Ductilizing refractory high entropy alloys

Thomas Dam and Sarmad Shaba

Chalmers University of Technology

Department of Materials and Manufacturing Technology

Supervisors: Sheng Guo and Saad Sheikh

Agenda

• Background

• Objective

• Introduction

• Strategy

• Binary alloys

• High entropy alloys

• Methods

• Experimental results with refractory alloys

• Conclusion

Background

• Higher-temperature materials are needed to shift performance towards the ideal limits

J. H. Perepezko, “The Hotter the Engine, the Better,” 2009

Objective

• Verify whether the electron theory is a valid strategy to ductilize refractory alloys

• Valid if:

• A ductile refractory high entropy alloy with single phase solid solution can be identified

Definition of high entropy alloy

• An alloy consisting with 5 or more metallic elements with 5 to 35% of each individual element

• Gibbs free energy

• ∆𝐺𝑚𝑖𝑥 = ∆𝐻𝑚𝑖𝑥 − 𝑇 ∗ ∆𝑆𝑚𝑖𝑥𝑖𝑛𝑔

• ∆𝑆𝑐𝑜𝑛𝑓𝑖𝑔𝑢𝑟𝑎𝑡𝑖𝑜𝑛𝑎𝑙 ≥ 1.5 R

One component Multiple components

Why refractory high entropy alloys?

Senkov, Intermetallics, V.19 Issue 5, 2011 p.698-706

Good high temperature strength But brittle

Senkov et al., J Alloy Compd, 509, 2011, 6043

Valence electron concentration (VEC)

Strategy

• Shear modulus for W53Xdecreases by alloying

• Mo instead of W in our case

• Group 4, Ti

• Group 5, Nb

• Mo-Ti and Mo-Nb Hu et al, Journal of Alloys and Compounds, Volume 671, 2016, 267–275

Phase diagram for Mo-Ti

Phase diagram for Mo-Nb

2400 °C

Phase diagram for Mo-Hf

Experimental methods

• Vacuum arc melting furnace

• Vickers hardness testing

• X-ray diffraction (XRD)

• Phase determination

• Scanning electron microscope (SEM)

• Microstructure, phase and fracture analysis

Arc melting furnace Copper molds

Cast sample

𝐻𝑉 ≈0.01819𝐹

𝑑2, 𝑑 =

𝑑1+𝑑2

2

• Vickers hardness (HV)

• 1 kg for 15 seconds

• 7-8 indents

• Average of the HV from each indent

Experimental result

Bending result of MoTi

503

424364

307

146 142 128 118

0

100

200

300

400

500

600

MoNb Mo0.5Nb MoTi Mo0.5Ti

HV and calculated HV

HV Calculated HV

Experimental result

• MoNb and Mo0.5Nb • MoTi and Mo0.5Ti

Strategy for HEAs

• Problems of alloying with Mo

• Brittleness

• Secondary phases with most refractory elements

• Two refractory HEAs will be prepared to verify the effects of alloying with Mo

• HfMoTiVZr for multiple phases

• MoNbTaVW for brittleness

Experimental result

• HfMoTiVZr

• Brittle and multiple phases

• VEC: 4.6

Experimental result

• MoNbTaVW

• Brittle and single phase

• VEC: 5.4

Cleavage

Strategy for HEA

• Composition

• Hf0.5 - High density

• Nb0.5 - High VEC

• Ta0.5 - High density

• Ti - Low VEC and density

• Zr - Low VEC and density

• Hf0.5Nb0.5Ta0.5TiZr

Hf0.5Nb0.5Ta0.5TiZr

• VEC: 4.29

376

112

0

100

200

300

400

500

Hf0.5Nb0.5Ta0.5TiZr

HV and calculated HV

HV Calculated HV

Hf0.5Nb0.5Ta0.5TiZr

• X-ray diffraction

• BCC phase

• SEM image

Interdendrite

Dendrite

VEC comparison between single phase refractory HEAs• VEC affects ductility

• Transition area

4 4,5 5 5,5 6

VEC

Hf0.5Nb0.5Ta0.5TiZr

HfNbTiZr

HfNbTaTiZr

MoNbTaVW

MoNbTaW

MoNbHfZrTi, as-cast

Ductile Brittle

Conclusion and discussion

• A ductile refractory HEA with single phase solid solution was identified

• Refractory HEAs with Mo are brittle

• High VEC

• Secondary phases

• Lowering VEC could be a valid strategy

• More compositions has to be tested

• Mechanical properties still unknown for Hf0.5Nb0.5Ta0.5TiZr

• Our composition can be improved