The Hall-Petch relationship is typically used to describe the relationship between the strength of a material (YS) and the average grain size (D) of that material. The relationship is generally described by a power law equation of the form YS = K*(D^n), where K and n are constants.

A negative intercept value in this equation is not physically meaningful as the Yield strength of a material can't be negative. It's most likely an indication of poor fit between the model and the data, or an error in the data or the analysis.

It is recommended to check the data used for fitting and also the fitting technique used. It is also recommended to compare the results with the experimental data or with other data available in the literature.