.2 to 3.5. The size effects on flow stress were analyzed considering boundary conditions, including free surfaces and constrained external boundaries. Then a model considering both free surface weakening and external boundary strengthening effects was proposed. The competition of free surface weakening and external boundary strengthening effects results in the flow stress size effects in micro-compression. There is a good agreement between the experimental and analytical results.