Effect of roll speed ratio on the texture and microstructural evolution of an FCC high-entropy alloy during differential speed rolling

A very coarse-grained(335μm)Fe_(41)Mn_(25)Ni_(24)Co_(8)Cr_(2)high-entropy alloy with a single FCC phase was cold rolling to a 80%reduction in thickness using the differential speed rolling technique with various speed ratios(SRs)ranging between 1 and 4.As the SR was increased,the volume fraction of the region of high-density micro-shear bands increased to accommodate the higher shear strain.At SR=4,the entire thickness of the sheet was covered with micro-shear bands,and ultrafine(sub)grains with a size of1.4μm were uniformly formed along the shear bands.A continuous dynamic recrystallization(CDRX)mechanism occurred during rolling,and a higher SR accelerated the CDRX process.During conventional rolling(at SR=1),a brass{110}<112>orientation texture with minor components of S{123}<634>and Cu{112}<111>orientations developed.At higher SRs,shear texture developed as the main type,while the development of rolling texture was suppressed.The microstructure at SR=4 obtained after annealing at973 K showed a fully recrystallized microstructure composed of a five times smaller grain size(4μm)with a higher intensity ofγfiber texture compared with that prepared by conventional rolling.The samples processed with high SRs exhibited better tensile properties compared with the conventionally rolled sample in terms of strength and ductility after annealing.The current results demonstrate that by using differential speed rolling with a high SR,one can achieve a significantly finer and more homogeneous microstructure,stronger shear texture,and superior tensile mechanical properties for an FCC high-entropy alloy compared to that obtained by conventional rolling.The strength of the as-rolled and annealed samples was quantitatively explained by considering the contribution of grain size and dislocation density to strengthening.

Using Connected Truck Trajectory Data to Compare Speeds in States with and without Differential Truck Speeds

Historically, researchers and practitioners have utilized spot speeds and microscopic simulation methodologies to evaluate the operational impact of differential or uniform speed limits for trucks and passenger vehicles. This paper presents a methodology that uses connected truck data to develop a statistical characterization of both passenger car and truck speeds. These techniques were applied to three adjacent states, Illinois, Indiana and Ohio. Illinois and Ohio have 70 mph speed limits for both trucks and cars. Indiana has a differential speed limit for heavy trucks (65 mph) and passenger cars (70 mph). The statistical distribution of truck speeds was then compared among Illinois, Indiana and Ohio. These speeds were derived from over 8 million connected truck records traveling along Interstate 70 in Illinois, Indiana and Ohio during a one-week period from May 8-14, 2022. Statistical test results over selected 20-mile sections in each state showed that median truck speeds in Indiana with its differential speed limit of 65 mph were only 1 - 2 mph lesser than the neighboring states of Illinois and Ohio who observe a uniform speed limit of 70 mph for all traffic.

Microstructure and tensile properties of magnesium nanocomposites fabricated using magnesium chips and carbon black

In this study,carbon black(0,0.01,0.03 and 0.08 wt%)and AZ31(Mg-3Al-lZn)magnesium chips were used to fabricate carbon black-reinforced magnesium matrix composites with extrusion or a combination of extrusion and high-ratio differential speed rolling.After hot pressing at 693 K and extrusion at 623 K with an extrusion ratio of 22,the magnesium chips coated with carbon black were soundly bonded into a bulk composite material.The grain sizes of the extruded materials were similar with a size of 48.2-51.5|im despite the difference in the amount of carbon black.The yield strength and ultimate tensile strength increased from 177 to 191 MPa and from 240 to 265 MPa,respectively,as a result of the addition of 0.01%carbon black;however,a further increase in the strength was marginal with additional carbon black.The same trend was observed in the strain hardening behavior.The tensile elongation increased by to the addition of 0.01%carbon black(from 15.8%to 17.4%)due to the increased work hardening effect,but decreased with additional carbon black due to its agglomeration and poor dispersion at higher concentration.After high-ratio differential speed rolling(HRDSR)on the extruded materials and subsequent annealing,the AZ31 and AZ31 composites had a similar fine grain size of 16.3-17.9 p.m.The annealed HRDSR composites showed the best mechanical properties at a higher content of carbon black(0.03%)compared to that(0.01%)for the extruded composites.This resulted from the enhanced dispersion effect of the carbon black due to the high shear flow induced during the HRDSR process.The extruded composites exhibited the three distinct hardening stages(stage II,stage III and stage IV),while the annealed HRDSR composites mainly displayed the stage III hardening.The addition of carbon black increased the strain hardening rate at all the strain hardening stages in both of the extruded and annealed HRDSR materials.At the initial hardening stage,the strain hardening rates of the extruded composites were higher than those of the annealed HRDSR composites,but this became reversed at the later stage of hardening.Possible explanations for this observation were discussed.The strength analysis suggests that dislocation-carbon black interaction by Orowan strengthening and dislocation generation due to a difference in thermal expansion between matrix and carbon black are the major strengthening mechanisms.

Differential variable speed limits to improve performance and safety of car-truck mixed traffic on freeways

This study develops a differential variable speed limit(DVSL)which assigns different speed limits for car and truck,and varies speed limits based on traffic conditions.The proposed DVSL algorithm changes speed limits in real time based on truck percentage and occupancy immediately upstream of the ramp and the average speed of the control road sections upstream of the ramp.DVSL algorithm also considers spatial coordination of speeds,which gradually changes the speed limits in successive road sections upstream of the ramp when the severe congestion occurs.The study tested the impacts of DVSL and three other speed limit strategies on delay and safety for a section of the Gardiner Expressway in Toronto,Canada using the VISSIM traffic simulation model.The other strategies are 1)uniform speed limit(USL),2)differential speed limit for car and truck(DSL),and 3)USL&DSL(U&D)-i.e.,USL at low truck percentage and DSL at high truck percentage.It was found that DVSL showed the lowest delays for both car and truck among the four strategies.This is mainly because DVSL increased the spacing between vehicles in the right lane upstream of the on-ramp and facilitated vehicles’merging into the mainline freeway.It was also found that DVSL showed the lowest likelihood of rear-end crash between the lead and following vehicles among the four strategies.This study demonstrates that the proposed DVSL algorithm can better control car and truck speeds to reduce delay and improve safety of car-truck mixed traffic flow on freeways.

The significance of the posted minimum speed limits along interstate highways in South Carolina on traffic operation and safety

This paper evaluated the significance of the posted differential speed limits(DSL)on traffic safety and operation along interstate highways in South Carolina.This paper defines DSL as the difference between the posted maximum and minimum speed limits.The paper used vehicle speed and traffic crashes for analysis.The evaluation results showed over 99%of all vehicles complied with the posted 45 mph minimum speed limit and more than 50%of drivers drove above the posted maximum speed limit regardless of the posted maximum speed limits.The results also indicated that speed variations increased as the DSL increased.However,the safety evaluation showed conflicting results on the impacts of the DSL on safety.The results showed the DSL had negative implications on crashes per AADT per lane.This is attributed to increased interactions between slower and faster-moving vehicles in the traffic stream.On the contrary,it was observed that segments with higher DSL had fewer crashes per mile per lane than the segments with the lower DSL.These results suggest there is a need to conduct more research to quantify the efficacy of posting the minimum speed limit because there were no problems observed with motorists driving too slowly on interstate highways.The results provide important information on the efficacy of posting minimum speed limits to improve the uniformity of traffic flow and safety.