# Investigation of Micromechanical Properties and Tribological Behavior of WE43 Magnesium Alloy after Deep Cryogenic Treatment Combined with Precipitation Hardening

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

_{max}: 100 mN, 250 mN, 500 mN and 1000 mN; the load hold and unloading time was set at 30 s, in accordance with the ISO 14577 standard [29], and the maximum load hold time was 10 s. For each sample, 24 indents were made. Hardness H

_{IT}and Young’s modulus E

_{IT}were determined using the standard Oliver-Pharr method [30,31], which uses the tangent slope to the initial part of the unload curve in the calculations. The initial part of the curve is described by the formula:

_{f}—indent depth after unloading, α—a constant comprising the modulus of elasticity and Poisson’s coefficient for the indenter material and the sample, m—exponent depending on the indenter geometry.

_{c}, was determined using the dependence:

_{max}—maximum load imposed on the indenter, S—system rigidity.

_{p}, and the depth of its penetration, h

_{c}, was determined: A

_{p}= f(h).

_{IT}, and the reduced modulus of elasticity, E

_{r}, were calculated from formulas:

_{max}—maximum load imposed on the indenter, A

_{p}—indent area after unloading (calibrated).

_{p}—indent area after unloading, h

_{c}—contact depth.

_{IT}:

_{IT}, ν—Young’s modulus and Poisson’s coefficient for the investigated material, E

_{i}, ν

_{i}—Young’s modulus and Poisson’s coefficient for the indenter material (diamond E

_{i}= 1141 GPa, ν

_{i}= 0.07)

^{3}) is shown in Figure 1.

- Load—F
_{n}: 10 (N) - Friction distance radius—r: 7 (mm)
- Linear velocity—v: 0.15 (m/s)
- Distance—s: 100 (m)
- Ambient temperature: 21 ± 1 (°C)
- Air humidity: 40% ± 5 (%)

_{2}, 6 mm in diameter, were used as counter-specimens.

_{w}, linear wear L

_{W}, and mean friction coefficient µ

_{mean}. The coefficient of friction was recorded continuously during the tests.

_{W}, was determined from the formula:

^{3}), P—average area of the wear trace (mm

^{2}), r—radius of the friction distance (mm), F

_{n}—the load applied (N), s—friction distance (m).

_{W}, was measured and verified during profilometric measurements.

## 3. Research Results and Discussion

#### 3.1. Micromechanical Tests

_{IT}, Young’s modulus E

_{IT}, maximum depth of indenter penetration h

_{max}, total work of indentation W

_{tot}and the percentage of the work of elastic recovery η

_{IT}on the maximum indenter load F

_{max}.

_{IT}, and the highest Young’s modulus, E

_{IT}. An approximate 15% increase in mechanical properties was observed compared to the alloy in its initial state (Figure 4). The sub-zero treatment alone, in turn, resulted in an approximate 5–8% increase in micromechanical properties. The observed effects are the result of changes in the structure of the Mg-Y-Nd alloy studied by the authors in previous articles [27,28], where it was shown that deep cryogenic treatment caused a twofold increase in the number of β-phase precipitates and decrease in the grain area compared to the alloy aged without sub-zero treatment. The large number of additional precipitates formed as a result of deep cryogenic treatment after solution treatment can be explained by the formation of additional nucleation sites in magnesium alloys [15], which also occurs, among others, in magnesium alloys with aluminum and with gadolinium [20,21,22,23,24,25,26,33,34].

_{max}, and the micromechanical properties determined from them (Figure 3, Figure 4, Figure 6 and Figure 7) also allowed noticing that for the WE43 magnesium alloy, there was a significant decrease in the measured quantities as the indenter load increased. This phenomenon is explained on the basis of the Taylor’s dislocation model and the relation proposed by the team of Nix, Gao [35] between the hardness, H, and the depth of indent, h (8), determined for crystalline materials and called “geometrically necessary dislocations (GNDs) model” underneath an indenter tip:

_{0}—the hardness in the limit of infinite depth; h*—characteristic length that depends on the shape of the indenter, the shear modulus and H

_{0}.

_{tot}, and the percentage of elastic deformation work, η

_{IT}, as a function of maximum indenter load. Analysis of the results reveals a direct correlation between the deformation resistance of the magnesium alloy studied, observed by changing parameters such as indenter penetration depth, surface area and volume of the indent. For precipitation hardening combined with DCT, the lowest values of total indentation work are observed as well as an approximately 14% share of elastic deformation work due to increase in hardness and Young’s modulus.

#### 3.2. Sliding Wear Tests on WE43 Magnesium Alloy

_{w}, calculated on their basis after different heat treatment variants. Figure 10 presents the linear wear, L

_{w}, and the mean stabilized friction coefficient, μ

_{mean}, of the investigated alloy.

_{w}= 2.32 × 10

^{−3}mm

^{3}/Nm) (Figure 8 and Figure 9). The application of deep cryogenic treatment alone allows for a 30% reduction in wear of the alloy, V

_{w}, while the combination of deep cryogenic treatment with precipitation hardening reduces the volumetric wear of the WE43 alloy by more than double to 1.14 × 10

^{−3}mm

^{3}/Nm.

_{w}(Figure 10a) confirms a significant reduction of this type of wear for the WE43 alloy subjected to sub-zero treatment at −196 °C/24 h, after solution treatment at 545 °C/8 h, and after aging for 24 h. The alloy in the initial state was characterized by linear wear of L

_{w}= 92.4 μm, while the treatment that was carried out allowed for reducing this value by more than 53%, to 43 μm. The mean stabilized coefficient of dry friction, μ

_{mean}, in rotary motion for the friction couple ZrO

_{2}ball/WE43 alloy (Figure 10b) oscillated around µ

_{mean}= 0.48 for the samples in the initial state. Lower values were recorded for samples after sub-zero treatment and after solution treatment combined with sub-zero treatment, µ

_{mean}= 0.41, and µ

_{mean}= 0.44 after sub-zero treatment combined with precipitation hardening. The above results corroborate that increasing the amount of β-phase lamellar precipitates by introducing deep cryogenic treatment to the precipitation hardening process effectively reduces the tribological wear of the investigated alloy. Similar observations can be found in the literature in works on magnesium alloys with aluminium [16,21] and gadolinium [22,23,24,25,26] additions, and in our previous papers concerning WE54 magnesium alloy [27,28].

_{2}balls, was observed on the surface, which was confirmed by the absence of their wear observed during profilometric and microscopic measurements. The analysis results are consistent with the alloy certificate provided by the manufacturer, Luxfer Mel Technologies.

#### 3.3. Determination of Wear Micromechanisms of WE43 Alloy in Rotary Motion

_{2}balls interacting with the studied alloy during tribological tests.

_{2}counter-specimens, on which no wear was observed. However, areas of material transferred from the tested WE43 magnesium alloy samples are visible (Figure 13a–c). Deep cryogenic treatment combined with precipitation hardening also facilitates reduction of this process (Figure 13d). The absence of wear of the ZrO

_{2}balls is mainly due to the large difference in hardness of the materials tested.

## 4. Conclusions

- Deep cryogenic treatment (DCT) combined with precipitation hardening by changing the structure effectively improves the micromechanical and tribological properties of alloy WE43. Among others, a more than 15% increase in hardness H
_{IT}and Young’s modulus E_{IT}, as well as a change in parameters such as maximum indenter penetration depth, surface area and indent volume were demonstrated. The lowest values of total indentation work W_{tot}were observed with an about 14% share of elastic deformation work η_{IT}. - As the maximum indenter load F
_{max}increased, a considerable decrease in the micromechanical properties (H_{IT}, E_{IT}) was observed, which indicates a strong effect of the increase in the surface area of the indents made on the WE43 magnesium alloy on the values measured by means of microindentation. The measurements showed a small scatter of results, 3% on average, and dependence of the tested quantities on the applied heat treatment was preserved for all loads. - The tribological tests and the parameters tested, such as: volumetric wear V
_{w}, linear wear L_{w}and stabilized friction coefficient μ_{mean}, indicate a twofold improvement in wear resistance of WE43 magnesium alloy subjected to deep cryogenic treatment in combination with precipitation hardening, compared to the alloy in the as-delivered condition. - Profilometric studies, microscopic observation and microanalysis of the chemical composition (EDS) showed that the proposed treatment (DCT + precipitation hardening) is effective in reducing the area (depth and width) of the wear traces of the magnesium-rare earth alloy and reduces the cutting process as well as the adhesion of alloy material to counter-specimens, i.e., ZrO
_{2}balls, The heat treatment applied and the friction process itself have no significant effect on the change in alloy composition. - The examination of the morphology of the wear traces allows for the conclusion that abrasive wear was the main wear mechanism of the WE43 alloy. The SEM images showed phenomena characteristic of this wear mechanism, such as microploughing, microcutting and adhesion.
- Further research is being conducted to understand the exact mechanism affecting the improvement of properties of magnesium alloys with rare earth metals under deep cryogenic treatment. The combination of deep cryogenic treatment and precipitation hardening is an effective method to improve the service life of WE43 magnesium alloy.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 1.**Performing an indentation measurement (

**a**); microscopic observation of the indents made (

**b**).

**Figure 2.**Diagram of a tribological couple (

**a**); example of a WE43 magnesium alloy sample in its initial state after tribological test (

**b**).

**Figure 3.**Load–unload curves F(h) for alloy WE43 in its initial state (

**a**), after deep cryogenic treatment (

**b**), after solution treatment and sub-zero treatment (

**c**) and after precipitation hardening combined with deep cryogenic treatment (

**d**).

**Figure 4.**Hardness H

_{IT}(

**a**) and Young’s modulus E

_{IT}(

**b**) of WE43 magnesium alloy as a function of maximum indenter load F

_{max}. The error bars in the figures represent a standard deviation (σ

_{HIT}= 9.97–38.26 MPa; σ

_{EIT}= 0.36–3.58 GPa).

**Figure 5.**Microstructure of the WE54 magnesium alloy in the initial state, after deep cryogenic treatment (DCT), after solution treatment and deep cryogenic treatment (S + DCT), after solution treatment and aging (S + A), and after precipitation hardening combined with deep cryogenic treatment (S + DCT + A + DCT).

**Figure 6.**Maximum depth of indenter penetration h

_{max}during microindentation tests of WE43 magnesium alloy. The error bars represent a standard deviation (σ

_{hmax}= 0.02–0.13 μm).

**Figure 7.**Total work of indentation W

_{tot}(

**a**) and percentage of elastic deformation work η

_{IT}(

**b**) of WE43 magnesium alloy as a function of maximum indenter load F

_{max}. The error bars in the figures represent a standard deviation (σ

_{Wtot}= 0.002–0.094 μJ; σ

_{ηIT}= 0.08–1.36%).

**Figure 8.**SEM microscope images and isometric 3D images of wear traces of WE43 magnesium alloy in the initial state (

**a**,

**b**) and after deep cryogenic treatment combined with precipitation hardening (

**c**,

**d**).

**Figure 9.**Volumetric wear V

_{w}of WE43 alloy in the initial state and after different heat treatment variants. The error bars represent a standard deviation (σ

_{Vw}= 0.43 × 10

^{4}–1.57 × 10

^{4}mm

^{3}/Nm).

**Figure 10.**Linear wear, L

_{w}—(

**a**) mean stabilized friction coefficient, μ

_{mean}—(

**b**) of WE43 alloy in the initial state and after different heat treatment variants. The error bars in the figures represent a standard deviation (σ

_{Lw}= 1.32–6.82 μm; σ

_{μmean}= 0.046–0.064).

**Figure 11.**Microanalysis of the chemical composition of wear traces of the as-delivered WE43 alloy—(

**a**); after deep cryogenic treatment—(

**b**) and after sub-zero treatment combined with precipitation hardening—(

**c**).

**Figure 12.**SEM images of the wear traces formed in rotary motion of as-delivered WE43 alloy—(

**a**); after deep cryogenic treatment—(

**b**); after solution treatment combined with DCT—(

**c**) and after sub-zero treatment combined with precipitation hardening—(

**d**).

**Figure 13.**SEM images of ZrO

_{2}balls after tribological tests in a pair with as-delivered WE43 alloy—(

**a**); after deep cryogenic treatment—(

**b**); after solution treatment combined with DCT—(

**c**) and after sub-zero treatment combined with precipitation hardening—(

**d**).

Content of Components, wt.-% | |||||||
---|---|---|---|---|---|---|---|

Y | Nd | Zr | Zn | Mn | Cu | RE | Mg |

4.0 | 2.3 | 0.49 | 0.01 | 0.02 | 0.002 | 3.0 | residue |

Sample | Heat Treatment Applied | |||
---|---|---|---|---|

Solution Treatment | Deep Cryogenic Treatment | Aging | Deep Cryogenic Treatment | |

WE43 in initial state | - | - | - | - |

WE43–DCT | - | −196 °C/24 h | - | - |

WE43–S + DCT | 545 °C/8 h | −196 °C/24 h | - | - |

WE43–S + A | 545 °C/8 h | - | 250 °C/24 h | - |

WE43–S + DCT + A + DCT | 545 °C/8 h | −196 °C/24 h | 250 °C/24 h | −196 °C/24 h |

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**MDPI and ACS Style**

Barylski, A.; Aniołek, K.; Dercz, G.; Kowalewski, P.; Kaptacz, S.; Rak, J.; Kupka, M.
Investigation of Micromechanical Properties and Tribological Behavior of WE43 Magnesium Alloy after Deep Cryogenic Treatment Combined with Precipitation Hardening. *Materials* **2021**, *14*, 7343.
https://doi.org/10.3390/ma14237343

**AMA Style**

Barylski A, Aniołek K, Dercz G, Kowalewski P, Kaptacz S, Rak J, Kupka M.
Investigation of Micromechanical Properties and Tribological Behavior of WE43 Magnesium Alloy after Deep Cryogenic Treatment Combined with Precipitation Hardening. *Materials*. 2021; 14(23):7343.
https://doi.org/10.3390/ma14237343

**Chicago/Turabian Style**

Barylski, Adrian, Krzysztof Aniołek, Grzegorz Dercz, Piotr Kowalewski, Sławomir Kaptacz, Jan Rak, and Marian Kupka.
2021. "Investigation of Micromechanical Properties and Tribological Behavior of WE43 Magnesium Alloy after Deep Cryogenic Treatment Combined with Precipitation Hardening" *Materials* 14, no. 23: 7343.
https://doi.org/10.3390/ma14237343