Thx Neil!This was my solution:
    public class AttributeTableManager
    {
        private IFeatureLayer _featureLayer;
        public AttributeTableManager(IFeatureLayer fl)
        {
            this._featureLayer = fl;
        }
        /// <summary>
        /// Altera o alias do campo
        /// </summary>
        /// <param name="fieldName">Name do campo na attributetable, ele é usado para procurar o campo correto e alterar o alias</param>
        /// <param name="alias">Alias que será definido para o campo</param>
        public void ChangeAliasName(string fieldName,string alias)
        {
            ILayerFields fields = this._featureLayer as ILayerFields;
            IFieldInfo fldInfo = fields.FieldInfo[fields.FindField(fieldName)];
            fldInfo.Alias = alias;
        }
    }