About

Kevin Hemker is the Alonzo G. Decker professor of mechanical engineering at Johns Hopkins University. He is known for his work explaining the underlying, atomic-level details that govern the mechanical response, performance, and reliability of disparate material systems. Hemker holds joint appointments in the departments of Materials Science and Engineering, and Earth and Planetary Sciences. His research group has made key observations and discoveries that have challenged the way the community thinks about and understands materials behavior in nanocrystalline materials, materials for microelectromechanical systems (MEMS), metallic micro-lattices, thermal barrier coatings for satellites and gas turbines, armor ceramics, extreme environments, and high-temperature structural materials. His current projects include deformation behavior and grain growth in nanocrystalline materials and thin films, characterization and modeling of multilayered thermal protection systems, development of metallic materials for MEMS, and the development of architected materials through textile and additive manufacturing.

Research topics

• Materials science
• Composite material
• Metallurgy
• Nanotechnology
• Chemistry
• Optoelectronics
• Crystallography
• Thermodynamics
• Geometry
• Condensed matter physics
• Physics

Selected publications

• Solidification behavior and cracking mechanisms of Ru-containing BCC-B2 superalloys

  Scripta Materialia · 2025-07-01 · 9 citations

  article
  PublisherDOI

• The thermal and mechanical response of refractory alloys at ultrahigh temperatures

  Acta Materialia · 2025-11-29

  articleSenior authorCorresponding
  PublisherDOI

• Stress and Temperature Dependence of Screw Dislocation Motion in Refractory Multi-Principal Element Alloys

  High Entropy Alloys & Materials · 2025-07-19 · 2 citations

  articleOpen access

  Abstract The motion of dislocations controls the strength of alloys with a body-centered cubic structure. Recently, new alloys with body-centered cubic structure, called refractory multi-principal element alloys, have demonstrated outstanding high-temperature strength, with some compositions exhibiting exceptional stability in strength with respect to temperature—a so-called athermal regime. Despite recent advances, the understanding of the impact and glide mechanisms of dislocations at high temperatures, particularly corresponding to the athermal regime, in these unusual yet superior alloys are lacking. Here, a phase field dislocation dynamics model is employed to predict the energetically favorable pathways taken by initially screw-character, long dislocations as a function of temperature and driving stress. The simulations resolve the critical stress at which glide becomes jerky to smooth, the changes in the glide mechanisms as temperature increases, and the local impact of variations in composition-dependent energetic barriers. It is shown that the jerky-to-smooth stress exhibits a two-stage response, where it decays with temperature at low temperatures and transitions to an athermal regime at high temperatures, like that measured for these alloys. The analysis elucidates the changes in the glide processes responsible for the onset of the athermal regime in critical stress and shows a close connection to experimentally measured athermal temperatures, suggesting that screw dislocations might impact the high-temperature strength of these alloys.

  PublisherOA PDFDOI

• Mechanical response and failure mechanisms of novel boron carbide ceramics under medium and high strain rates

  Journal of the European Ceramic Society · 2025-11-21

  article
  PublisherDOI

• Achieving High Tensile Strength and Ductility in Refractory Alloys by Tuning Electronic Structure

  Research Square · 2025-04-11 · 1 citations

  preprintOpen access
  PublisherOA PDFDOI

• Tailoring the nanotwin spacing of Ni-Mo-W alloys via composition and substrate temperature control

  Scripta Materialia · 2024-01-15 · 4 citations

  articleOpen accessSenior authorCorresponding
  PublisherOA PDFDOI

• Capturing the ultrahigh temperature response of materials with sub-scale tensile testing

  Materials Today · 2024-09-13 · 9 citations

  articleOpen accessSenior authorCorresponding

  Materials that can maintain their strength at extreme temperatures are in great demand. Efforts to develop ultrahigh temperature materials are underway, but ultrahigh temperature data is hard to find, and tensile tests conducted above 1400 °C are expensive and extremely rare. Here, we demonstrate Joule heating of sub-scale specimens as a promising alternative for conducting ultrahigh temperature tensile tests. Challenges associated with testing at extreme temperatures have been addressed, and unique advantages of the new methodology include rapid heating (and cooling) of specimens to temperatures as high as their melting temperatures, in vacuum, with in situ temperature and strain measurement. Proof-of-concept tensile tests on ATI C103™ were conducted at temperatures ranging from 25 to 2,000 °C, and the results are shown to be in excellent agreement with proprietary datasets. This new test methodology has unveiled a new ultrahigh temperature plateau in the ATI C103™ alloy above 1500 °C and opens the door for exploring ultrahigh temperature deformation mechanisms in a wide variety of materials.

  PublisherDOI

• The deformation mechanisms responsible for strain localization in nanotwinned nickel alloys

  Acta Materialia · 2024-10-21 · 4 citations

  articleOpen accessSenior authorCorresponding
  PublisherOA PDFDOI

• Elucidating the mechanical and thermal response of nanotwinned Ni alloys

  2024-07-09

  reportOpen access

  Transformative advances in nanoscale materials synthesis, characterization and modeling are enabling the synthesis of materials with unprecedent properties and greater understanding of the nanoscale mechanisms that underpin these properties. Nanotwinned Cu alloys have received considerable attention due to their impressive balance of strength and ductility, but they have limited microstructural stability. Recent instantiation of nanotwins in sputter deposited Ni-Mo-W, and several commercial Ni-based superalloys, point to the potential development of a new class of high temperature materials with a very beneficial suite of mechanical and physical properties. The experimental study described in this report was undertaken to elucidate the nanoscale origins of the thermal and mechanical behavior of nanotwinned Ni alloys. Micropillar compression experiments have shown nanotwinned Ni85Mo15-xWx alloys to possess unusually high strengths above 3.5GPa and enhanced microstructural stability. The strength of these nanotwinned alloys is determined by the abrupt formation of shear bands. This study focused on identifying the nanoscale trigger, or triggers, for shear banding in nanotwinned materials using in situ experiments and atomic-scale postmortem analysis. Contrasting and comparing the mechanical response and postmortem nanostructures of “Mo-rich” and “W-rich” nanotwinned specimens elucidated the importance of the lateral motion of easy glide defects that results in erasure of twins and local coarsening of the nanotwinned microstructure. This twin coarsening was then associated with very localized plasticity, strain softening, and shear band formation. The difference between alloys was further studied by considering the role of various material factors: stacking fault energy, coherent twin boundary spacing and flatness, grain size, and the interactions of solute atoms with twin and grain boundaries. The effect of alloy composition and sputtering power and temperature were also investigated and used to control twin spacing and geometry. Combined with atomic-scale simulations, these experiments insights should allow us to identify strategies for achieving concomitant ultrahigh strength and ductility in nanotwinned Ni alloys. In parallel but synergistic studies, our work was buoyed by collaborative state-of-the-art nanoscale orientation and strain mapping at the DOE National Center for Electron Microscopy (NCEM). For example, novel 4D-STEM techniques were used to acquire nanoscale strain maps. The extremely fine twin spacing limited our measurements of atomic-scale thermal expansion within twins, but recent results from NCEM suggest that it will soon be possible to conduct such measurements and to unravel the origins of the novel thermal expansion characteristics of nanotwinned Ni alloys.

  PublisherOA PDFDOI

• Dynamic Failure Mechanisms and Impact Performance Of Microstructurally Engineered Boron Carbide Ceramics

  SSRN Electronic Journal · 2024-01-01

  preprintOpen access
  PublisherDOI

Recent grants

• Experimental Characterization of Deformation Mechanisms in Magnesium Rare Earth Alloys

  NSF · $529k · 2017–2020

• Materials World Network: NSF-Germany (DFG) Materials Collaboration: LIGA Ni-base Superalloys for MEMS Applications

  NSF · $363k · 2008–2012

• NSF-Germany Materials Collaboration: High Temperature Materials for Microelectromechanical Systems

  NSF · $210k · 2005–2009

• NIRT: Uncovering Deformation Mechanisms of Nanostructured Materials

  NSF · $1.2M · 2002–2007

• GOALI: Development of Metallic MEMS Materials for Extreme Environments

  NSF · $420k · 2014–2018

Frequent coauthors

• Amy J. Clarke

  160 shared

• S. Howard

  160 shared

• Alan A. Luo

  The Ohio State University

  144 shared

• James A. Robinson

  144 shared

• Michele V. Manuel

  144 shared

• David Deyoung

  South Dakota School of Mines and Technology

  124 shared

• Alonzo Decker Chair

  Johns Hopkins University

  108 shared

• Joy Forsmark

  Johns Hopkins University

  108 shared

Labs

• Hemker LabPI

Education

• PhD, Materials Science and Engineering

  Stanford University

  1990

Awards & honors

• Materials Science Research Silver Medal from ASM Internation…
• Fellow of the American Association for the Advancement of Sc…
• Fellow of the American Society of Mechanical Engineers (ASME…
• Fellow of the American Society of Metals (ASM International)
• Fellow of The Minerals, Metals & Materials Society (TMS)