Nonlinear ultrasonic (NLU) techniques have emerged as a potential solution to improve the resolution of nondestructive measurements to detect microstructural changes of cyclically loaded materials. However, current NLU methods need power-demanding instrumentation that is useful only in the laboratory settings. On the other hand, phased array systems provide the capability of sensing such changes when the later portion of the elastic waveforms, called diffuse field, is analyzed. Moreover, phased array systems are an excellent solution for field test measurement and imaging of material damage. This study explores the use of NLU metrics based on ratios of harmonic amplitudes and frequencies to map the buildup of damage precursors, such as crystal dislocations, under cyclic loading within the microstructure of fatigued 2024-T3 aluminum specimens. The results show that these metrics are highly sensitive to microstructural fatigue damage making them significantly important to measure mechanical properties, such as fracture toughness, that are extremely useful in predicting the remaining useful life of a studied material. A nonlinear metric of elastic energy that encapsulates the nonlinear effects of subharmonic and higher-harmonic generations and frequency ratio is proposed. These effects of spectral energy shifts are combined making this metric highly sensitive to nano- and micro-scale damage within the fatigued medium.