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The gas phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing boron carbides via the precursors of BCl 3– C 3 H 6(propene)– H 2 is investigated with a set of 325 gaseous species, in which the data for 135 species are evaluated in this work. The thermochemistry data are calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels. The concentration distribution of all of the 325 species is obtained with the principle of chemical equilibrium. The thermochemistry data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. The heat capacities and entropies at temperatures in 298.15–2000 K are evaluated with the standard statistical thermodynamics. The Gibbs free energies of formation in 298.15–2000 K are calculated with the classical thermodynamics based on the developed heat capacities and entropies. By including the crystal B 4 C , C (graphite), and B , the results for an example of the 3:1:2 precursors of BCl 3: C 3 H 6: H 2 show that the crystal B 4 C can be produced at temperatures higher than 700 K while the graphite has a higher molar value and can be produced at lower temperatures. It is also examined that the production of graphite can be controlled by changing the ratio of the injected reactants or pressure. It is interesting that BHCl 2, BCH 3 Cl 2, BH 2 Cl , and B 2 Cl 4 are found to be the most effective species in the CVD process, which is similar to those in the BCl 3– CH 4– H 2 system. The results predicted in this work are consistent with the experiments.

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The hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) tests cycling from 400°C to 700°C under full reverse strain-controlled condition were investigated. Stress-mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high-temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi-cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter. KEYWORDS cyclic softening, cyclic stress-strain behavior, fatigue life, hot work die steel, thermomechanical fatigue | INTRODUCTIONDuring the die filling process, like die casting, hot extrusion, hot forging or hot stamping, the service life of hot work die is limited because of their extreme working environments in terms of the cyclic alternating high temperature, which results from the close contact between hot work die and hot work piece (1200°C for steel forging) or molten metal (675°C for Al-alloy casting, 930°C for Cu-alloy casting) and mechanical stress, leading to cyclic plastic deformation, which gives rise to crack initiation. [1][2][3][4] All along, the hot work die is damaged through a complex oxidation/wear/non-isothermal low cycle fatigue interaction. 5,6 Therefore, non-isothermal low cycle fatigue damage (occurrence of the cyclic reversed plasticity at low and high temperatures) of hot work die is usually called thermomechanical fatigue (TMF). According to incomplete statistics 7 , thermal cracking caused by TMF accounts for approximately 80% of the total failure die-casting dies. Thus, good TMF behavior can prolong the service life of the hot work die. However, the literatures about TMF behavior of hot work die steel are rarely found. 5,6,8,9 As TMF tests are time consuming and require expensive equipment, the fatigue life prediction with varying Nomenclature: ϕ, mechanical strain/temperature true phase angle; Δε mech , mechanical strain range; Δε mech /2, mechanical strain amplitude; R ε , strain ratio; (ε mech ) max , maximum mechanical strain; (ε mech ) min , minimum mechanical strain; T 0 , reference temperature; T max , maximum temperature; T min , minimum temperature; σ max , maximum stress; σ min , minimum stress; σ m , mean stress; Δσ, st...