半固态LC4合金的屈服应力

以SIMA法制备的非枝晶LC4合金为例 ,进行压缩试验 ,测出了应变速率、变形温度及保温时间不同时的屈服应力。结果表明 :当应变速率相同时 ,在固态区 ,屈服应力随着变形温度的增加稍有减小 ;而在半固态区 ,屈服应力随着变形温度的提高明显减小 ,且在共晶点附近急剧下降。屈服应力随着保温时间增加而略有减小的主要原因是由于随着保温时间的增加 ,晶粒的尺寸、球形化程度、固相晶粒间的结合程度及晶粒内俘获的液体不同所致。当变形温度及保温时间相同时 ,随着应变速率的升高 ,屈服应力明显增加。通过分析不同变形温度及保温时间的材料显微组织进化规律 ,阐明了屈服应力与显微组织之间的内在联系     

Ti-6Al-4V合金等温压缩变形时的温度敏感性指数

研究变形工艺对Ti-6Al-4V合金微观组织演变和温度敏感性指数的影响。等温压缩实验选取的变形温度为1123～1213 K、应变速率为0.01～10.0 s-1、变形程度为30%,50%,70%。结果表明,初生α相晶粒尺寸和体积分数随着变形温度升高和应变速率增加逐渐减小;温度敏感性指数随着变形温度升高和应变速率增加逐渐增大;应变增加时,初生α相晶粒尺寸和体积分数变化不明显,温度敏感性指数减小。     

TC4钛合金高温变形时的微观组织演变

基于TC4钛合金压缩变形时的微观组织观察和定量金相实验,研究了变形工艺参数(变形温度、应变速率和变形程度)对微观组织演变和组织参数(初生α相晶粒尺寸和体积分数)的影响。结果表明: 在α+β两相区,随着变形温度的升高,初生α相晶粒尺寸呈波浪状变化,初生α相逐渐减少;随着应变速率的增加,初生α相形貌由等轴状转变为长条状,微观组织参数的变化规律与温度有关,当变形温度高于1203 K时,初生α相晶粒尺寸逐渐减小,而低于1203 K时,初生α相晶粒尺寸呈波浪状变化。当变形温度高于1223 K时,初生α相体积分数呈波浪状变化,而低于1223 K时,初生α相体积分数逐渐减小;随着变形程度的增加,二次α相逐渐减少,初生α相晶粒尺寸呈先减小后略有增大的趋势,而初生α相体积分数变化较小     

TA15钛合金双道次热压缩变形软化行为及等轴α相组织演变规律

对TA15钛合金进行了双道次热压缩实验,研究了该合金在变形温度910、940和970℃,应变速率0.01、0.1和1 s1及道次间隙保温时间600、1000、1400、1800和2200 s等不同变形参数下的软化行为,定量计算了相应的等轴0晶粒尺寸及其含量.结果 表明:TA15钛合金在双道次热压缩过程中,较高的变形温度和较小的应变速率会促使变形过程中的流动应力减小;在道次间隙保温过程中,材料呈现出了静态软化现象,其静态软化率随着变形温度的升高、应变速率的增大及道次间隙保温时间的延长而增加,且受应变速率的影响程度最大;此外,从合金的微观组织中观察到等轴d相晶粒在道次间隙保温过程中得到了明显的细化,且细化程度与其静态软化呈正相关性.     

Cu-P-Cr-Ni-Mo耐候钢两相区铁素体晶粒细化机理

采用Gleeble-3500热模拟试验机和背散射电子衍射(EBSD)技术分析研究了Cu-P-Cr-Ni-Mo耐候钢在(γ+α)两相区不同变形量和不同变形温度下的组织演变,探究了铁素体晶粒的细化机理。结果表明,Cu-P-Cr-Ni-Mo耐候钢在750℃,应变速率为0.01 s-1变形时,当应变超过0.69后,铁素体的转变量增加不明显,但等轴铁素体晶粒数量增加,新形成的细小等轴再结晶铁素体晶粒尺寸达到1.4~3μm;在850~750℃、应变速率为0.01 s-1变形时,随着变形温度的降低,铁素体转变量增加,铁素体晶粒尺寸减小,但均匀性降低。Cu-P-Cr-Ni-Mo耐候钢(γ+α)两相区铁素体晶粒细化机制为铁素体的连续动态再结晶,并且随着变形温度的降低,发生铁素体的连续动态再结晶的能力增强。     

热变形条件对Ti60合金微观组织的影响

研究变形工艺参数对Ti60合金微观组织(初生α相尺寸和体积分数)的影响。实验时选取的变形温度为900、930、960和980℃,应变速率为0.001、0.01、0.1、1.0和10.0 s?1,变形程度为50%、60%和70%。结果表明:变形温度对Ti60合金微观组织有着显著影响。在(α β)两相区,随着变形温度的升高,初生α相含量减少,而α相尺寸呈先增大后减小的趋势;应变速率对Ti60合金变形组织中初生α相的形态有较大影响。随着应变速率的增加,晶粒尺寸呈先减小后略有增大的趋势,初生α相含量呈逐渐减小的趋势;变形程度存在一临界值,超过这一临界值后,变形程度的增加有利于晶粒的细化;初生α相含量随着变形程度的增加呈先增大后减小的趋势。     

热压缩变形对Al-1Mg-1.0Si-0.5Cu-0.2Zr合金组织的影响

利用Gleeble-1500热模拟机对Al-Mg-Si-Cu合金进行高温压缩变形模拟实验,分析了变形温度、应变速率和应变量对显微组织的影响。结果表明:在应变速率和应变量一定时,变形温度较低时(380、430℃),合金变形带中未出现明显的再结晶晶粒,且有大量的位错杂乱的缠结在第二相粒子周围,形成位错塞积;变形温度较高时(480、530℃),变形带间出现了许多动态再结晶晶粒,晶粒的数量随着温度的升高而增加同时位错数量显著减少。在合金变形温度为480℃、应变量0.76时,再结晶晶粒随着应变速率升高,其数量有所增加而平均尺寸有所减小,再结晶程度增强。当合金在较低温度、较高应变速率和较小应变量下变形时,Al-Mg-Si-Cu合金主要软化机制为动态回复;在变形温度较高,应变速率较低,应变量较大时,合金的动态软化机制主要是动态再结晶。     

高固相率MB15镁合金半固态压缩力学行为研究

通过MB15镁合金半固态等温压缩试验,研究了高固相率半固态材料的力学行为,提出了触变强度是高固相率半固态金属在稳态变形过程中的触变点的应力,即半固态金属固体骨架所能承受的最大正应力,并分析了加热温度、应变速率、保温时间、固相晶粒大小、晶粒圆整度及材料本身的强度等因素对半固态触变强度的影响,提出了触变强度的存在条件.结果表明,半固态触变强度随着加热温度的升高、应变速率的降低及保温时间的延长而降低,随着固相晶粒的减小、晶粒圆整度的增加及材料本身强度的增加而增加,当半固态材料内部的固相颗粒相互连结形成固体骨架时,存在触变强度.     

热变形参数对Ti6246钛合金显微组织的影响

使用Gleeble-3800热模拟机对Ti6246钛合金进行了等温热压缩试验,研究了变形温度、应变速率以及变形程度对合金显微组织的影响。结果表明:Ti6246合金经不同相区变形时,显微组织对热变形参数敏感性不同。变形温度对两相区变形后初生α相含量,β相区变形后β晶粒尺寸、数量的影响较为显著;应变速率则对两相区变形后初生α相的形态、β相区变形后β晶粒的取向和晶界再结晶有较大影响,且在低温,大应变速率时,观察到合金局部塑性流动现象;随着变形程度的增大,两相区变形后拉长的初生α相发生破断、球化,β相区变形后粗化的β晶粒呈现等轴形态。     

MB15镁合金半固态压缩力学行为研究

通过MB15镁合金半固态等温压缩试验，研究了半固态材料的力学行为，提出了触变强度是半固态金属在稳态变形过程中触变点的应力，即半固态金属固体骨架所能承受的最大正应力，并分析了加热温度、应变速率、保温时间、固相晶粒大小、晶粒圆整度及材料本身的强度等因素对半固态触变强度的影响，提出了触变强度的存在条件。结果表明，半固态触变强度随着加热温度的升高、应变速率的降低及保温时间的延长而降低，随着固相晶粒的减小、晶粒圆整度的增加及材料本身强度的增加而增加；当半固态材料内部的固相颗粒相互连结形成固体骨架时，存在触变强度。     

Microstructure development and thixoextrusion of magnesium alloy prepared by repetitive upsetting-extrusion

Thixoextrusion involves processing alloys with a spheroidal microstructure in the semi-solid state. Before thixoextrusion, repetitive upsetting-extrusion (RUE) is introduced into the strain induced metal activation (SIMA) process to predeform AZ80 magnesium alloy. Microstructure evolution of RUE formed AZ80 magnesium alloy during partial remelting is studied at temperatures for times. Tensile mechanical properties of thixoextruded components are determined and compared with those of AZ80 magnesium alloy thixoextruded from starting material produced by casting. The results show that with increasing number of RUE passes solid grain size decreases and the rate of liquation is improved. Prolonged holding time results in grain coarsening and the improvement of degree of spheroidization. The variation of the solid grains with holding time obeys the Lifshitz, Slyozov and Wagner law. Increasing the heating temperature is favorable for the formation of spheroidal solid grains. The tensile properties for AZ80 magnesium alloy thixoextruded from starting material produced by RUE are better than those of AZ80 magnesium alloy thixoextruded from starting material produced by casting.     

High temperature deformation behavior and microstructure evolution of wrought nickel-based superalloy GH4037 in solid and semi-solid states

Cylindrical samples of Ni-based GH4037 alloy were compressed at solid temperatures (1200, 1250 and 1300 °C) and semi-solid temperatures (1340, 1350, 1360, 1370 and 1380 °C) with different strain rates of 0.01, 0.1 and 1 s⁻¹. High temperature deformation behavior and microstructure evolution of GH4037 alloy were investigated. The results indicated that flow stress decreased rapidly at semi-solid temperatures compared to that at solid temperatures. Besides, the flow stress continued to increase after reaching the initial peak stress at semi-solid temperatures when the strain rate was 1 s⁻¹. With increasing the deformation temperature, the size of initial solid grains and recrystallized grains increased. At semi-solid temperatures, the grains were equiaxed, and liquid phase existed at the grain boundaries and inside the grains. Discontinuous dynamic recrystallization (DDRX) characterized by grain boundary bulging was the main nucleation mechanism for GH4037 alloy.     

Deformation mechanism and forming properties of 6061Al alloys during compression in semi-solid state

The 6061 semi-solid aluminium alloy feedstocks prepared by near-liquidus casting were compressed in semi-solid state by means of Gleeble-3500 thermal-mechanical simulator. The relationship between the true stress and the true strain at different temperatures and strain rates was studied with the deformation degree of 70%. The microstructures during the deformation process were characterized. The deformation mechanism and thixo-forming properties of the semi-solid alloys were analyzed. The results show that the homogeneous and non-dendrite microstructures of semi-solid 6061Al alloy manufactured by near-liquidus casting technology could be transformed into semi-solid state with the microstructure suitable for thixo-forming which are composed of near-spherical grains and liquid phase with eutectic composition through reheating process. The deformation temperature and strain rate affect the peak stress significantly rather than steady flow stress. The resistance to deformation in semi-solid state decreases with the increase of the deformation temperature and decrease of the strain rate. At steady thixotropic deformation stage, the thixotropic property is uniform, and the main deformation mechanism is the rotating or sliding between the solid particles and the plastic deformation of the solid particles.     

超细晶铁素体基体Q&P钢恒应力下的变形行为

采用预淬火热处理工艺得到了超细晶Q&P钢,通过力保载程序控制试验研究了试验钢在某一恒定应力作用下的组织演变和变形行为。结果表明,随着保载应力不断增大,应变量逐渐增加,应变随保载时间的延长略微增加;当加载应力低于屈服强度时,随保载时间延长,应变无变化;加载应力高于屈服强度时,变形量剧烈增加,在变形过程中应变诱导的残留奥氏体向马氏体转变;同时随着应变量不断增加,残留奥氏体的稳定性呈先降低再升高趋势。     

Kinetic Analysis of the Austenite Grain Growth in GCr15 Steel

The growth behavior of austenite grains in GCr15 steel was investigated through the isothermal annealing tests of the steel under different heating temperatures and holding times. The tests were performed on a Gleeble-3800 thermo-mechanical simulation machine. Austenitizing temperatures 1223, 1323, 1373, and 1423 K were chosen, and holding time varied from 0 to 480 s. Experimental results suggest that austenite grains grow gradually with the increase of heating temperature, and holding time has an important effect on the growth of austenite grains. The time exponent for the growth is bigger at higher temperature, and the growth rate decreases with increasing time. On the basis of previous models and experimental results, a mathematical model that can describe the growth behavior of austenite grains in the tested steel under different heating temperatures and holding times was obtained using regression analysis. The predicted grain sizes by the model are in good agreement with measured ones.     

Characteristics of the tensile flow behavior of Ti–46Al–9Nb sheet material – Analysis of thermally activated processes of plastic deformation

Flow behavior, strain hardening and activation parameters, i.e. activation volume, stress exponents and normalized free enthalpy of activation, of Ti–46Al–9Nb sheet with near-gamma microstructure have been investigated in tension tests between 700 and 1000 °C. The dependence of yield stress on temperature and strain rate, the course of the strain hardening curves and the values of activation parameters show that thermally activated dislocation mechanisms are mainly involved in the tensile deformation process of the investigated material. At constant temperature the value of the activation volume depends both on plastic strain and strain rate. The activation volume generally decreases with increasing strain. The decrease is particularly well observable for higher strain rates, thus indicating a growing role of thermally activated climb mechanisms governing the process of dynamic recovery. The activation volume calculated for a constant plastic strain (2% in case of this study) is a function of temperature and strain rate. At lower deformation rates, or alternatively at higher temperatures, the activation volume increases. Such behavior indicates a decrease in dislocation density due to the onset of dynamic recrystallization. The analysis of stress exponents and the obtained free enthalpy of activation confirm that different thermally activated processes are acting during deformation under the tensile test conditions studied.     

The mechanical behavior of a cryomilled Al–10Ti–2Cu alloy

The mechanical behavior of a cryomilled Al–10Ti–2Cu (wt.%) alloy has been studied by performing uniaxial tension tests at temperatures ranging from room temperature to 525°C. Elastic–nearly perfectly plastic stress–strain behavior is observed at all temperatures. Tension–compression asymmetry of the room temperature yield stress is also observed. These characteristics are in agreement with those recently reported in the literature for single-phase NC materials. The flow stress (700 MPa at room temperature) decreases dramatically with increasing temperature. Testing of material following thermal exposures suggests that microstructural coarsening alone cannot account for the decrease in strength with increasing temperature. From a coarsening standpoint, this material appears to be very thermally stable. The ductility is influenced by several factors. Low levels of internal porosity along with the presence of fine oxide and carbide dispersoids contribute to lower ductility. The absence of work hardening exhibited by the Al–10Ti–2Cu also leads to reduced strain to failure. The features observed on fracture surfaces suggest that fracture occurs by the nucleation and growth of voids at particle–matrix interfaces. Evidence of fracture along prior powder particle boundaries is present as well. The microstructure consists primarily of regions containing grains measuring in the range 30–70 nm. Large grained regions consisting of nominally pure Al ranging in size from 300 to 500 nm are also present. No evidence of dislocation activity within either the fine or large grained regions can be found in the as extruded material. Specimens deformed at room temperature and 93°C reveal evidence of dislocation activity within the large grain regions. Dislocation configurations suggest an Orowan bypass mechanism. No dislocations are found within the 30–70 nm size grains following tensile deformation.     

加热速度和保温时间对X70钢组织与性能的影响

用Gleeble-3500型热模拟试验机研究了加热速度和保温时间对X70钢组织和性能的影响。结果表明,加热速度对奥氏体相变点及冷却后的组织和性能有显著影响。随加热速度的增加,相变点呈线性增加,而钢的强度和韧性降低。随着保温时间的延长,钢的强度在保温时间为60 s时达到最大值,随保温时间的进一步增加,强度呈降低的趋势;而且随加热温度的升高,降低的趋势更明显。而冲击韧性则随保温时间的延长呈降低趋势。     

铝合金搅拌摩擦焊管液压胀形性能及变形规律

采用热模拟实验研究了Ni50Ti46-xAl4Hfx (x=0～6,at％)合金的微观组织特点和高温变形行为.合金由基体相NiTi和第二相Ti2Ni组成.Hf全部溶解在基体相和第二相中,并且促进了第二相的析出,显著提高了合金的高温强度.Ni-Ti-Al-Hf合金在700～800℃下变形行为的研究表明:合金具有良好的高温压缩性能,流变应力随着应变速率的减小或变形温度的增加而降低,其高温变形行为符合温度补偿幂规律描述.Hf的添加使合金的应力指数(n)和变形激活能(Q)上升,这主要是由于Hf的固溶强化、促进Ti2Ni相析出强化和高温稳定性造成的.